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Main Event Loop of Logging Server
int main (int argc, char *argv[])
{
initialize_acceptor
(argc 1 ? atoi (argv[1]) : PORT);
// Loop forever performing logging
// server processing.
for (;;) {
// struct assignment.
ready_handles = activity_handles;
// Wait for client I/O events.
select (maxhp1, ready_handles, 0, 0, 0);
// First receive pending logging records.
handle_data ();
// Then accept pending connections.
handle_connections ();
}
}
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![Advanced ACE Tutorial Do
Conventional Client Logging
Daemon Implementation
The main() method receives logging records from
client applications and forwards them on to the
logging server
int main (int argc, char *argv[])
{
HANDLE stream = initialize_stream_endpoint
(argc 1
? atoi (argv[1])
: PORT);
Log_Record lr;
// Loop forever performing client
// logging daemon processing.
for (;;) {
// ... get logging records from client
// application processes ...
size_t size = htonl (lr.size);
send (stream, size, sizeof size);
encode_log_record (lr);
send (stream, ((char *) lr), sizeof lr);
}
}
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![Advanced ACE Tutorial Douglas C. Schmidt
Problem with Sockets: Lack of Type-safety
int buggy_echo_server (u_short port_num) I/O handles are
{ // Error checking omitted. not amenable to
sockaddr_in s_addr;
strong type
int acceptor =
socket (PF_UNIX, SOCK_DGRAM, 0); checking at
s_addr.sin_family = AF_INET; compile-time
s_addr.sin_port = port_num;
s_addr.sin_addr.s_addr = INADDR_ANY; The adjacent
bind (acceptor, (sockaddr *) s_addr, code contains
sizeof s_addr); many subtle,
int handle = accept (acceptor, 0, 0);
for (;;) { common bugs
char buf[BUFSIZ];
ssize_t n = read (acceptor, buf, sizeof buf);
if (n = 0) break;
write (handle, buf, n);
}
}
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![Advanced ACE Tutorial Douglas C. Schmidt
ACE Connection-Oriented Socket Wrapper Facade
Streaming and Addressing Classes
class ACE_SOCK_Stream class ACE_INET_Addr
: public ACE_SOCK { : public ACE_Addr
public: {
// Trait. public:
typedef ACE_INET_Addr PEER_ADDR; ACE_INET_Addr (u_short port,
ssize_t send (const void *buf, const char host[]);
int n); u_short get_port_number (void);
ssize_t recv (void *buf, ACE_UINT_32 get_ip_addr (void);
int n); // ...
ssize_t send_n (const void *buf, };
int n);
ssize_t sendv_n (const iovec *iov,
int n);
ssize_t recv_n (void *buf, int n);
int close (void);
// ...
};
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![Advanced ACE Tutorial Do
An Echo Server Written using
ACE C++ Socket Wrapper Facades
int echo_server (u_short port_num)
{
// Local server address.
ACE_INET_Addr my_addr (port_num);
// Initialize the acceptor mode server.
ACE_SOCK_Acceptor acceptor (my_addr);
// Data transfer object.
ACE_SOCK_Stream new_stream;
// Accept a new connection.
acceptor.accept (new_stream);
for (;;) {
char buf[BUFSIZ];
// Error caught at compile time!
ssize_t n =
acceptor.recv (buf, sizeof buf);
new_stream.send_n (buf, n);
}
}
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A Generic Version of the Echo Server
template class ACCEPTOR
int echo_server (u_short port)
{
// Local server address (note traits).
typename
ACCEPTOR::PEER_ADDR my_addr (port);
// Initialize the acceptor mode server.
ACCEPTOR acceptor (my_addr);
// Data transfer object (note traits).
typename ACCEPTOR::PEER_STREAM stream;
// Accept a new connection.
acceptor.accept (stream);
for (;;) {
char buf[BUFSIZ];
ssize_t n =
stream.recv (buf, sizeof buf);
stream.send_n (buf, n);
}
}
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![Advanced ACE Tutorial Do
Main Event Loop of Logging Server
int main (int argc, char *argv[])
{
initialize_acceptor
(argc 1 ? atoi (argv[1]) : PORT);
// Loop forever performing logging
// server processing.
for (;;) {
// object assignment.
ready_handles = activity_handles;
// Wait for client I/O events.
ACE::select (int (maxhp1),
// calls operator fd_set *().
ready_handles);
// First receive pending logging records.
handle_data ();
// Then accept pending connections.
handle_connections ();
}
}
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![Advanced ACE Tutorial Do
OO Client Logging
Daemon Implementation
int main (int argc, char *argv[])
{
ACE_SOCK_Stream stream;
ACE_SOCK_Connector con; // Establish connection.
con.connect (stream, ACE_INET_Addr (argc 1
? atoi (argv[1]) : PORT));
ACE_Log_Record lr;
// Loop forever performing client
// logging daemon processing.
for (;;) {
// ... get logging records from client
// application processes ...
ACE_UINT_32 size = lr.size;
lr.size = htonl (lr.size);
encode_log_record (lr);
iovec iov[2];
iov[0].iov_len = sizeof (ACE_UINT_32);
iov[0].iov_base = lr.size;
iov[1].iov_len = size;
iov[1].iov_base = lr;
// Uses writev(2);
stream.sendv_n (iov, 2);
}
}
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![Advanced ACE Tutorial Do
Define Parsimonious Interfaces
e.g., use LSOCK to pass socket handles:
ACE_LSOCK_Stream stream;
ACE_LSOCK_Acceptor acceptor (/tmp/foo);
acceptor.accept (stream);
stream.send_handle (stream.get_handle ());
versus the less parsimonious BSD 4.3 socket code
ACE_LSOCK::send_handle
(const ACE_HANDLE sd) const {
u_char a[2]; iovec iov; msghdr send_msg;
a[0] = 0xab, a[1] = 0xcd;
iov.iov_base = (char *) a;
iov.iov_len = sizeof a;
send_msg.msg_iov = iov;
send_msg.msg_iovlen = 1;
send_msg.msg_name = (char *) 0;
send_msg.msg_namelen = 0;
send_msg.msg_accrights = (char *) sd;
send_msg.msg_accrightslen = sizeof sd;
return sendmsg (this-get_handle (),
send_msg, 0);
Note that SVR4 and BSD 4.4 APIs are different
than BSD 4.3!
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![Advanced ACE Tutorial Douglas C. Schmidt
The Reactor Pattern
Reactor select (handles); Intent
foreach h in handles loop
handle_events() table[h].handle_event(type)
register_handler(h) end loop Demuxes dispatches
remove_handler(h) handlers
1 Event Handler
requests that are
N
uses Handle owns handle_event(type) delievered concurrency
get_handle() to an application by one
notifies
or more clients
Synchronous Event Concrete
Demultiplexer Event
Forces Resolved
select() Handler
Serially demux events
www.cs.wustl.edu/˜schmidt/ synchronously
POSA/ efficiently
Extend applications
without changing
demuxing code
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The Logging_Handler and
Logging_Acceptor Classes
// Performs I/O with client logging daemons.
class Logging_Handler : public
ACE_Svc_HandlerACE_SOCK_Acceptor::PEER_STREAM,
// Trait!
ACE_NULL_SYNCH
{
public:
// Recv and process remote logging records.
virtual int handle_input (ACE_HANDLE);
};
// Logging_Handler factory.
class Logging_Acceptor : public
ACE_AcceptorLogging_Handler, ACE_SOCK_Acceptor
{
public:
// Dynamic linking hooks.
virtual int init (int argc, char *argv[]);
virtual int fini (void);
};
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Logging_Acceptor Initialization
and Termination
// Automatically called when a Logging_Acceptor
// object is linked dynamically.
Logging_Acceptor::init (int argc, char *argv[])
{
ACE_Get_Opt get_opt (argc, argv, p:, 0);
ACE_INET_Addr addr (DEFAULT_PORT);
for (int c; (c = get_opt ()) != -1; )
switch (c) {
case ’p’:
addr.set (atoi (getopt.optarg));
break;
default:
break;
}
// Initialize endpoint and register
// with the ACE_Reactor.
open (addr, ACE_Reactor::instance ());
}
// Automatically called when object is unlinked.
Logging_Acceptor::fini (void) { handle_close (); }
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![Advanced ACE Tutorial Douglas C. Schmidt
Service Configuration
The logging service is configured Generic event-loop to
via scripting in a svc.conf file: dynamically configure service
% cat ./svc.conf
daemons
# Dynamically configure int main (int argc, char *argv[])
# the logging service {
dynamic Logger // Initialize the daemon and
Service_Object * // configure services
logger:_make_Logger() -p 2001 ACE_Service_Config::open (argc,
# Note, .dll or .so suffix argv);
# added to the logger // Run forever, performing the
# automatically // configured services
ACE_Reactor::instance ()-
run_reactor_event_loop ();
/* NOTREACHED */
}
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Master Server Driver Function
// Thread function prototype.
typedef void *(*THR_FUNC)(void *);
int main (int argc, char *argv[]) {
parse_args (argc, argv);
// Queue client requests.
MESSAGE_QUEUE msg_queue;
// Spawn off NUM_THREADS to run in parallel.
for (int i = 0; i NUM_THREADS; i++)
thr_create (0, 0,
THR_FUNC (worker),
(void *) msg_queue,
THR_BOUND, 0);
// Initialize network device and
// recv HTTP work requests.
thr_create (0, 0, THR_FUNC (recv_requests),
(void *) msg_queue,
THR_BOUND, 0);
// Wait for all threads to exit (BEWARE)!
while (thr_join (0, t_id, (void **) 0) == 0)
continue; // ...
}
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![Advanced ACE Tutorial Douglas C. Schmidt
HTTP_Acceptor Class Interface
template class ACCEPTOR The HTTP_Acceptor
class HTTP_Acceptor : class implements the
public ACE_AcceptorHTTP_Handler
Acceptor role
ACCEPTOR::PEER_STREAM,
// Note use of a trait. i.e., it accepts
ACCEPTOR
{ connections/initializes
public: HTTP_Handlers
// Called when HTTP_Acceptor is
// dynamically linked.
virtual int init (int argc, char *argv[]);
// Called when HTTP_Acceptor is
// dynamically unlinked.
virtual int fini (void);
// ...
};
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HTTP_Acceptor Class Implementation
// Initialize service when dynamically linked.
template class PA int
HTTP_AcceptorPA::init (int argc, char *argv[])
{
Options::instance ()-parse_args (argc, argv);
// Initialize the communication endpoint and
// register to accept connections.
peer_acceptor ().open (typename
PA::PEER_ADDR (Options::instance ()-port ()),
Reactor::instance ());
}
// Terminate service when dynamically unlinked.
template class PA int
HTTP_AcceptorPA::fini (void)
{
// Shutdown threads in the pool.
HTTP_ProcessorPA::instance ()-
msg_queue ()-deactivate ();
// Wait for all threads to exit.
HTTP_ProcessorPA::instance ()-
thr_mgr ()-wait ();
}
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![Advanced ACE Tutorial Douglas C. Schmidt
Main Program for the Web Server
int main (int argc, char *argv[]) The main()
{ function is totally
// Initialize the daemon and generic!
// dynamically configure services.
ACE_Service_Config::open (argc, Dynamically
argv); configure execute
// Loop forever, running services Web Server
// and handling reconfigurations.
ACE_Reactor::instance ()- Make any
run_reactor_event_loop (); application
/* NOTREACHED */ “Web-enabled”
}
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ACE_Connector Class Implementation
// Initiate connection using specified
// blocking semantics.
template class SH, class PC int
ACE_ConnectorSH, PC::connect
(SH *sh,
const PC::PEER_ADDR r_addr,
ACE_Synch_Options options)
{
ACE_Time_Value *timeout = 0;
int use_reactor =
options[ACE_Synch_Options::USE_REACTOR];
if (use_reactor)
timeout = ACE_Time_Value::zero;
else
timeout =
options[ACE_Synch_Options::USE_TIMEOUT]
? (Time_Value *) options.timeout () : 0;
// Hook methods.
if (sh == 0)
sh = make_svc_handler ();
if (connect_svc_handler (sh, raddr,
timeout) != -1)
activate_svc_handler (sh);
}
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![Advanced ACE Tutorial Do
Peer_Message Schema
// Peer address is used to identify the
// source/destination of a Peer message.
class Peer_Addr {
public:
CONN_ID conn_id_; // Unique connection id.
u_char logical_id_; // Logical ID.
u_char payload_; // Payload type.
};
// Fixed sized header.
class Peer_Header { public: /* ... */ };
// Variable-sized message (sdu_ may be
// between 0 and MAX_MSG_SIZE).
class Peer_Message {
public:
// The maximum size of a message.
enum { MAX_PAYLOAD_SIZE = 1024 };
Peer_Header header_; // Fixed-sized header.
char sdu_[MAX_PAYLOAD_SIZE]; // Message payload.
};
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![Advanced ACE Tutorial Douglas C. Schmidt
Dynamically Configuring Gateway into an Application
Parameterized by proxy handler Example of the Component
template Configurator pattern
class SUPPLIER_HANDLER, int main (int argc, char *argv[])
class CONSUMER_HANDLER {
class Gateway // Initialize the daemon and
: public Service_Object // dynamically configure services.
{ ACE_Service_Config::open (argc,
public: argv);
// Perform initialization. // Run forever, performing the
virtual int init // configured services.
(int argc, char *argv[]); ACE_Reactor::instance ()-
run_reactor_event_loop ();
// Perform termination. /* NOTREACHED */
virtual int fini (void); }
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Main Driver Function for the Stream
Create Producer and Consumer Modules and
push them onto the Stream
int main (int argc, char *argv[])
{
// Control hierarchically-related
// active objects.
MT_Stream stream;
// All processing is performed in the
// Stream after push’s complete.
stream.push (new MT_Module
(Consumer, new Consumer);
stream.push (new MT_Module
(Producer, new Producer));
// Barrier synchronization: wait for
// the threads, to exit, then exit
// the main thread.
ACE_Thread_Manager::instance ()-wait ();
}
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Ace tutorial c
- 1. C++ Network Programmingwith Patterns, Frameworks, and ACE Douglas C. Schmidt Professor Department of EECS [email protected] Vanderbilt University www.cs.wustl.edu/schmidt/ (615) 343-8197 Sponsors NSF, DARPA, ATD, BBN, Boeing, Cisco, Comverse, GDIS, Experian, Global MT, Hughes, Kodak, Krones, Lockheed, Lucent, Microsoft, Mitre, Motorola, NASA, Nokia, Nortel, OCI, Oresis, OTI, QNX, Raytheon, SAIC, Siemens SCR, Siemens MED, Siemens ZT, Sprint, Telcordia, USENIX
- 2. Advanced ACE Tutorial Douglas C. Schmidt Roadmap to Levels of Middleware APPLICATIONS Observations DOMAIN-SPECIFIC MIDDLEWARE SERVICES HUD WTS AVIONICS REPLICATION – Historically, apps Nav SERVICE built atop OS COMMON MIDDLEWARE SERVICES EVENT CHANNEL Cons Cons – Today, apps built Cons atop middleware DISTRIBUTION MIDDLEWARE – Middleware has multiple layers HOST INFRASTRUCTURE MIDDLEWARE Just like network OPERATING SYSTEMS PROTOCOLS protocol stacks HARDWARE DEVICES www.cs.wustl.edu/˜schmidt/PDF/ middleware-chapter.pdf Vanderbilt University 1
- 3. Advanced ACE Tutorial Douglas C. Schmidt Motivation for Concurrency SERVER maxfdp1 Leverage hardware/software read_fds CLIENT WORK REQUEST – e.g., multi-processors and OS WORK WORK WORK thread support CLIENT REQUEST REQUEST REQUEST CLIENT CLIENT Increase performance (1) ITERATIVE SERVER – e.g., overlap computation and SERVER communication CLIENT Improve response-time WORK REQUEST – e.g., GUIs and network servers WORK WORK WORK CLIENT REQUEST REQUEST REQUEST Simplify program structure CLIENT CLIENT – e.g., sync vs. async (2) CONCURRENT SERVER Vanderbilt University 2
- 4. Advanced ACE Tutorial Douglas C. Schmidt Motivation for Distribution PRINTER Collaboration ! connectivity and COMPUTER interworking CD ROM FILE SYSTEM Performance ! multi-processing and locality ! (1) STAND-ALONE APPLICATION ARCHITECTURE Reliability and availability TIME NAME SERVICE SERVICE CYCLE replication SERVICE DISPLAY SERVICE Scalability and portability ! FI LE modularity NETWORK ! SERVICE PRINT Extensibility dynamic SERVICE configuration and reconfiguration CD ROM PRINTER FILE SYSTEM Cost effectiveness ! open (2) DISTRIBUTED APPLICATION ARCHITECTURE systems and resource sharing Vanderbilt University 3
- 5. Advanced ACE Tutorial Douglas C. Schmidt Challenges and Solutions Developing efficient, robust, and extensible concurrent networking applications is hard – e.g., must address complex topics that are less problematic or not relevant for non-concurrent, stand-alone applications OO techniques and OO language features help to enhance software quality factors – Key OO techniques include patterns and frameworks – Key OO language features include classes, inheritance, dynamic binding, and parameterized types – Key software quality factors include modularity, extensibility, portability, reusability, and correctness Vanderbilt University 4
- 6. Advanced ACE Tutorial Douglas C. Schmidt Caveats OO is not a panacea – Though when used properly it helps minimize “accidental” complexity and improve software quality factors It’s also essential to understand advanced OS features to enhance functionality and performance, e.g., – Multi-threading – Multi-processing – Synchronization – Shared memory – Explicit dynamic linking – Communication protocols and IPC mechanisms Vanderbilt University 5
- 7. Advanced ACE Tutorial Douglas C. Schmidt Tutorial Outline Brief overview of key OO networking and concurrency concepts and OS platform mechanisms – Emphasis is on practical solutions Examine a range of examples in detail – Networked Logging Service – Concurrent Web Server – Application-level Telecom Gateway – Call Center Manager Event Server Discuss general concurrent programming strategies Provide URLs for further reading on the topic Vanderbilt University 6
- 8. Advanced ACE Tutorial Douglas C. Schmidt Software Development Environment The topics discussed here are largely independent of OS, network, and programming language – Currently used successfully on UNIX/POSIX, Windows, and RTOS platforms, running on TCP/IP networks using C++ Examples are illustrated using freely available ADAPTIVE Communication Environment (ACE) OO framework components – Although ACE is written in C++, the principles covered in this tutorial apply to other OO languages – e.g., Java, Eiffel, Smalltalk, etc. In addition, other networks and backplanes can be used, as well Vanderbilt University 7
- 9. Advanced ACE Tutorial Douglas C. Schmidt Sources of Complexity PRINTER Inherent complexity COMPUTER – Latency FILE CD ROM SYSTEM – Reliability – Synchronization (1) STAND-ALONE APPLICATION ARCHITECTURE – Deadlock TIME NAME CYCLE SERVICE SERVICE SERVICE Accidental Complexity DISPLAY SERVICE – Low-level APIs FI LE NETWORK SERVICE – Poor debugging tools PRINT – Algorithmic SERVICE CD ROM decomposition PRINTER FILE SYSTEM – Continuous (2) DISTRIBUTED APPLICATION ARCHITECTURE re-invention Vanderbilt University 8
- 10. Advanced ACE Tutorial Douglas C. Schmidt Sources of Inherent Complexity Inherent complexity results from fundamental domain challenges, e.g.: Concurrent programming Distributed programming Eliminating “race conditions” Addressing the impact of latency Deadlock avoidance Fault tolerance and high availability Fair scheduling Load balancing and service partitioning Performance optimization and tuning Consistent ordering of distributed events Vanderbilt University 9
- 11. Advanced ACE Tutorial Douglas C. Schmidt Sources of Accidental Complexity Accidental complexity results from limitations with tools and techniques used to develop concurrent applications, e.g., Lack of portable, reentrant, type-safe and extensible system call interfaces and component libraries Inadequate debugging support and lack of concurrent and distributed program analysis tools Widespread use of algorithmic decomposition – Fine for explaining concurrent programming concepts and algorithms but inadequate for developing large-scale concurrent network applications Continuous rediscovery and reinvention of core concepts and components Vanderbilt University 10
- 12. Advanced ACE Tutorial Douglas C. Schmidt OO Contributions to Concurrent and Distributed Applications Concurrent network programming is Patterns and frameworks elevate traditionally performed using development level to focus on low-level OS mechanisms, e.g., application concerns, e.g., fork/exec Service functionality and policies Shared memory and semaphores Service configuration Memory-mapped files Concurrent event Signals demultiplexing and event sockets/select handler dispatching Low-level thread APIs Service concurrency and synchronization Vanderbilt University 11
- 13. Advanced ACE Tutorial Douglas C. Schmidt Overview of Patterns Patterns represent solutions to problems that arise when developing software within a particular context – i.e., “Patterns == problem/solution pairs within a context” Patterns capture the static and dynamic structure and collaboration among key participants in software designs – They are particularly useful for articulating how and why to resolve non-functional forces Patterns facilitate reuse of successful software architectures and designs Vanderbilt University 12
- 14. Advanced ACE Tutorial Do Example: the Proxy Pattern 1: METHOD CALL : BROKER : QUOTER PROXY 4: METHOD RETURN 2: FORWARD REQUEST 3: RESPONSE CLIENT NETWORK : QUOTER SERVER Intent: Provide a surrogate for another object that controls access to it Vanderbilt University
- 15. Advanced ACE Tutorial Douglas C. Schmidt Overview of Frameworks and Components A framework is: – “An integrated collection of components that collaborate to produce a reusable architecture for a family of related applications” Frameworks differ from conventional class libraries: 1. Frameworks are “semi-complete” applications 2. Frameworks address a particular application domain 3. Frameworks provide “inversion of control” Frameworks facilitate reuse of successful networked application software designs and implementations – Applications inherit from and instantiate framework components Vanderbilt University 14
- 16. Advanced ACE Tutorial Douglas C. Schmidt Class Libraries versus Frameworks APPLICATION- LOCAL SPECIFIC INVOCATIONS MATH CLASSES Key distinctions FUNCTIONALITY ADT CLASSES DATABASE Class libraries CLASSES EVENT GLUE GUI NETWORK – Reusable building blocks CLASSES CODE IPC LOOP CLASSES – Domain-independent (A) CLASS LIBRARY ARCHITECTURE – Limited in scope – Passive EVENT NETWORKING LOOP ADT Frameworks CLASSES GUI APPLICATION- CALL – Reusable, “semi-complete” INVOKES SPECIFIC BACKS FUNCTIONALITY EVENT applications MATH LOOP CLASSES – Domain-specific DATABASE EVENT LOOP – Broader in scope (B) FRAMEWORK ARCHITECTURE – Active Vanderbilt University 15
- 17. Advanced ACE Tutorial Douglas C. Schmidt The ADAPTIVE Communication Environment (ACE) - JAWS ADAPTIVE STANDARDS BASED MIDDLEWARE NETWORKED WEB SERVER THE ACE ORB SERVICE TOKEN GATEWAY COMPONENTS (TAO) SERVER SERVER LAYER LOGGING NAME TIME SERVER SERVER SERVER SERVICE CORBA FRAMEWORK ACCEPTOR CONNECTOR HANDLER HANDLER LAYER PROCESS/ STREAMS C++ THREAD LOG SHARED SERVICE WRAPPER MANAGERS MSG CONFIG- MALLOC FACADE REACTOR/ PROACTOR URATOR LAYER SYNCH SPIPE SOCK SAP/ FIFO MEM FILE WRAPPERS SAP TLI SAP SAP MAP SAP OS ADAPTATION LAYER C PROCESSES/ WIN32 NAMED SOCKETS/ UNIX SELECT/ DYNAMIC SHARED FILE SYS PIPES UNIX APIS THREADS STREAM PIPES TLI FIFOS IO COMP LINKING MEMORY APIS PROCESS/THREAD COMMUNICATION VIRTUAL MEMORY FILE SUBSYSTEM SUBSYSTEM SUBSYSTEM GENERAL OPERATING SYSTEM SERVICES www.cs.wustl.edu/˜schmidt/ACE.html Vanderbilt University 16
- 18. Advanced ACE Tutorial Douglas C. Schmidt ACE Statistics ACE library contains 250,000 Currently used by lines of C++ dozens of companies – Over 40 person-years of effort – Bellcore, BBN, Boeing, Ericsson, Ported to UNIX, Windows, MVS, and Hughes, Kodak, RT/embedded platforms Lockheed, Lucent, – e.g., VxWorks, LynxOS, Chorus Motorola, Nokia, Nortel, Raytheon, Large user and open-source SAIC, Siemens, etc. developer community Supported commercially – ˜schmidt/ACE-users.html by Riverace – www.riverace.com Vanderbilt University 17
- 19. Advanced ACE Tutorial Douglas C. Schmidt The Key Frameworks in ACE Acceptor- Reactor Proactor Connector Service Streams Task Configurator ACE contains a number of frameworks that can be used separately or together This design permits fine-grained subsetting of ACE components – Subsetting helps minimize ACE’s memory footprint – $ACE_ROOT/doc/ACE-subsets.html Vanderbilt University 18
- 20. Advanced ACE Tutorial Douglas C. Schmidt Patterns for Communication Middleware Thread-per Active Acceptor Observation Request Object Thread-per Connector Failures rarely result from Session Half-Sync/ unknown scientific Thread Half-Async Component Pool Configurator principles, but from failing Thread Leader/ to apply proven Object Specific Followers Lifetime engineering practices and Storage Manager patterns Asynchronous Reactor Completion Double Thread- Token Checked Safe Benefits of Patterns Proactor Locking Interface Facilitate design reuse External Wrapper Scoped Strategized Polymorphism Facade Locking Locking Preserve crucial design Concurrency Event Initialization Synchronization information Patterns Patterns Patterns Patterns Guide design choices Vanderbilt University 19
- 21. Advanced ACE Tutorial Douglas C. Schmidt The ACE ORB (TAO) in args TAO Overview ! CLIENT OBJ operation() OBJECT REF out args + return (SERVANT) A real-time, value high-performance IDL SKELETON REAL-TIME ORB IDL ORB RUN-TIME STUBS SCHEDULER OBJECT ADAPTER Leverages ACE REAL-TIME ORB CORE – Runs on POSIX, IOP IOP PLUGGABLE ORB XPORT PLUGGABLE ORB XPORT Windows, PROTOCOLS PROTOCOLS RTOSs OS KERNEL OS KERNEL REAL-TIME I/O ACE REAL-TIME I/O Related efforts ! SUBSYSTEM COMPONENTS SUBSYSTEM HIGH-SPEED HIGH-SPEED NETWORK INTERFACE NETWORK NETWORK INTERFACE QuO at BBN MIC/GME at www.cs.wustl.edu/˜schmidt/TAO. Vanderbilt html XOTS Vanderbilt University 20
- 22. Advanced ACE Tutorial Douglas C. Schmidt TAO Statistics TAO order of magnitude 50 person-years of effort – Core ORB 300,000 LOC Currently used by many – IDL compiler 200,000 companies LOC – e.g., Boeing, BBN, Lockheed, – CORBA Object Services Lucent, Motorola, Raytheon, 250,000 LOC SAIC, Siemens, etc. – Leverages ACE heavily Supported commercially by OCI Ported to UNIX, Windows, and PrismTech RT/embedded platforms – www.ociweb.com – e.g., VxWorks, LynxOS, – www.prismtechnologies.com Chorus, WinCE Vanderbilt University 21
- 23. Advanced ACE Tutorial Douglas C. Schmidt JAWS Adaptive Web Server 1: GET ~schmidt JAWS Overview WWW HTTP/1.0 WWW CLIENT SERVER 2: index.html – A high-performance PROTOCOL HANDLERS Web server HTML PARSER Flexible concurrency GUI DISPATCHER and dispatching REQUESTER mechanisms – Leverages the ACE COMMUNICATION PROTOCOL GRAPHICS ADAPTER (E.G., HTTP) framework Ported to most OS OS KERNEL OS KERNEL platforms OS I/O SUBSYSTEM OS I/O SUBSYSTEM – Used commercially by NETWORK ADAPTERS NETWORK ADAPTERS CacheFlow NETWORK www.cacheflow.com www.cs.wustl.edu/˜jxh/ research/ Vanderbilt University 22
- 24. Advanced ACE Tutorial Douglas C. Schmidt Java ACE DISTRIBUTED SERVICES AND Java ACE TOKEN LOGGING NAME TIME COMPONENTS SERVER SERVER SERVER SERVER Overview FRAMEWORKS AND CLASS SERVICE A Java version ACCEPTOR CONNECTOR CATEGORIES HANDLER of ACE ADAPTIVE SERVICE EXECUTIVE (ASX) JAVA SYNCH SOCK_SAP THREAD LOG TIMER SERVICE – Used for WRAPPERS WRAPPERS MANAGER CONFIGURATOR MSG QUEUE medical JAVA VIRTUAL MACHINE (JVM) imaging www.cs.wustl.edu/˜schmidt/JACE.html prototype www.cs.wustl.edu/˜schmidt/C++2java. html www.cs.wustl.edu/˜schmidt/PDF/ MedJava.pdf Vanderbilt University 23
- 25. Advanced ACE Tutorial Do Networked Logging Service PRINTER P1 CONSOLE CLIENT LOCAL IPC LOGGING P2 DAEMON RE MO P3 TE IPC SERVER LOGGING DAEMON HA A OST B HOST B HOST CLIENT SERVER A IPC TE MO NETWORK RE STORAGE DEVICE HOST P1 CLIENT B CLIENT LOCAL IPC LOGGING P2 DAEMON P3 Intent: Server logging daemon collects, formats, and outputs logging records forwarded from client logging daemons residing throughout a network or Internet Vanderbilt University
- 26. Advanced ACE Tutorial Douglas C. Schmidt Networked Logging Service Programming API The logging API is similar to printf(), e.g.: ACE_ERROR ((LM_ERROR, (%t) fork failed)); Generates on logging server host: Oct 31 14:50:13 [email protected]@2766@LM_ERROR@client ::(4) fork failed and ACE_DEBUG ((LM_DEBUG, (%t) sending to server %s, server_host)); generates on logging server host: Oct 31 14:50:28 [email protected]@18352@LM_DEBUG@drwho ::(6) sending to server bastille Vanderbilt University 25
- 27. Advanced ACE Tutorial Douglas C. Schmidt Conventional Logging Server Design Typical algorithmic pseudo-code for The “grand mistake:” networked logging server: Avoid the temptation to void logging_server (void) { “step-wise refine” this initialize acceptor endpoint algorithmically decomposed loop forever { pseudo-code directly into wait for events the detailed design and handle data events implementation of the handle connection events logging server! } } Vanderbilt University 26
- 28. Advanced ACE Tutorial Do The select()-based Logging Server Implementation SERVER SERVER LOGGING DAEMON maxhandlep1 acceptor NETWORK read_handles CLIENT CONNECTION REQUEST LOGGING RECORDS LOGGING LOGGING RECORDS RECORDS CLIENT CLIENT CLIENT Serializes server processing at select() demuxing level Vanderbilt University
- 29. Advanced ACE Tutorial Do Conventional Logging Server Implementation Note the excessive amount of detail required to program at the socket level... // Main program static const int PORT = 10000; typedef u_long COUNTER; typedef int HANDLE; // Counts the # of logging records processed static COUNTER request_count; // Acceptor-mode socket handle static HANDLE acceptor; // Highest active handle number, plus 1 static HANDLE maxhp1; // Set of currently active handles static fd_set activity_handles; // Scratch copy of activity_handles static fd_set ready_handles; Vanderbilt University
- 30. Advanced ACE Tutorial Do Main Event Loop of Logging Server int main (int argc, char *argv[]) { initialize_acceptor (argc 1 ? atoi (argv[1]) : PORT); // Loop forever performing logging // server processing. for (;;) { // struct assignment. ready_handles = activity_handles; // Wait for client I/O events. select (maxhp1, ready_handles, 0, 0, 0); // First receive pending logging records. handle_data (); // Then accept pending connections. handle_connections (); } } Vanderbilt University
- 31. Advanced ACE Tutorial Do Initialize Acceptor Socket static void initialize_acceptor (u_short port) { struct sockaddr_in saddr; // Create a local endpoint of communication. acceptor = socket (PF_INET, SOCK_STREAM, 0); // Set up the address info. to become server. memset ((void *) saddr, 0, sizeof saddr); saddr.sin_family = AF_INET; saddr.sin_port = htons (port); saddr.sin_addr.s_addr = htonl (INADDR_ANY); // Associate address with endpoint bind (acceptor, (struct sockaddr *) saddr, sizeof saddr); // Make endpoint listen for connection requests. listen (acceptor, 5); // Initialize handle sets. FD_ZERO (ready_handles); FD_ZERO (activity_handles); FD_SET (acceptor, activity_handles); maxhp1 = acceptor + 1; } Vanderbilt University
- 32. Advanced ACE Tutorial Do Handle Data Processing static void handle_data (void) { // acceptor + 1 is the lowest client handle for (HANDLE h = acceptor + 1; h maxhp1; h++) if (FD_ISSET (h, ready_handles)) { ssize_t n = handle_log_record (h, 1); // Guaranteed not to block in this case! if (n 0) ++request_count; // Count the # of logging records else if (n == 0) { // Handle connection shutdown. FD_CLR (h, activity_handles); close (h); if (h + 1 == maxhp1) { // Skip past unused handles while (!FD_ISSET (--h, activity_handles)) continue; maxhp1 = h + 1; } } } } Vanderbilt University
- 33. Advanced ACE Tutorial Do Receive and Process Logging Records static ssize_t handle_log_record (HANDLE in_h, HANDLE out_h) { ssize_t n; size_t len; Log_Record lr; // The first recv reads the length (stored as a // fixed-size integer) of adjacent logging record. n = recv (in_h, (char *) len, sizeof len, 0); if (n = 0) return n; len = ntohl (len); // Convert byte-ordering // The second recv then reads len bytes to // obtain the actual record. for (size_t nread = 0; nread len; nread += n n = recv (in_h, ((char *) lr) + nread, len - nread, 0); // Decode and print record. decode_log_record (lr); if (write (out_h, lr.buf, lr.size) == -1) return -1; else return 0; } Vanderbilt University
- 34. Advanced ACE Tutorial Do Handle Connection Acceptance static void handle_connections (void) { if (FD_ISSET (acceptor, ready_handles)) { static struct timeval poll_tv = {0, 0}; HANDLE h; // Handle all pending connection requests // (note use of select’s polling feature) do { // Beware of subtle bug(s) here... h = accept (acceptor, 0, 0); FD_SET (h, activity_handles); // Grow max. socket handle if necessary. if (h = maxhp1) maxhp1 = h + 1; } while (select (acceptor + 1, ready_handles, 0, 0, poll_tv) == 1); } Vanderbilt University
- 35. Advanced ACE Tutorial Do Conventional Client Logging Daemon Implementation The main() method receives logging records from client applications and forwards them on to the logging server int main (int argc, char *argv[]) { HANDLE stream = initialize_stream_endpoint (argc 1 ? atoi (argv[1]) : PORT); Log_Record lr; // Loop forever performing client // logging daemon processing. for (;;) { // ... get logging records from client // application processes ... size_t size = htonl (lr.size); send (stream, size, sizeof size); encode_log_record (lr); send (stream, ((char *) lr), sizeof lr); } } Vanderbilt University
- 36. Advanced ACE Tutorial Do Client Connection Establishment static HANDLE initialize_stream_endpoint (const char *host, u_short port) { struct sockaddr_in saddr; // Create a local endpoint of communication. HANDLE stream = socket (PF_INET, SOCK_STREAM, 0); // Set up the address info. to become client. memset ((void *) saddr, 0, sizeof saddr); saddr.sin_family = AF_INET; saddr.sin_port = htons (port); hostent *hp = gethostbyname (host); memcpy ((void *) saddr, htonl (hp-h_addr), hp-h_length); // Associate address with endpoint connect (stream, (struct sockaddr *) saddr, sizeof saddr); return stream; } Vanderbilt University
- 37. Advanced ACE Tutorial Douglas C. Schmidt Limitations with Algorithmic Decomposition Algorithmic decomposition tightly couples application-specific functionality and the following configuration-related characteristics: Application Structure – The number of services per process – Time when services are configured into a process Communication and Demultiplexing Mechanisms – The underlying IPC mechanisms that communicate with other participating clients and servers – Event demultiplexing and event handler dispatching mechanisms Concurrency and Synchronization Model – The process and/or thread architecture that executes service(s) at run-time Vanderbilt University 36
- 38. Advanced ACE Tutorial Douglas C. Schmidt Overcoming Limitations via OO The algorithmic decomposition illustrated above specifies many low-level details – Moreover, the excessive coupling impedes reusability, extensibility, and portability... In contrast, OO focuses on application-specific behavior, e.g., int Logging_Handler::handle_input (void) { ssize_t n = handle_log_record (peer ().get_handle (), ACE_STDOUT); if (n 0) ++request_count; // Count the # of logging records return n = 0 ? -1 : 0; } Vanderbilt University 37
- 39. Advanced ACE Tutorial Douglas C. Schmidt OO Contributions to Software Patterns facilitate the large-scale reuse of software architecture – Even when reuse of algorithms, detailed designs, and implementations is not feasible Frameworks achieve large-scale design and code reuse – In contrast, traditional techniques focus on the functions and algorithms that solve particular requirements Note that patterns and frameworks are not unique to OO! – However, objects and classes are useful abstraction mechanisms Vanderbilt University 38
- 40. Advanced ACE Tutorial Douglas C. Schmidt Patterns in the Networked Logging Server Active Object Component Configurator Acceptor STRATEGIC Reactor PATTERNS TACTICAL PATTERNS Adapter Iterator Factory Template Wrapper Method Method Facade Strategic and tactical are relative to the context and abstraction level Vanderbilt University 39
- 41. Advanced ACE Tutorial Douglas C. Schmidt Summary of Pattern Intents Wrapper Facade ! “Encapsulates the functions and data provided by existing non-OO APIs within more concise, robust, portable, maintainable, and cohesive OO class interfaces” Reactor ! “Demultiplexes and dispatches requests that are delivered concurrently to an application by one or more clients” Acceptor ! “Decouple the passive connection and initialization of a peer service in a distributed system from the processing performed once the peer service is connected and initialized” Component Configurator ! “Decouples the implementation of services from the time when they are configured” Active Object ! “Decouples method execution from method invocation to enhance concurrency and simplify synchronized access to an object that resides in its own thread of control” Vanderbilt University 40
- 42. Advanced ACE Tutorial Douglas C. Schmidt Components in the OO Logging Server Application-specific components – Process logging records received from clients Connection-oriented application components – ACE_Svc_Handler (service handler) Performs I/O-related tasks with clients – ACE_Acceptor factory Passively accepts connection requests Dynamically creates a service handler for each client and “activates” it Application-independent ACE framework components – Perform IPC, explicit dynamic linking, event demultiplexing, event handler dispatching, multi-threading, etc. Vanderbilt University 41
- 43. Advanced ACE Tutorial Do Class Diagram for OO Logging Server Logging_Handler SOCK_Stream COMPONENTS APPLICATION- SOCK_Acceptor Null_Synch SPECIFIC 1 Logging n Logging Acceptor activates Handler COMPONENTS CONNECTION- SVC_HANDLER PEER_STREAM ORIENTED PEER_ACCEPTOR SYNCH_STRAT Acceptor Svc Handler PEER PEER ACCEPTOR STREAM COMPONENTS FRAMEWORK Stream Connection ACE IPC_SAP Service Configurator Concurrency Reactor Vanderbilt University
- 44. Advanced ACE Tutorial Douglas C. Schmidt Addressing Robustness, Portability, and Maintainability Challenges Problem – Building distributed applications using low-level APIs is hard Forces – Low-level APIs are verbose, tedious, and error-prone to program – Low-level APIs are non-portable and non-maintainable Solution – Apply the Wrapper Facade pattern to encapsulate low-level functions and data structures Vanderbilt University 43
- 45. Advanced ACE Tutorial Douglas C. Schmidt The Wrapper Facade Pattern Intent client Encapsulates the functions and data provided by 1: method_k() existing lower-level, Wrapper 2: function_k() non-OO APIs within more Facade concise, robust, portable, Functions method_1() function_1() maintainable, and cohesive ... ... higher-level OO class method_m() function_n() interfaces POSA2 (www.cs.wustl.edu/ ˜schmidt/POSA/) Forces Resolved Avoid tedious, error-prone, and non-portable system APIs Create cohesive abstractions Vanderbilt University 44
- 46. Advanced ACE Tutorial Do Motivating the Wrapper Facade Pattern: the Socket API 2: ACTIVE ROLE 1: PASSIVE ROLE socket() bind() socket() (optional) bind() connect() listen() send()/recv() accept() 3: SERVICE send()/recv() close() PROCESSING close() CLIENT NETWORK SERVER Sockets are the most common network programming API and are available on most OS platforms Vanderbilt University
- 47. Advanced ACE Tutorial Douglas C. Schmidt Problem with Sockets: Lack of Type-safety int buggy_echo_server (u_short port_num) I/O handles are { // Error checking omitted. not amenable to sockaddr_in s_addr; strong type int acceptor = socket (PF_UNIX, SOCK_DGRAM, 0); checking at s_addr.sin_family = AF_INET; compile-time s_addr.sin_port = port_num; s_addr.sin_addr.s_addr = INADDR_ANY; The adjacent bind (acceptor, (sockaddr *) s_addr, code contains sizeof s_addr); many subtle, int handle = accept (acceptor, 0, 0); for (;;) { common bugs char buf[BUFSIZ]; ssize_t n = read (acceptor, buf, sizeof buf); if (n = 0) break; write (handle, buf, n); } } Vanderbilt University 46
- 48. Advanced ACE Tutorial Douglas C. Schmidt Problem with Sockets: Steep Learning Curve Many socket/TLI API functions have complex semantics, e.g.: Multiple protocol families and address families – e.g., TCP, UNIX domain, OSI, XNS, etc. Infrequently used features, e.g.: – Broadcasting/multicasting – Passing open file handles – Urgent data delivery and reception – Asynch I/O, non-blocking I/O, I/O-based and timer-based event multiplexing Vanderbilt University 47
- 49. Advanced ACE Tutorial Douglas C. Schmidt Problem with Sockets: Portability Having multiple “standards,” i.e., sockets and TLI, makes portability difficult, e.g., – May require conditional compilation – In addition, related functions are not included in POSIX standards e.g., select(), WaitForMultipleObjects(), and poll() Portability between UNIX and Windows Sockets is problematic, e.g.: – Header files – Error numbers – Handle vs. descriptor types – Shutdown semantics – I/O controls and socket options Vanderbilt University 48
- 50. Advanced ACE Tutorial Douglas C. Schmidt Problem with Sockets: Poorly Structured gethostbyname() getservbyname() getsockname() getpeername() getsockopt() setsockopt() sendmsg() recvfrom() recvmsg() connect() sendto() socket() accept() writev() readv() listen() write() send() bind() read() recv() Limitations Socket API is linear rather than hierarchical There is no consistency among names... Non-portable Vanderbilt University 49
- 51. Advanced ACE Tutorial Do Socket Taxonomy N/ COMMUNICATION DOMAIN TIO ION EC AT E LOCAL LOCAL/REMOTE NN NIC SIV CO MU LE PA S M RO CO ER XF IVE A CT TYPE OF COMMUNICATION SERVICE socket(PF_UNIX)/bind() socket(PF_INET)/bind() GRAM DATA sendto()/recvfrom() sendto()/recvfrom() socket(PF_UNIX)/bind() socket(PF_INET)/bind() STREAM CONNECTED DATAGRAM socket(PF_UNIX) socket(PF_INET) bind()/connect() bind()/connect() send()/recv() send()/recv() socket(PF_UNIX) socket(PF_INET) bind()/connect() bind()/connect() socket(PF_UNIX) socket(PF_INET) bind()/listen()/accept() bind()/listen()/accept() send()/recv() send()/recv() socket(PF_UNIX) socket(PF_INET) bind()/connect() bind()/connect() The Socket API can be classified along three dimensions 1. Connection role 2. Communication domain 3. Type of service Vanderbilt University
- 52. Advanced ACE Tutorial Do Solution: ACE Socket Wrapper Facades N/ COMMUNICATION DOMAIN IO ION CT T LOCAL/REMOTE NE ICA E LOCAL N N SIV CO MU LE PA S M RO ER CO XF E TIV AC TYPE OF COMMUNICATION SERVICE m gra ram _D GRAM CK DATA _Dg SO OCK SOCK_Dgram_Bcast LS SOCK_Dgram_Mcast CONNECTED DATAGRAM LSOCK_Dgram SOCK_Dgram LSOCK_CODgram SOCK_CODgram LSOCK_Acceptor SOCK_Acceptor STREAM LSOCK_Stream SOCK_Stream LSOCK_Connector SOCK_Connector The ACE C++ wrapper facades more explicitly model the key socket components using OO classes Vanderbilt University
- 53. Advanced ACE Tutorial Do The ACE Connection-Oriented Socket Wrapper Facades ACE_IPC_SAP ACE_Addr ACE_SOCK_IO ACE_SOCK ACE_SOCK_Acceptor ACE_INET_Addr ACE_SOCK_Stream ACE_SOCK_Connector Participants Passive and active connection factories – ACE_SOCK_Acceptor and ACE_SOCK_Connector Streaming classes – ACE_SOCK_Stream and ACE_SOCK_IO Addressing classes – ACE_Addr and ACE_INET_Addr Vanderbilt University
- 54. Advanced ACE Tutorial Douglas C. Schmidt The ACE Connection-Oriented Socket Wrapper Facade Factories class ACE_SOCK_Connector class ACE_SOCK_Acceptor { : public ACE_SOCK public: { // Traits public: typedef ACE_INET_Addr PEER_ADDR; // Traits typedef ACE_SOCK_Stream PEER_STREAM; typedef ACE_INET_Addr PEER_ADDR; typedef ACE_SOCK_Stream PEER_STREAM; int connect (ACE_SOCK_Stream new_sap, ACE_SOCK_Acceptor (const ACE_INET_Addr ); const ACE_INET_Addr raddr, int open (const ACE_INET_Addr addr); ACE_Time_Value *timeout, int accept const ACE_INET_Addr laddr); (ACE_SOCK_Stream new_sap, // ... ACE_INET_Addr *, }; ACE_Time_Value *); //... }; Vanderbilt University 53
- 55. Advanced ACE Tutorial Douglas C. Schmidt ACE Connection-Oriented Socket Wrapper Facade Streaming and Addressing Classes class ACE_SOCK_Stream class ACE_INET_Addr : public ACE_SOCK { : public ACE_Addr public: { // Trait. public: typedef ACE_INET_Addr PEER_ADDR; ACE_INET_Addr (u_short port, ssize_t send (const void *buf, const char host[]); int n); u_short get_port_number (void); ssize_t recv (void *buf, ACE_UINT_32 get_ip_addr (void); int n); // ... ssize_t send_n (const void *buf, }; int n); ssize_t sendv_n (const iovec *iov, int n); ssize_t recv_n (void *buf, int n); int close (void); // ... }; Vanderbilt University 54
- 56. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Motivating the Socket Wrapper Facade Structure Q: Why decouple the ACE_SOCK_Acceptor and the ACE_SOCK_Connector from ACE_SOCK_Stream? A: For the same reasons that ACE_Acceptor and ACE_Connector are decoupled from ACE_Svc_Handler, e.g., – An ACE_SOCK_Stream is only responsible for data transfer Regardless of whether the connection is established passively or actively – This ensures that the ACE_SOCK* components aren’t used incorrectly... e.g., you can’t accidentally read() or write() on ACE_SOCK_Connectors or ACE_SOCK_Acceptors, etc. Vanderbilt University 55
- 57. Advanced ACE Tutorial Do An Echo Server Written using ACE C++ Socket Wrapper Facades int echo_server (u_short port_num) { // Local server address. ACE_INET_Addr my_addr (port_num); // Initialize the acceptor mode server. ACE_SOCK_Acceptor acceptor (my_addr); // Data transfer object. ACE_SOCK_Stream new_stream; // Accept a new connection. acceptor.accept (new_stream); for (;;) { char buf[BUFSIZ]; // Error caught at compile time! ssize_t n = acceptor.recv (buf, sizeof buf); new_stream.send_n (buf, n); } } Vanderbilt University
- 58. Advanced ACE Tutorial Do A Generic Version of the Echo Server template class ACCEPTOR int echo_server (u_short port) { // Local server address (note traits). typename ACCEPTOR::PEER_ADDR my_addr (port); // Initialize the acceptor mode server. ACCEPTOR acceptor (my_addr); // Data transfer object (note traits). typename ACCEPTOR::PEER_STREAM stream; // Accept a new connection. acceptor.accept (stream); for (;;) { char buf[BUFSIZ]; ssize_t n = stream.recv (buf, sizeof buf); stream.send_n (buf, n); } } Vanderbilt University
- 59. Advanced ACE Tutorial Douglas C. Schmidt Scope of the ACE IPC Wrapper Facades A ACE IPC SAP SOCK TLI SPIPE SSL FIFO MEM SysV SAP SAP SAP SAP SAP SAP IPC SOCKET TLI STREAM SSL NAMED MMAP SYSTEM V API API PIPE API API PIPE API API IPC API C++NPv1 (www.cs.wustl.edu/˜schmidt/ACE/book1/) Vanderbilt University 58
- 60. Advanced ACE Tutorial Do Using the Wrapper Facade Pattern for the Logging Server Note we haven’t improved the overall design (yet) // ... Same as before ... // Acceptor-mode socket handle. static ACE_SOCK_Acceptor acceptor; // Set of currently active handles static ACE_Handle_Set activity_handles; // Scratch copy of activity_handles static ACE_Handle_Set ready_handles; static void initialize_acceptor (u_short port) { // Set up address info. to become server. ACE_INET_Addr saddr (port); // Create a local endpoint of communication. acceptor.open (saddr); // Set the SOCK_Acceptor into non-blocking mode. acceptor.enable (ACE_NONBLOCK); activity_handles.set_bit (acceptor.get_handle ()); } Vanderbilt University
- 61. Advanced ACE Tutorial Do Main Event Loop of Logging Server int main (int argc, char *argv[]) { initialize_acceptor (argc 1 ? atoi (argv[1]) : PORT); // Loop forever performing logging // server processing. for (;;) { // object assignment. ready_handles = activity_handles; // Wait for client I/O events. ACE::select (int (maxhp1), // calls operator fd_set *(). ready_handles); // First receive pending logging records. handle_data (); // Then accept pending connections. handle_connections (); } } Vanderbilt University
- 62. Advanced ACE Tutorial Do Handling Connections and Data Processing static void handle_connections (void) { if (ready_handles.is_set (acceptor.get_handle ())) ACE_SOCK_Stream str; // Handle all pending connection requests. while (acceptor.accept (str) != -1) activity_handles.set_bit (str.get_handle ()); } } static void handle_data (void) { ACE_HANDLE h; ACE_Handle_Set_Iterator iter (ready_handles); while ((h = iter ()) != ACE_INVALID_HANDLE) { ACE_SOCK_Stream str (h); ssize_t n = handle_log_record (str, ACE_STDOUT); if (n 0) // Count # of logging records. ++request_count; else if (n == 0) { // Handle connection shutdown. activity_handles.clr_bit (h); s.close (); } } Vanderbilt University
- 63. Advanced ACE Tutorial Do Receive and Process Logging Records static ssize_t handle_log_record (ACE_SOCK_Stream s, ACE_HANDLE out_h) ACE_UINT_32 len; ACE_Log_Record lr; // The first recv reads the length (stored as a // fixed-size integer) of adjacent logging record. ssize_t n = s.recv_n ((char *) len, sizeof len); if (n = 0) return n; len = ntohl (len); // Convert byte-ordering // Perform sanity check! if (len sizeof (lr)) return -1; // The second recv then reads len bytes to // obtain the actual record. s.recv_n ((char *) lr, sizeof lr); // Decode and print record. decode_log_record (lr); if (ACE_OS::write (out_h, lr.buf, lr.size) == -1) return -1; else return 0; } Vanderbilt University
- 64. Advanced ACE Tutorial Do OO Client Logging Daemon Implementation int main (int argc, char *argv[]) { ACE_SOCK_Stream stream; ACE_SOCK_Connector con; // Establish connection. con.connect (stream, ACE_INET_Addr (argc 1 ? atoi (argv[1]) : PORT)); ACE_Log_Record lr; // Loop forever performing client // logging daemon processing. for (;;) { // ... get logging records from client // application processes ... ACE_UINT_32 size = lr.size; lr.size = htonl (lr.size); encode_log_record (lr); iovec iov[2]; iov[0].iov_len = sizeof (ACE_UINT_32); iov[0].iov_base = lr.size; iov[1].iov_len = size; iov[1].iov_base = lr; // Uses writev(2); stream.sendv_n (iov, 2); } } Vanderbilt University
- 65. Advanced ACE Tutorial Douglas C. Schmidt Evaluating the Wrapper Facade Solution Benefits Liabilities More concise Potentially more indirection More robust Additional learning curve More portable Still haven’t solved the overall design problem More maintainable – i.e., the overall design is More efficient still based on step-wise refinement of functions Vanderbilt University 64
- 66. Advanced ACE Tutorial Douglas C. Schmidt ACE C++ Wrapper Facade Design Refactoring Principles Enforce typesafety at compile-time Allow controlled violations of typesafety Simplify for the common case Replace one-dimensional interfaces with hierarchical class categories Enhance portability with parameterized types Inline performance critical methods Define auxiliary classes to hide error-prone details Vanderbilt University 65
- 67. Advanced ACE Tutorial Douglas C. Schmidt Enforce Typesafety at Compile-Time Sockets cannot detect certain errors at compile-time, e.g., int acceptor = socket (PF_INET, SOCK_STREAM, 0); // ... bind (acceptor, ...); // Bind address. listen (acceptor); // Make a acceptor-mode socket. HANDLE n_sd = accept (acceptor, 0, 0); // Error not detected until run-time. read (acceptor, buf, sizeof buf); ACE enforces type-safety at compile-time via factories, e.g.: ACE_SOCK_Acceptor acceptor (port); // Error: recv() not a method of ACE_SOCK_Acceptor. acceptor.recv (buf, sizeof buf); Vanderbilt University 66
- 68. Advanced ACE Tutorial Douglas C. Schmidt Allow Controlled Violations of Typesafety Make it easy to use the C++ Socket wrapper facades correctly, hard to use it incorrectly, but not impossible to use it in ways the class designers did not anticipate e.g., it may be necessary to retrieve the underlying socket handle: ACE_SOCK_Acceptor acceptor; // ... ACE_Handle_Set ready_handles; // ... if (ready_handles.is_set (acceptor.get_handle ()) ACE::select (acceptor.get_handle () + 1, ready_handles); Vanderbilt University 67
- 69. Advanced ACE Tutorial Douglas C. Schmidt Supply Default Parameters ACE_SOCK_Connector (ACE_SOCK_Stream new_stream, const ACE_Addr remote_sap, ACE_Time_Value *timeout = 0, const ACE_Addr local_sap = ACE_Addr::sap_any, int protocol_family = PF_INET, int protocol = 0); The result is extremely concise for the common case: ACE_SOCK_Stream stream; // Compiler supplies default values. ACE_SOCK_Connector con (stream, ACE_INET_Addr (port, host)); Vanderbilt University 68
- 70. Advanced ACE Tutorial Do Define Parsimonious Interfaces e.g., use LSOCK to pass socket handles: ACE_LSOCK_Stream stream; ACE_LSOCK_Acceptor acceptor (/tmp/foo); acceptor.accept (stream); stream.send_handle (stream.get_handle ()); versus the less parsimonious BSD 4.3 socket code ACE_LSOCK::send_handle (const ACE_HANDLE sd) const { u_char a[2]; iovec iov; msghdr send_msg; a[0] = 0xab, a[1] = 0xcd; iov.iov_base = (char *) a; iov.iov_len = sizeof a; send_msg.msg_iov = iov; send_msg.msg_iovlen = 1; send_msg.msg_name = (char *) 0; send_msg.msg_namelen = 0; send_msg.msg_accrights = (char *) sd; send_msg.msg_accrightslen = sizeof sd; return sendmsg (this-get_handle (), send_msg, 0); Note that SVR4 and BSD 4.4 APIs are different than BSD 4.3! Vanderbilt University
- 71. Advanced ACE Tutorial Douglas C. Schmidt Combine Multiple Operations into One Operation Creating a conventional acceptor-mode socket requires multiple calls: int acceptor = socket (PF_INET, SOCK_STREAM, 0); sockaddr_in addr; memset (addr, 0, sizeof addr); addr.sin_family = AF_INET; addr.sin_port = htons (port); addr.sin_addr.s_addr = INADDR_ANY; bind (acceptor, addr, addr_len); listen (acceptor); // ... ACE_SOCK_Acceptor combines this into a single operation: ACE_SOCK_Acceptor acceptor ((ACE_INET_Addr) port); Vanderbilt University 70
- 72. Advanced ACE Tutorial Douglas C. Schmidt Create Hierarchical Class Categories ACE ACE IPC SOCK SAP ACE ACE ACE ACE ACE ACE SOCK SOCK SOCK SOCK SOCK SOCK Dgram Dgram CODgram Stream Connector Acceptor Bcast ACE SOCK ACE ACE ACE ACE ACE Dgram LSOCK LSOCK LSOCK LSOCK LSOCK Mcast Dgram CODgram Stream Connector Acceptor GROUP DATAGRAM ACE STREAM CONNECTION COMM COMM LSOCK COMM ESTABLISHMENT Vanderbilt University 71
- 73. Advanced ACE Tutorial Douglas C. Schmidt Enhance Portability with Parameterized Types // Conditionally select IPC mechanism. DISTRIBUTED 1 APPLICATION DISTRIBUTED 2 APPLICATION DISTRIBUTED 3 APPLICATION #if defined (USE_SOCKETS) APPLICATION1 APPLICATION2 APPLICATION3 typedef ACE_SOCK_Acceptor PEER_ACCEPTOR; COMMON INTERFACE #elif defined (USE_TLI) (PARAMETERIZED TYPES) typedef ACE_TLI_Acceptor PEER_ACCEPTOR; SOCK_SAP TLI_SAP #endif // USE_SOCKETS. USER SOCKET BSDSOCKET SYSTEM V BSDSOCKET int main (void) SPACE API API TLI API API { // ... OS KERNEL KERNEL // Invoke with appropriate PROTOCOL MECHANISMS SPACE // network programming interface. (TCP/IP, OSI, ETC.) echo_serverPEER_ACCEPTOR (port); NETWORK INTERFACE } Switching wholesale between sockets and TLI simply requires instantiating a different ACE C++ wrapper facade Vanderbilt University 72
- 74. Advanced ACE Tutorial Douglas C. Schmidt Inline Performance Critical Methods Inlining is time and space efficient since key methods are very short: class ACE_SOCK_Stream : public ACE_SOCK { public: ssize_t send (const void *buf, size_t n) { return ACE_OS::send (this-get_handle (), buf, n); } ssize_t recv (void *buf, size_t n) { return ACE_OS::recv (this-get_handle (), buf, n); } }; Vanderbilt University 73
- 75. Advanced ACE Tutorial Douglas C. Schmidt Define Auxiliary Classes to Hide Error-Prone Details Standard C socket addressing is awkward and error-prone e.g., easy to neglect to zero-out a sockaddr_in or convert port numbers to network byte-order, etc. ACE C++ Socket wrapper facades define classes to handle details class ACE_INET_Addr : public ACE_Addr { public: ACE_INET_Addr (u_short port, long ip_addr = 0) { memset (this-inet_addr_, 0, sizeof this-inet_addr_); this-inet_addr_.sin_family = AF_INET; this-inet_addr_.sin_port = htons (port); memcpy (this-inet_addr_.sin_addr, ip_addr, sizeof ip_addr); } // ... private: sockaddr_in inet_addr_; }; Vanderbilt University 74
- 76. Advanced ACE Tutorial Douglas C. Schmidt Demultiplexing and Dispatching Events Problem – The logging server must process several different types of events simultaneously from different sources of events Forces – Multi-threading is not always available – Multi-threading is not always efficient – Multi-threading can be error-prone – Tightly coupling event demuxing with server-specific logic is inflexible Solution – Use the Reactor pattern to decouple event demuxing/dispatching from server-specific processing Vanderbilt University 75
- 77. Advanced ACE Tutorial Douglas C. Schmidt The Reactor Pattern Reactor select (handles); Intent foreach h in handles loop handle_events() table[h].handle_event(type) register_handler(h) end loop Demuxes dispatches remove_handler(h) handlers 1 Event Handler requests that are N uses Handle owns handle_event(type) delievered concurrency get_handle() to an application by one notifies or more clients Synchronous Event Concrete Demultiplexer Event Forces Resolved select() Handler Serially demux events www.cs.wustl.edu/˜schmidt/ synchronously POSA/ efficiently Extend applications without changing demuxing code Vanderbilt University 76
- 78. Advanced ACE Tutorial Douglas C. Schmidt Collaboration in the Reactor Pattern callback : main Concrete Reactor Note inversion of program Event_Handler Reactor() control INITIALIZATION INITIALIZE register_handler(callback) Also note how MODE REGISTER HANDLER get_handle() long-running EXTRACT HANDLE handle_events() event handlers START EVENT LOOP select() can degrade FOREACH EVENT DO handle_input() quality of service EVENT HANDLING DATA ARRIVES since callbacks handle_output() OK TO SEND “steal” Reactor’s MODE handle_signal() thread of SIGNAL ARRIVES handle_timeout() control... TIMER EXPIRES remove_handler(callback) REMOVE HANDLER CLEANUP handle_close() Vanderbilt University 77
- 79. Advanced ACE Tutorial Do Structure and Implementations of the ACE Reactor Framework Reactor framework participants ACE_Timer_Queue ACE_Event_Handler 0..1 ACE_Reactor Application Event ACE_Time_Value Handler Common Reactor implementations in ACE 1 ACE_Reactor ACE_Reactor_Impl ACE_WFMO_Reactor TOKEN ACE_Select_Reactor_Impl ACE_Select_Reactor_T «bind» ACE_Select_Reactor_Token ACE_TP_Reactor ACE_Select_Reactor Vanderbilt University
- 80. Advanced ACE Tutorial Do Using the ACE Reactor Framework in the Logging Server REGISTERED APPLICATION 2: sh = new Logging_Handler : Logging OBJECTS 3: accept (sh-peer()) LEVEL : Logging Handler Handler 4: sh-open() : Logging : Event : Event Acceptor 5: handle_input() Handler Handler 6: recv(msg) 7:process(msg) : Event Handler 1: handle_input() FRAMEWORK LEVEL :Timer : Handle Queue Table : Reactor OS EVENT DEMULTIPLEXING INTERFACE KERNEL LEVEL Benefits Liabilities Straightforward to Callbacks are “brittle” program Can’t leverage Concurrency control multi-processors is easy Vanderbilt University
- 81. Advanced ACE Tutorial Douglas C. Schmidt Addressing Acceptor Endpoint Connection and Initialization Challenges Problem – The communication protocol used between applications is often orthogonal to its connection establishment and service handler initialization protocols Forces – Low-level connection APIs are error-prone and non-portable – Separating initialization from processing increases software reuse Solution – Use the Acceptor pattern to decouple passive connection establishment and connection handler initialization from the subsequent logging protocol Vanderbilt University 80
- 82. Advanced ACE Tutorial Do The Acceptor-Connector Pattern (Acceptor Role) Svc Svc Handler Handler Acceptor peer_stream_ ACTIVATES peer_acceptor_ open() accept() APPLICATION- DEFINED APPLICATION- INDEPENDENT Reactor www.cs.wustl.edu/˜schmidt/POSA/ Intent of Acceptor Role Forces resolved Decouple the passive Reuse passive connection and connection setup initialization of a peer and service service in a distributed initialization code system from the Ensure that processing performed acceptor-mode once the peer service is handles aren’t used connected and to read/write data initialized Vanderbilt University
- 83. Advanced ACE Tutorial Do Structure of the ACE Acceptor-Connector Framework ACE_Event_Handler SYNCH_STRATEGY SVC_HANDLER, PEER_CONNECTOR ACE_Task ACE_Connector PEER_STREAM, SYNCH_STRATEGY SVC_HANDLER, PEER_ACCEPTOR ACE_Svc_Handler ACE_Acceptor «bind» Application Service Framework characteristics Uses C++ parameterized types to strategize IPC and service aspects Uses Template Method pattern to strategize creation, connection establishment, and concurrency policies Vanderbilt University
- 84. Advanced ACE Tutorial Do Using the ACE_Acceptor in the Logging Server : Logging : Logging Handler Handler : Logging : Logging Handler Acceptor : Svc : Svc Handler Handler : Svc : Acceptor Handler : Logging PASSIVE ACTIVE Handler LISTENER CONNECTIONS 1: handle_input() : Svc 2: sh = make_svc_handler() Handler 3: accept_svc_handler(sh) : Reactor 4: activate_svc_handler(sh) The ACE_Acceptor is a factory – i.e., it creates, connects, and activates an ACE_Svc_Handler There’s often one ACE_Acceptor per-service/per-port Vanderbilt University
- 85. Advanced ACE Tutorial Do ACE_Acceptor Class Public Interface template class SVC_HANDLER, // Service aspect class PEER_ACCEPTOR // IPC aspect class ACE_Acceptor : public ACE_Service_Object { // Inherits indirectly from ACE_Event_Handler public: // Initialization. virtual int open (typename const PEER_ACCEPTOR::PEER_ADDR , ACE_Reactor * = ACE_Reactor::instance ()); // Template Method. virtual int handle_input (ACE_HANDLE); protected: // Factory method creates a service handler. virtual SVC_HANDLER *make_svc_handler (void); // Accept a new connection. virtual int accept_svc_handler (SVC_HANDLER *); // Activate a service handler. virtual int activate_svc_handler (SVC_HANDLER *); private: // Acceptor IPC connection strategy. PEER_ACCEPTOR peer_acceptor_; }; Vanderbilt University
- 86. Advanced ACE Tutorial Do ACE_Acceptor Class Implementation // Shorthand names. #define SH SVC_HANDLER #define PA PEER_ACCEPTOR // Template Method that creates, connects, // and activates service handlers. template class SH, class PA int ACE_AcceptorSH, PA::handle_input (ACE_HANDLE) { // Factory method that makes a service handler. SH *svc_handler = make_svc_handler (); // Accept the connection. accept_svc_handler (svc_handler); // Delegate control to the service handler. activate_svc_handler (svc_handler); } Vanderbilt University
- 87. Advanced ACE Tutorial Douglas C. Schmidt The Template Method Pattern Intent Abstract Class Define the skeleton of template_method() ... primitive_operation1() an algorithm in an primitive_operation1() primitive_operation2() ... operation, deferring primitive_operation2() ... some steps to subclasses Concrete Class Gamma et al., Design primitive_operation1() Patterns: Elements of primitive_operation2() Reusable Object-Oriented Software AW, ’94 Vanderbilt University 86
- 88. Advanced ACE Tutorial Douglas C. Schmidt Using the Template Method Pattern in the ACE Acceptor Implementation Benefits Acceptor handle_input() ... Straightforward to make_svc_handler() make_svc_handler() program via inheritance accept_svc_handler() ... activate_svc_handler() accept_svc_handler() and dynamic binding ... activate_svc_handler() Liabilities My Acceptor Design is “brittle” and make_svc_handler() can cause “explosion” of activate_svc_handler() subclasses due to “whitebox” design Vanderbilt University 87
- 89. Advanced ACE Tutorial Douglas C. Schmidt The Strategy Pattern Intent STRATEGY Define a family of Context Strategy context_interface() algorithm_interface() algorithms, encapsulate each one, and make them interchangeable Gamma et al., Design Concrete Concrete Patterns: Elements of Strategy A Strategy C Reusable Object-Oriented algorithm_interface() algorithm_interface() Software AW, ’94 Concrete Strategy B algorithm_interface() Vanderbilt University 88
- 90. Advanced ACE Tutorial Douglas C. Schmidt Using the Strategy Pattern in the ACE Acceptor Implementation Benefits Acceptor Concurrency delegates handle_input() Strategy More 2: activate_svc_handler(sh) activate_svc_handler() extensible due sh = create_svc_handler () to “blackbox” ... design accept_svc_handler (sh) ... 1: activate_svc_handler(sh) Reactive Thread Process Liabilities ... Strategy Strategy Strategy More complex and harder to develop initially Vanderbilt University 89
- 91. Advanced ACE Tutorial Do ACE_Acceptor Template Method Hook Implementations Template method hooks can be overridden // Factory method for creating a service handler. template class SH, class PA SH * ACE_AcceptorSH, PA::make_svc_handler (ACE_HANDLE) return new SH; // Default behavior. } // Accept connections from clients. template class SH, class PA int ACE_AcceptorSH, PA::accept_svc_handler (SH *sh) { peer_acceptor_.accept (sh-peer ()); } // Activate the service handler. template class SH, class PA int ACE_AcceptorSH, PA::activate_svc_handler (SH *sh) { if (sh-open () == -1) sh-close (); } Vanderbilt University
- 92. Advanced ACE Tutorial Do ACE_Acceptor Initialization Implementation Note how the PEER_ACCEPTOR’s open() method hides all the details associated with passively initializing communication endpoints // Initialization. template class SH, class PA int ACE_AcceptorSH, PA::open (typename const PA::PEER_ADDR addr, ACE_Reactor *reactor) { // Forward initialization to the concrete // peer acceptor. peer_acceptor_.open (addr); // Register with Reactor. reactor-register_handler (this, ACE_Event_Handler::ACCEPT_MASK); } Vanderbilt University
- 93. Advanced ACE Tutorial Do ACE_Svc_Handler Class Public Interface Note how IPC and synchronization aspects are strategized template class PEER_STREAM, // IPC aspect class SYNCH_STRAT // Synch aspect class ACE_Svc_Handler : public ACE_TaskSYNCH_STRAT // Task is-a Service_Object, // which is-an Event_Handler { public: // Constructor. ACE_Svc_Handler (Reactor * = ACE_Reactor::instance ()); // Activate the handler (called by the . // ACE_Acceptor or ACE_Connector). virtual int open (void *); // Return underlying IPC mechanism. PEER_STREAM peer (void); // ... private: PEER_STREAM peer_; // IPC mechanism. virtual ˜ACE_Svc_Handler (void); }; Vanderbilt University
- 94. Advanced ACE Tutorial Douglas C. Schmidt ACE_Svc_Handler Implementation #define PS PEER_STREAM By default, a #define SS SYNCH_STRAT ACE_Svc_Handler template class PS, class SS object is registered ACE_Svc_HandlerPS, SS::ACE_Svc_Handler with the singleton (ACE_Reactor *r): ACE_Service_Object (r) ACE_Reactor {} template class PS, class SS – This makes the int ACE_Svc_HandlerPS, SS::open service “reactive” (void *) { so that no other // Enable non-blocking I/O. peer ().enable (ACE_NONBLOCK); synchronization mechanisms are // Register handler with the Reactor. necessary reactor ()-register_handler (this, ACE_Event_Handler::READ_MASK); } Vanderbilt University 93
- 95. Advanced ACE Tutorial Do Object Diagram for OO Logging Server SERVER LOGGING Service DAEMON Config Logging Acceptor Service Repository Logging Handler Service Manager Logging Handler Reactor LOGGING RECORDS CONNECTION SERVER REMOTE REQUEST CONTROL OPERATIONS CLIENT CLIENT CLIENT CLIENT Vanderbilt University
- 96. Advanced ACE Tutorial Do The Logging_Handler and Logging_Acceptor Classes // Performs I/O with client logging daemons. class Logging_Handler : public ACE_Svc_HandlerACE_SOCK_Acceptor::PEER_STREAM, // Trait! ACE_NULL_SYNCH { public: // Recv and process remote logging records. virtual int handle_input (ACE_HANDLE); }; // Logging_Handler factory. class Logging_Acceptor : public ACE_AcceptorLogging_Handler, ACE_SOCK_Acceptor { public: // Dynamic linking hooks. virtual int init (int argc, char *argv[]); virtual int fini (void); }; Vanderbilt University
- 97. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Parameterizing IPC Mechanisms Q: How can you switch between different IPC mechanisms? A: By parameterizing IPC Mechanisms with C++ Templates, e.g.: #if defined (ACE_USE_SOCKETS) typedef ACE_SOCK_Acceptor PEER_ACCEPTOR; #elif defined (ACE_USE_TLI) typedef ACE_TLI_Acceptor PEER_ACCEPTOR; #endif /* ACE_USE_SOCKETS */ class Logging_Handler : public ACE_Svc_HandlerPEER_ACCEPTOR::PEER_STREAM, // Trait! ACE_NULL_SYNCH { /* ... /* }; class Logging_Acceptor : public ACE_Acceptor Logging_Handler, PEER_ACCEPTOR { /* ... */ }; Vanderbilt University 96
- 98. Advanced ACE Tutorial Douglas C. Schmidt Logging_Handler Input Method Callback routine that receives logging records Implementation of application-specific int logging method Logging_Handler::handle_input (ACE_HANDLE) { This is the main code // Call existing function to recv supplied by a // logging record and print to stdout. ssize_t n = developer! handle_log_record (peer ().get_handle (), ACE_STDOUT); if (n 0) // Count the # of logging records ++request_count; return n = 0 ? -1 : 0; } Vanderbilt University 97
- 99. Advanced ACE Tutorial Do Logging_Acceptor Initialization and Termination // Automatically called when a Logging_Acceptor // object is linked dynamically. Logging_Acceptor::init (int argc, char *argv[]) { ACE_Get_Opt get_opt (argc, argv, p:, 0); ACE_INET_Addr addr (DEFAULT_PORT); for (int c; (c = get_opt ()) != -1; ) switch (c) { case ’p’: addr.set (atoi (getopt.optarg)); break; default: break; } // Initialize endpoint and register // with the ACE_Reactor. open (addr, ACE_Reactor::instance ()); } // Automatically called when object is unlinked. Logging_Acceptor::fini (void) { handle_close (); } Vanderbilt University
- 100. Advanced ACE Tutorial Douglas C. Schmidt Putting the Pieces Together at Run-time Problem – Prematurely committing ourselves to a particular logging server configuration is inflexible and inefficient Forces – It is useful to build systems by “scripting” components – Certain design decisions can’t be made efficiently until run-time – It is a bad idea to force users to “pay” for components they do not use Solution – Use the Component Configurator pattern to assemble the desired logging server components dynamically Vanderbilt University 99
- 101. Advanced ACE Tutorial Douglas C. Schmidt The Component Configurator Pattern APPLICATION Intent LAYER Concrete Component Decouples the implementation of services from the time when they are Component Component Config configured CONFIGURATION suspend() 1 1 resume() n Forces Resolved LAYER init() A fini() 1 1 info() Reduce resource utilization Component Repository Support dynamic (re)configuration REACTIVE LAYER Event www.cs.wustl.edu/ Handler n 1 Reactor ˜schmidt/POSA/ Vanderbilt University 100
- 102. Advanced ACE Tutorial Do Structure of the ACE Service Configurator Framework ACE_Event_Handler ACE_Service_Config ACE_Service_Object ACE_Service_Repository Application Service ACE_Service_Repository_Iterator Framework characteristics ACE_Service_Config uses a variant of the Monostate pattern Can be accessed either via a script or programmatically Vanderbilt University
- 103. Advanced ACE Tutorial Do Using the ACE Service Configurator Framework for the Logging Server SERVICE Thread CONFIGURATOR Logger RUNTIME Reactive Logger Service Object Service Service DLLS Repository Object Thread Pool Logger Service Reactor Service Config Object svc.conf dynamic Logger Service_Object * FILE logger:make_Logger() -p 2001 The existing Logging Server service is single-threaded Other versions could be multi-threaded Note how we can script this via the svc.conf file Vanderbilt University
- 104. Advanced ACE Tutorial Douglas C. Schmidt Dynamically Linking a Service Dynamically linked factory function Application-specific factory that allocates a new function used to dynamically Logging_Acceptor create a service extern C ACE_Service_Object * The make_Logger() function make_Logger (void); provides a hook between an application-specific service ACE_Service_Object * and the make_Logger (void) { application-independent ACE return new Logging_Acceptor; mechanisms // Framework automatically // deletes memory. – ACE handles all memory } allocation and deallocation Vanderbilt University 103
- 105. Advanced ACE Tutorial Douglas C. Schmidt Service Configuration The logging service is configured Generic event-loop to via scripting in a svc.conf file: dynamically configure service % cat ./svc.conf daemons # Dynamically configure int main (int argc, char *argv[]) # the logging service { dynamic Logger // Initialize the daemon and Service_Object * // configure services logger:_make_Logger() -p 2001 ACE_Service_Config::open (argc, # Note, .dll or .so suffix argv); # added to the logger // Run forever, performing the # automatically // configured services ACE_Reactor::instance ()- run_reactor_event_loop (); /* NOTREACHED */ } Vanderbilt University 104
- 106. Advanced ACE Tutorial Douglas C. Schmidt State Chart for the Service Configurator Framework CONFIGURE/ Service_Config::process_directives() INITIALIZED IDLE START EVENT LOOP/ Reactor::run_event_loop() SHUTDOWN/ NETWORK EVENT/ Service_Config::close() AWAITING EVENTS Reactor::dispatch() PERFORM RECONFIGURE/ CALLBACK Service_Config::process_directives() CALL HANDLER/ Event_Handler::handle_input() Vanderbilt University 105
- 107. Advanced ACE Tutorial Douglas C. Schmidt Advantages of OO Logging Server The OO architecture illustrated thus far decouples application-specific service functionality from: – Time when a service is configured into a process – The number of services per-process – The type of IPC mechanism used – The type of event demultiplexing mechanism used We can use the techniques discussed thus far to extend applications without: – Modifying, recompiling, and relinking existing code – Terminating and restarting executing daemons The remainder of the Logging Server slides examine a set of techniques for decoupling functionality from concurrency mechanisms, as well Vanderbilt University 106
- 108. Advanced ACE Tutorial Douglas C. Schmidt Concurrent OO Logging Server The structure of the Logging Server can benefit from concurrent execution on a multi-processor platform This section examines ACE C++ classes and patterns that extend the logging server to incorporate concurrency – Note how most extensions require minimal changes to the existing OO architecture... This example also illustrates additional ACE components involving synchronization and multi-threading Vanderbilt University 107
- 109. Advanced ACE Tutorial Do Concurrent OO Logging Server Architecture SERVER LOGGING SERVER 1: SOCK Acceptor 2: accept() 4: handle_input() 5: spawn() 7: recv() 7: recv() 8: write() 8: write() Logging Acceptor Logging Logging Reactor Handler Handler 3: connect() 6: send() 6: send() CLIENT CLIENT A NETWORK B Runs each client connection in a separate thread Vanderbilt University
- 110. Advanced ACE Tutorial Douglas C. Schmidt Pseudo-code for Concurrent Server Pseudo-code for multi-threaded Logging_Handler factory Logging Server void handler_factory (void) { initialize acceptor endpoint foreach (pending connection event) { accept connection spawn a thread to handle connection and run logging_handler() entry point } } Pseudo-code for logging_handler() function void logging_handler (void) { foreach (incoming logging records from client) call handle_log_record() exit thread } Vanderbilt University 109
- 111. Advanced ACE Tutorial Douglas C. Schmidt Concurrency Overview A thread is a sequence of SHARED ADDRESS SPACE instructions executed in one or more processes – One process ! THREADS stand-alone systems – More than one process ! distributed systems PROCESS Traditional OS processes contain a single thread of control This simplifies programming since a sequence of execution steps is protected from unwanted interference by other execution sequences... Vanderbilt University 110
- 112. Advanced ACE Tutorial Douglas C. Schmidt Traditional Approaches to OS Concurrency 1. Device drivers and programs with signal handlers utilize a limited form of concurrency e.g., asynchronous I/O Note that concurrency encompasses more than multi-threading... 2. Many existing programs utilize OS processes to provide “coarse-grained” concurrency e.g., – Client/server database applications – Standard network daemons like UNIX INETD Multiple OS processes may share memory via memory mapping or shared memory and use semaphores to coordinate execution The OS kernel scheduler dictates process behavior Vanderbilt University 111
- 113. Advanced ACE Tutorial Douglas C. Schmidt Evaluating Traditional OS Process-based Concurrency Advantages – Easy to keep processes from interfering A process combines security, protection, and robustness Disadvantages – Complicated to program, e.g., – Signal handling may be tricky – Shared memory may be inconvenient Inefficient – The OS kernel is involved in synchronization and process management – Difficult to exert fine-grained control over scheduling and priorities Vanderbilt University 112
- 114. Advanced ACE Tutorial Douglas C. Schmidt Modern OS Concurrency Modern OS platforms typically provide a standard set of APIs that handle – Process/thread creation and destruction – Various types of process/thread synchronization and mutual exclusion – Asynchronous facilities for interrupting long-running processes/threads to report errors and control program behavior Once the underlying concepts are mastered, it’s relatively easy to learn different concurrency APIs – e.g., traditional UNIX process operations, Solaris threads, POSIX pthreads, WIN32 threads, Java threads, etc. Vanderbilt University 113
- 115. Advanced ACE Tutorial Douglas C. Schmidt Lightweight Concurrency Modern operating systems provide lightweight mechanisms that manage and synchronize multiple threads within a process – Some systems also allow threads to synchronize across multiple processes Benefits of threads 1. Relatively simple and efficient to create, control, synchronize, and collaborate – Threads share many process resources by default 2. Improve performance by overlapping computation and communication – Threads may also consume less resources than processes 3. Improve program structure – e.g., compared with using asynchronous I/O Vanderbilt University 114
- 116. Advanced ACE Tutorial Do Example: Single-threaded vs. Multi-threaded Applications CLIENT CLIENT SERVER KERNEL USER REQUEST KERNEL USER SERVICE THREAD EXECUTES BLOCKED RESPONSE SINGLE- THREADED RPC KERNEL USER SERVER KERNEL USER REQUEST SERVICE EXECUTES RESPONSE SERVER KERNEL REQUEST USER RESPONSE SERVICE EXECUTES MULTI- CLIENT THREADED RPC Vanderbilt University
- 117. Advanced ACE Tutorial Douglas C. Schmidt Hardware and OS Concurrency Support Four typical USER-LEVEL abstractions LWP LWP LWP LWP LWP LWP LWP 1. Application threads 2. Lightweight KERNEL-LEVEL processes PE PE PE PE PE PE PE PE 3. Kernel threads SHARED MEMORY 4. Processing elements PE LWP UNIX PROCESSING LIGHTWEIGHT PROCESS THREAD ELEMENT PROCESS Vanderbilt University 116
- 118. Advanced ACE Tutorial Douglas C. Schmidt Application Threads Most process resources are Each thread also contains unique equally accessible to all threads information, e.g., in a process, e.g., Identifier Virtual memory Register set (e.g., PC and SP) User permissions and access Run-time stack control privileges Signal mask Open files Priority Signal handlers Thread-specific data (e.g., errno) Note, there is no MMU protection for threads in a single process Vanderbilt University 117
- 119. Advanced ACE Tutorial Douglas C. Schmidt Kernel-level vs. User-level Threads Application and system characteristics influence the choice of user-level vs. kernel-level threading A high degree of “virtual” application concurrency implies user-level threads (i.e., unbound threads) – e.g., desktop windowing system on a uni-processor A high degree of “real” application parallelism implies lightweight processes (LWPs) (i.e., bound threads) – e.g., video-on-demand server or matrix multiplication on a multi-processor Vanderbilt University 118
- 120. Advanced ACE Tutorial Douglas C. Schmidt Overview of OS Synchronization Mechanisms Threads share resources in a process address space Therefore, they must use synchronization mechanisms to coordinate their access to shared data Traditional OS synchronization mechanisms are very low-level, tedious to program, error-prone, and non-portable ACE encapsulates these mechanisms with wrapper facades and higher-level patterns/components Vanderbilt University 119
- 121. Advanced ACE Tutorial Douglas C. Schmidt Common OS Synchronization Mechanisms Mutual exclusion (mutex) locks – Serialize thread access to a shared resource Counting semaphores – Synchronize thread execution Readers/writer (R/W) locks – Serialize resources that are searched more than changed Condition variables – Used to block threads until shared data changes state File locks – System-wide R/W locks accessed by processes Vanderbilt University 120
- 122. Advanced ACE Tutorial Douglas C. Schmidt Additional ACE Synchronization Mechanism Events – Gates and latches Barriers – Allows threads to synchronize their completion Token – Provides FIFO scheduling order Task – Provides higher-level “active object” for concurrent applications Thread-specific storage – Low-overhead, contention-free storage Vanderbilt University 121
- 123. Advanced ACE Tutorial Do Concurrency Mechanisms in ACE MANAGERS SYNCH CONDITIONS ACTIVE Thread OBJECTS Task Null Process Manager Manager Condition Thread MUTEX ADVANCED SYNCH LOCK TYPE TYPE Condition Atomic Token Barrier Op TSS GUARDS SYNCH WRAPPERS RW Semaphore Guard Mutex Null Mutex Mutex Thread Thread Read Mutex Semaphore Guard File Process Lock Process Write Events Mutex Semaphore Guard All ACE Concurrency mechanisms are ported to all OS platforms www.cs.wustl.edu/˜schmidt/ACE/ book1/ Vanderbilt University
- 124. Advanced ACE Tutorial Douglas C. Schmidt Addressing Logger Server Concurrency Challenges Problem – Multi-threaded logging servers may be necessary when single-threaded reactive servers inefficient, non-scalable, or non-robust Forces – Multi-threading can be very hard to program – No single multi-threading model is always optimal Solution – Use the Active Object pattern to allow multiple concurrent logging server operations using an OO programming style Vanderbilt University 123
- 125. Advanced ACE Tutorial Do The Active Object Pattern loop { m = act_list.dequeue() Proxy if (m.guard()) m.call() else act_list.enqueue (m); Future m1() 1: enqueue(new M1) } Future m2() Future m3() 3: dispatch() Activation Scheduler List dispatch() enqueue() VISIBLE enqueue() 2: enqueue(M1) dequeue() TO 1 CLIENTS n Servant Method M1 HIDDEN m1() 1 1 Request M2 m2() FROM guard() CLIENTS m3() 4: m1() call() M3 www.cs.wustl.edu/˜schmidt/POSA/ Intent Forces Resolved Decouples method Allow blocking execution from method operations invocation to enhance Permit flexible concurrency and simplify concurrency synchronized access to strategies an object that resides in its own thread of control Vanderbilt University
- 126. Advanced ACE Tutorial Do ACE Support for Active Objects t2 : : TASK Task STATE 2: enqueue (msg) 3: svc () : Message 4: dequeue (msg) Queue 5: do_work(msg) ACTIVE 1: put (msg) 6: put (msg) t3 : Task : TASK t1 : STATE Task : TASK STATE : Message Queue : Message Queue ACTIVE ACTIVE The ACE Task framework can be used to implement the complete Active Object pattern or lighterweight subsets Vanderbilt University
- 127. Advanced ACE Tutorial Douglas C. Schmidt The ACE Task Framework An ACE_Task binds a separate thread of control together with an object’s data and methods – Multiple active objects may execute in parallel in separate lightweight or heavyweight processes ACE_Task objects communicate by passing typed messages to other ACE_Task objects – Each ACE_Task maintains a queue of pending messages that it processes in priority order ACE_Task is a low-level mechanism to support active objects Vanderbilt University 126
- 128. Advanced ACE Tutorial Do Structure of the ACE Task Framework ACE_Event_Handler ACE_Service_Object SYNCH 0..1 * ACE_Thread_Manager ACE_Task SYNCH ACE_Message_Block ACE_Message_Queue * 1 Framework characteristics 1. ACE_Tasks can register with an ACE_Reactor 2. They can be dynamically linked 3. They can queue data 4. They can run as active objects in 1 or more threads Vanderbilt University
- 129. Advanced ACE Tutorial Douglas C. Schmidt The ACE_Task Class Public Interface template class SYNCH_STRAT // Synchronization aspect class ACE_Task : public ACE_Service_Object { public: // Initialization/termination hooks. virtual int open (void *args = 0) = 0; virtual int close (u_long = 0) = 0; // Transfer msg to queue for immediate processing. virtual int put (ACE_Message_Block *, ACE_Time_Value * = 0) = 0; // Run by a daemon thread for deferred processing. virtual int svc (void) = 0; // Turn task into active object. int activate (long flags, int threads = 1); Vanderbilt University 128
- 130. Advanced ACE Tutorial Douglas C. Schmidt ACE_Task Class Protected Interface Many of the following methods are used by put() and svc() // Accessors to internal queue. ACE_Message_QueueSYNCH_STRAT *msg_queue (void); void msg_queue (ACE_Message_QueueSYNCH_STRAT *); // Accessors to thread manager. ACE_Thread_Manager *thr_mgr (void); void thr_mgr (ACE_Thread_Manager *); // Insert message into the message list. int putq (ACE_Message_Block *, ACE_Time_Value *tv = 0); // Extract the first message from the list (blocking). int getq (ACE_Message_Block *mb, ACE_Time_Value *tv = 0); // Hook into the underlying thread library. static void *svc_run (ACE_TaskSYNCH_STRAT *); Vanderbilt University 129
- 131. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Combining Threads C++ Objects Q: What is the svc_run() function and why is it a static method? A: OS thread APIs require C-style functions as entry point The ACE Task framework encapsulates the svc_run() function within the ACE_Task::activate() method: template class SYNCH_STRAT int ACE_TaskSYNCH_STRAT::activate (long flags, int n_threads) { if (thr_mgr () == NULL) thr_mgr (ACE_Thread_Manager::instance ()); thr_mgr ()-spawn_n (n_threads, ACE_TaskSYNCH_STRAT::svc_run, (void *) this, flags); } 1. ACE_Task::activate () 4. template SYNCH_STRATEGY void * 2. ACE_Thread_Manager::spawn ACE_TaskSYNCH_STRATEGY::svc_run (svc_run, this); (ACE_TaskSYNCH_STRATEGY *t) { 3. _beginthreadex // ... (0, 0, void *status = t-svc (); svc_run, this, // ... 0, thread_id); R UN-T I ME return status; // Thread return. T HR EA D ST A CK } Vanderbilt University 130
- 132. Advanced ACE Tutorial Douglas C. Schmidt The svc_run() Adapter Function ACE_Task::svc_run() is static method used as the entry point to execute an instance of a service concurrently in its own thread template class SYNCH_STRAT void * ACE_TaskSYNCH_STRAT::svc_run (ACE_TaskSYNCH_STRAT *t) { // Thread added to thr_mgr() automatically on entry. // Run service handler and record return value. void *status = (void *) t-svc (); t-close (u_long (status)); // Status becomes return value of thread... return status; // Thread removed from thr_mgr() automatically on return. } Vanderbilt University 131
- 133. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Motivation for the ACE_Thread_Manager Q: How can groups of collaborating threads be managed atomically? A: Develop the ACE_Thread_Manager class that: – Supports the notion of thread groups i.e., operations on all threads in a group – Implements barrier synchronization on thread exits – Shields applications from incompatibilities between different OS thread libraries e.g., detached threads and thread joins Vanderbilt University 132
- 134. Advanced ACE Tutorial Douglas C. Schmidt Using ACE Task Framework for Logging Server Process remote logging records The OO implementation localizes by looping until the client the application-specific part of terminates connection the logging service in a single int point, while leveraging off Thr_Logging_Handler::svc (void) reusable ACE components { while (handle_input () != -1) Compare with original, which // Call existing function borrow’s the Reactor thread // to recv logging record int // and print to stdout. Logging_Handler::handle_input (void) continue; { handle_log_record return 0; (peer ().get_handle (), } ACE_STDOUT); // ... } Vanderbilt University 133
- 135. Advanced ACE Tutorial Do Class Diagram for Concurrent OO Logging Server Thr_Logging_Handler COMPONENTS SOCK_Stream APPLICATION- SOCK_Acceptor NULL_Synch SPECIFIC 1 Thr n Thr Logging Logging activates Acceptor Handler SVC_HANDLER COMPONENTS CONNECTION- PEER_ACCEPTOR PEER_STREAM ORIENTED SYNCH Acceptor Svc Handler PEER PEER ACCEPTOR STREAM COMPONENTS FRAMEWORK Stream Connection ACE IPC_SAP Service Configurator Concurrency Reactor global Vanderbilt University
- 136. Advanced ACE Tutorial Do Thr_Logging_Acceptor and Thr_Logging_Handler Interfaces Template classes that create, connect, and activate a new thread to handle each client class Thr_Logging_Handler : public Logging_Handler // Inherits handle_input { public: // Override definition in ACE_Svc_Handler // class to spawn a new thread! This method // is called by the ACE_Acceptor. virtual int open (void *); // Process remote logging records. virtual int svc (void); }; class Thr_Logging_Acceptor : public ACE_AcceptorThr_Logging_Handler, ACE_SOCK_Acceptor { // Same as Logging_Acceptor... }; Vanderbilt University
- 137. Advanced ACE Tutorial Do Thr_Logging_Handler Implementation Override definition in the ACE_Svc_Handler class to spawn a new thread int Thr_Logging_Handler::open (void *) { // Spawn a new thread to handle // logging records with the client. activate (THR_DETACHED); } Process remote logging records by looping until client terminates connection int Thr_Logging_Handler::svc (void) { while (handle_input () != -1) // Call existing function to recv // logging record and print to stdout. continue; } Vanderbilt University
- 138. Advanced ACE Tutorial Douglas C. Schmidt Dynamically Reconfiguring the Logging Server The logging service is Dynamically linked factory configured via scripting in a function that allocates a new svc.conf file: threaded Logging_Acceptor extern C % cat ./svc.conf ACE_Service_Object *make_Logger (void); # Dynamically reconfigure # the logging service ACE_Service_Object * remove Logger make_Logger (void) dynamic Logger { Service_Object * return new Thr_Logging_Acceptor; thr_logger:_make_Logger() } -p 2002 # .dll or .so suffix added to # thr_logger automatically Logging service is reconfigured by changing the svc.conf file and sending SIGHUP signal to server Vanderbilt University 137
- 139. Advanced ACE Tutorial Douglas C. Schmidt Caveats for the Concurrent Logging Server The concurrent Logging Server has several problems – Output in the handle_log_record() function is not serialized – The auto-increment of global variable request_count is also not serialized Lack of serialization leads to errors on many shared memory multi-processor platforms... – Note that this problem is indicative of a large class of errors in concurrent programs... The following slides compare and contrast a series of techniques that address this problem Vanderbilt University 138
- 140. Advanced ACE Tutorial Douglas C. Schmidt Explicit Synchronization Mechanisms One approach for serialization uses OS mutual exclusion mechanisms explicitly, e.g., // at file scope mutex_t lock; // SunOS 5.x synchronization mechanism // ... handle_log_record (ACE_HANDLE in_h, ACE_HANDLE out_h) { // in method scope ... mutex_lock (lock); if (ACE_OS::write (out_h, lr.buf, lr.size) == -1) return -1; mutex_unlock (lock); // ... } However, adding these mutex calls explicitly causes problems... Vanderbilt University 139
- 141. Advanced ACE Tutorial Douglas C. Schmidt Problem: Explicit mutex_* Calls Inelegant ! “Impedance mismatch” with C/C++ Obtrusive – Must find and lock all uses of write() – Can yield inheritance anomaly Error-prone – C++ exception handling and multiple method exit points – Thread mutexes won’t work for separate processes – Global mutexes may not be initialized correctly Non-portable ! Hard-coded to Solaris 2.x Inefficient ! e.g., expensive for certain platforms/designs Vanderbilt University 140
- 142. Advanced ACE Tutorial Do Solution: Synchronization Wrapper Facades class ACE_Thread_Mutex { public: ACE_Thread_Mutex (void) { mutex_init (lock_, USYNCH_THREAD, 0); } ˜ACE_Thread_Mutex (void) { mutex_destroy (lock_); int acquire (void) { return mutex_lock (lock_); } int tryacquire (void) { return mutex_trylock (lock_); } int release (void) { return mutex_unlock (lock_); private: // SunOS 5.x serialization mechanism. mutex_t lock_; void operator= (const ACE_Thread_Mutex ); ACE_Thread_Mutex (const ACE_Thread_Mutex ); }; Note how we prevent improper copying and assignment by using C++ access control specifiers Vanderbilt University
- 143. Advanced ACE Tutorial Do Porting ACE_Thread_Mutex to Windows NT class ACE_Thread_Mutex { public: ACE_Thread_Mutex (void) { lock_ = CreateMutex (0, FALSE, 0); } ˜ACE_Thread_Mutex (void) { CloseHandle (lock_); } int acquire (void) { return WaitForSingleObject (lock_, INFINITE); } int tryacquire (void) { return WaitForSingleObject (lock_, 0); } int release (void) { return ReleaseMutex (lock_); } private: ACE_HANDLE lock_; // Windows locking mechanism. // ... Vanderbilt University
- 144. Advanced ACE Tutorial Douglas C. Schmidt Using the C++ Mutex Wrapper Facade Using C++ wrapper facades improves portability and elegance // at file scope. ACE_Thread_Mutex lock; // Implicitly unlocked. // ... handle_log_record (ACE_HANDLE in_h, ACE_HANDLE out_h) { // in method scope ... lock.acquire (); if (ACE_OS::write (out_h, lr.buf, lr.size) == -1) return -1; lock.release (); // ... However, this doesn’t really solve the tedium or error-proneness problems – www.cs.wustl.edu/˜schmidt/PDF/ObjMan.pdf Vanderbilt University 143
- 145. Advanced ACE Tutorial Douglas C. Schmidt Automated Mutex Acquisition and Release To ensure mutexes are locked and unlocked, we’ll define a template class that acquires and releases a mutex automatically template class LOCK class ACE_Guard { public: ACE_Guard (LOCK m): lock_ (m) { lock_.acquire (); } ˜ACE_Guard (void) { lock_.release (); } // ... other methods omitted ... private: LOCK lock_; } ACE_Guard uses the Scoped Locking idiom whereby a constructor acquires a resource and the destructor releases the resource Vanderbilt University 144
- 146. Advanced ACE Tutorial Douglas C. Schmidt The ACE_GUARD Macros ACE defines a set of macros that simplify the use of the ACE_Guard, ACE_Write_Guard, and ACE_Read_Guard classes – These macros test for deadlock and detect when operations on the underlying locks fail #define ACE_GUARD(MUTEX,OB,LOCK) ACE_GuardMUTEX OB (LOCK); if (OB.locked () == 0) return; #define ACE_GUARD_RETURN(MUTEX,OB,LOCK,RET) ACE_GuardMUTEX OB (LOCK); if (OB.locked () == 0) return RET; #define ACE_WRITE_GUARD(MUTEX,OB,LOCK) ACE_Write_GuardMUTEX OB (LOCK); if (OB.locked () == 0) return; #define ACE_WRITE_GUARD_RETURN(MUTEX,OB,LOCK,RET) ACE_Write_GuardMUTEX OB (LOCK); if (OB.locked () == 0) return RET; #define ACE_READ_GUARD(MUTEX,OB,LOCK) ACE_Read_GuardMUTEX OB (LOCK); if (OB.locked () == 0) return; #define ACE_READ_GUARD_RETURN(MUTEX,OB,LOCK,RET) ACE_Read_GuardMUTEX OB (LOCK); if (OB.locked () == 0) return RET; Vanderbilt University 145
- 147. Advanced ACE Tutorial Do Thread-safe handle_log_record() Function template class LOCK = ACE_Thread_Mutex ssize_t handle_log_record (ACE_HANDLE in, ACE_HANDLE out) { // beware static initialization... static LOCK lock; ACE_UINT_32 len; ACE_Log_Record lr; // The first recv reads the length (stored as a // fixed-size integer) of adjacent logging record. ssize_t n = s.recv_n ((char *) len, sizeof len); if (n = 0) return n; len = ntohl (len); // Convert byte-ordering // Perform sanity check! if (len sizeof (lr)) return -1; // The second recv then reads len bytes to // obtain the actual record. s.recv_n ((char *) lr, sizeof lr); // Decode and print record. decode_log_record (lr); // Automatically acquire mutex lock. ACE_GUARD_RETURN (LOCK, guard, lock, -1); if (ACE_OS::write (out, lr.buf, lr.size) == -1) return -1; // Automatically release mutex lock. return 0; } Vanderbilt University
- 148. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Motivating the ACE_Guard Design Q: Why is ACE_Guard parameterized by the type of LOCK? A: since many different flavors of locking can benefit from the Scoped Locking protocol – e.g., non-recursive vs. recursive mutexes, intra-process vs. inter-process mutexes, readers/writer mutexes, POSIX and System V semaphores, file locks, and the null mutex Q: Why are templates used, as opposed to inheritance/polymorphism? A: since they are more efficient and can reside in shared memory All ACE synchronization wrapper facades use the Adapter pattern to provide identical interfaces to facilitate parameterization Vanderbilt University 147
- 149. Advanced ACE Tutorial Douglas C. Schmidt The Adapter Pattern Intent 1: request () Adapter Convert the interface client of a class into another request() interface client expects 2: specific_request() Adaptee1 Force resolved: Adaptee2 specific_request() Provide an interface Adaptee3 specific_request() that captures specific_request() similarities between different OS mechanisms, e.g., locking or IPC Vanderbilt University 148
- 150. Advanced ACE Tutorial Douglas C. Schmidt Remaining Caveats int Logging_Handler::handle_input (void) There is a race { condition when ssize_t n = handle_log_record incrementing the (peer ().get_handle (), ACE_STDOUT); if (n 0) request_count // Count # of logging records. variable ++request_count; // Danger, race condition!!! Solving this problem using the return n = 0 ? -1 : 0; ACE_Thread_Mutex } or ACE_Guard classes A more elegant solution incorporates is still tedious, parameterized types, overloading, and the low-level, and Strategized Locking pattern, as discussed in error-prone C++NPv1 Vanderbilt University 149
- 151. Advanced ACE Tutorial Do Transparently Parameterizing Synchronization Using C++ Use the Strategized Locking pattern, C++ templates, and operator overloading to define “atomic operators” template class LOCK = ACE_Thread_Mutex, class TYPE = u_long class ACE_Atomic_Op { public: ACE_Atomic_Op (TYPE c = 0) { count_ = c; } TYPE operator++ (void) { ACE_GUARD (LOCK, guard, lock_); return ++count_; } operator TYPE () { ACE_GUARD (LOCK, guard, lock_); return count_; } // Other arithmetic operations omitted... private: LOCK lock_; TYPE count_; }; Vanderbilt University
- 152. Advanced ACE Tutorial Douglas C. Schmidt Final Version of Concurrent Logging Server Using the Atomic_Op class, only one change is made // At file scope. typedef ACE_Atomic_Op COUNTER; // Note default parameters... COUNTER request_count; request_count is now serialized automatically for (; ; ++request_count) // ACE_Atomic_Op::operator++ handle_log_record (get_handle (), ACE_STDOUT); The original non-threaded version may be supported efficiently as follows: typedef ACE_Atomic_OpNull_Mutex COUNTER; //... for (; ; ++request_count) handle_log_recordNull_Mutex (get_handle (), ACE_STDOUT); Vanderbilt University 151
- 153. Advanced ACE Tutorial Douglas C. Schmidt Concurrent Web Client/Server Example The following example illustrates a concurrent OO architecture for a high-performance Web client/server Key functional and non-functional system requirements are: – Robust implementation of HTTP 1.0 protocol i.e., resilient to incorrect or malicious Web clients/servers – Extensible for use with other protocols e.g., DICOM, HTTP 1.1, CORBA Simple Flow Protocol (SFP) – Leverage multi-processor hardware and OS software e.g., Support various concurrency patterns Vanderbilt University 152
- 154. Advanced ACE Tutorial Do General Web Client/Server Interactions 1: GET ~schmidt WWW HTTP/1.0 WWW CLIENT SERVER 2: index.html PROTOCOL HANDLERS HTML PARSER GUI DISPATCHER REQUESTER GRAPHICS COMMUNICATION PROTOCOL ADAPTER (E.G., HTTP) OS KERNEL OS KERNEL OS I/O SUBSYSTEM OS I/O SUBSYSTEM NETWORK ADAPTERS NETWORK ADAPTERS NETWORK www.cs.wustl.edu/˜jxh/research/ Vanderbilt University
- 155. Advanced ACE Tutorial Do Pseudo-code for Concurrent Web Server Pseudo-code for master server void master_server (void) { initialize queue and acceptor at port 80 spawn pool of worker threads foreach (pending work request from clients) { receive and queue request on queue } exit process } Pseudo-code for thread pool workers void worker (void) { foreach (work request on queue) dequeue and process request exit thread } As usual, make sure to avoid the “grand mistake” Vanderbilt University
- 156. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Motivating a Request Queue Q: Why use a request queue to store messages, rather than directly reading from I/O handles? A: – Promotes more efficient use of multiple CPUs via load balancing – Enables transparent interpositioning and prioritization – Makes it easier to shut down the server correctly and portably – Improves robustness to “denial of service” attacks – Moves queueing into the application process rather than OS Drawbacks – Using a message queue may lead to greater context switching and synchronization overhead... – Single point for bottlenecks Vanderbilt University 155
- 157. Advanced ACE Tutorial Do Thread Entry Point typedef ACE_Unbounded_QueueMessage MESSAGE_QUEUE; typedef u_long COUNTER; // Track the number of requests COUNTER request_count; // At file scope. // Entry point into the Web HTTP 1.0 protocol, // which runs in each thread in the thread pool. void *worker (MESSAGE_QUEUE *msg_queue) { Message mb; // Message containing HTTP request. while (msg_queue-dequeue_head (mb)) 0) { // Keep track of number of requests. ++request_count; // Print diagnostic cout got new request ACE_OS::thr_self () endl; // Identify and perform Web Server // request processing here... } return 0; } Vanderbilt University
- 158. Advanced ACE Tutorial Do Master Server Driver Function // Thread function prototype. typedef void *(*THR_FUNC)(void *); int main (int argc, char *argv[]) { parse_args (argc, argv); // Queue client requests. MESSAGE_QUEUE msg_queue; // Spawn off NUM_THREADS to run in parallel. for (int i = 0; i NUM_THREADS; i++) thr_create (0, 0, THR_FUNC (worker), (void *) msg_queue, THR_BOUND, 0); // Initialize network device and // recv HTTP work requests. thr_create (0, 0, THR_FUNC (recv_requests), (void *) msg_queue, THR_BOUND, 0); // Wait for all threads to exit (BEWARE)! while (thr_join (0, t_id, (void **) 0) == 0) continue; // ... } Vanderbilt University
- 159. Advanced ACE Tutorial Do Pseudo-code for recv_requests() void recv_requests (MESSAGE_QUEUE *msg_queue) { initialize socket acceptor at port 80 foreach (incoming request}) { use select to wait for new connections or data if (connection) establish connections using accept() else if (data) { use sockets calls to read() HTTP requests into msg msg_queue.enqueue_tail (msg); } } } This is the “supplier” thread Vanderbilt University
- 160. Advanced ACE Tutorial Douglas C. Schmidt Limitations with the Web Server The algorithmic decomposition tightly couples application-specific functionality with various configuration-related characteristics, e.g., – The HTTP 1.0 protocol – The number of services per process – The time when services are configured into a process The solution is not portable since it hard-codes – SunOS 5.x threading – sockets and select() There are race conditions in the code Vanderbilt University 159
- 161. Advanced ACE Tutorial Douglas C. Schmidt Overcoming Limitations via OO The algorithmic decomposition illustrated above specifies too many low-level details – Moreover, the excessive coupling complicates reusability, extensibility, and portability... In contrast, OO focuses on decoupling application-specific behavior from reusable application-independent mechanisms The OO approach described below uses reusable framework components and commonly recurring patterns Vanderbilt University 160
- 162. Advanced ACE Tutorial Douglas C. Schmidt Eliminating Race Conditions Problem – A naive implementation of MESSAGE_QUEUE will lead to race conditions e.g., when messages in different threads are enqueued and dequeued concurrently Forces – Producer/consumer concurrency is common, but requires careful attention to avoid overhead, deadlock, and proper control Solution – Utilize the Monitor Object pattern and condition variables Vanderbilt University 161
- 163. Advanced ACE Tutorial Douglas C. Schmidt The Monitor Object Pattern Intent Monitor Object + synchronized_method_1() Synchronizes method execution to ensure ... + synchronized_method_m() only one method runs within an object at a # monitor_lock_ time. It also allows an object’s methods to # monotor_condition_1_ ... cooperatively schedule their execution # monitor_condition_n_ sequences. ˜schmidt/POSA/ Forces Resolved Synchronization corresponds to methods Objects, not clients, are responsible for synchronization Cooperative method scheduling Vanderbilt University 162
- 164. Advanced ACE Tutorial Douglas C. Schmidt Overview of Condition Variables Condition variables (CVs) are used to “sleep/wait” until a particular condition involving shared data is signaled – CVs can wait on arbitrarily complex C++ expressions – Sleeping is often more efficient than busy waiting... This allows more complex scheduling decisions, compared with a mutex – i.e., a mutex makes other threads wait, whereas a condition variable allows a thread to make itself wait for a particular condition involving shared data Vanderbilt University 163
- 165. Advanced ACE Tutorial Douglas C. Schmidt Condition Variable Usage Patterns // Initially unlocked. Note how the use of the Scoped static ACE_Thread_Mutex lock; static ACE_Condition_Thread_Mutex Locking idiom simplifies the cond (lock); solution since we can’t forget to // synchronized release the lock! void acquire_resources (void) { // synchronized // Automatically acquire lock. void release_resources (void) { ACE_GUARD (ACE_Thread_Mutex, g, lock); // Automatically acquire lock. ACE_GUARD (ACE_Thread_Mutex, g, lock); // Check condition in loop while (condition expression false) // Atomically modify shared // Sleep. // information... cond.wait (); cond.signal (); // Atomically modify shared // Could use cond.broadcast() here. // information. // guard automatically // Destructor releases lock. // releases lock. } } Vanderbilt University 164
- 166. Advanced ACE Tutorial Douglas C. Schmidt ACE Condition Variable Interface class ACE_Condition_Thread_Mutex The ACE_Condition_ public: Thread_Mutex class is // Initialize the CV. ACE_Condition_Thread_Mutex a wrapper for the native (const ACE_Thread_Mutex ); OS condition variable // Implicitly destroy the CV. abstraction ˜ACE_Condition_Thread_Mutex (void); // Block on condition, or until // time passes. If time == 0 block. e.g., cond_t on int wait (ACE_Time_Value *time = 0); SunOS 5.x, // Signal one waiting thread. pthread_cond_t int signal (void); // Signal *all* waiting threads. for POSIX, and a int broadcast (void) const; custom private: implementation on cond_t cond_; // Solaris CV. const ACE_Thread_Mutex mutex_; Windows and }; VxWorks Vanderbilt University 165
- 167. Advanced ACE Tutorial Douglas C. Schmidt Overview of ACE_Message_Queue and ACE_Message_Block SYNCH STRATEGY An ACE_Message_Queue is a list of Message Message Queue Block ACE_Message_Blocks next() head_ prev() tail_ cont() – Efficiently handles arbitrarily-large Data_Block message payloads Message Block next() An ACE_Message_Block is a prev() Message cont() Composite Block Data_Block next() prev() – Similar to BSD mbufs or SVR4 cont() Message Data_Block Block STREAMS m_blks next() prev() cont() Design parameterizes Data_Block synchronization and allocation aspects Vanderbilt University 166
- 168. Advanced ACE Tutorial Do The ACE_Message_Block Class ACE_Message_Block # rd_ptr_ : size_t # wr_ptr_ : size_t # cont_ : ACE_Message_Block * # next_ : ACE_Message_Block * # prev_ : ACE_Message_Block * # data_block_ : ACE_Data_Block * + init (size : size_t) : int + msg_type (type : ACE_Message_Type) ACE_Data_Block + msg_type () : ACE_Message_Type + msg_priority (prio : u_long) # base_ : char * + msg_priority () : u_long # refcnt_ : int * 1 + clone () : ACE_Message_Block * + duplicate () : ACE_Message_Block * + release () : ACE_Message_Block * + set_flags (flags : u_long) : u_long + clr_flags (flags : u_long) : u_long + copy (buf : const char *,n : size_t) : int + rd_ptr (n : size_t) + rd_ptr () : char * + wr_ptr (n : size_t) + wr_ptr () : char * + length () : size_t + total_length () : size_t + size () : size_t Class characteristics Hide messaging implementations from clients ACE_Message ACE_Message ACE_Message _Block _Block _Block cont() cont() cont() data_block() data_block() data_block() wr_ptr() wr_ptr() rd_ptr() rd_ptr() rd_ptr() wr_ptr() ACE_Data _Block ACE_Data_Block PAYLOAD reference_count() = 2 (1) SIMPLE MESSAGE STRUCTURE (2) COMPOSITE MESSAGE STRUCTURE Vanderbilt University
- 169. Advanced ACE Tutorial Do The ACE_Message_Queue Class SYNCH_STRATEGY ACE_Message_Queue # head_ : ACE_Message_Block * # tail_ : ACE_Message_Block * # high_water_mark_ : size_t # low_water_mark_ : size_t + ACE_Message_Queue (high_water_mark : size_t = DEFAULT_HWM, low_water_mark : size_t = DEFAULT_LWM, notify : ACE_Notification_Strategy * = 0) + open (high_water_mark : size_t = DEFAULT_HWM, low_water_mark : size_t = DEFAULT_LWM, notify : ACE_Notification_Strategy * = 0) : int + flush () : int + notification_strategy (s : ACE_Notification_Strategy *) : void + is_empty () : int + is_full () : int + enqueue_tail (item : ACE_Message_Block *, timeout : ACE_Time_Value * = 0) : int + enqueue_head (item : ACE_Message_Block *, timeout : ACE_Time_Value * = 0) : int + enqueue_prio (item : ACE_Message_Block *, timeout : ACE_Time_Value * = 0) : int + dequeue_head (item : ACE_Message_Block *, timeout : ACE_Time_Value * = 0) : int + dequeue_tail (item : ACE_Message_Block *, timeout : ACE_Time_Value * = 0) : int + high_water_mark (new_hwm : size_t) : void + high_water_mark (void) : size_t + low_water_mark (new_lwm : size_t) : void + low_water_mark (void) : size_t + close () : int + deactivate () : int + activate () : int + pulse () : int + state () : int Class characteristics Note how the synchronization aspect can be strategized! Vanderbilt University
- 170. Advanced ACE Tutorial Do The ACE_Message_Queue Public Interface template class SYNCH_STRAT = ACE_MT_SYNCH // Synchronization aspect class ACE_Message_Queue { public: // Default high and low water marks. enum { DEFAULT_LWM = 0, DEFAULT_HWM = 4096 }; // Initialize a Message_Queue. Message_Queue (size_t hwm = DEFAULT_HWM, size_t lwm = DEFAULT_LWM); // Check if full or empty (hold locks) int is_empty (void) const; int is_full (void) const; // Enqueue and dequeue Message_Block *’s. int enqueue_prio (ACE_Message_Block *, ACE_Time_Value *); int enqueue_tail (ACE_Message_Block *, ACE_Time_Value *); int dequeue_head (ACE_Message_Block *, ACE_Time_Value *); int dequeue_tail (ACE_Message_Block *, ACE_Time_Value *); Vanderbilt University
- 171. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Parameterizing Synchronization Strategies Q: What is ACE_MT_SYNCH and Note the use of traits: how does it work? struct ACE_MT_SYNCH { typedef ACE_Thread_Mutex A: ACE_MT_SYNCH provides a MUTEX; thread-safe synchronization strategy typedef for a ACE_Svc_Handler ACE_Condition_Thread_Mutex COND; – e.g., it ensures that an }; ACE_Svc_Handler’s struct ACE_NULL_SYNCH { ACE_Message_Queue is typedef ACE_Null_Mutex thread-safe MUTEX; – Any ACE_Task that accesses typedef ACE_Null_Condition COND; shared state can use the }; ACE_MT_SYNCH traits Vanderbilt University 170
- 172. Advanced ACE Tutorial Do ACE_Message_Queue Class Private Interface private: // Check boundary conditions don’t hold locks. int is_empty_i (void) const; int is_full_i (void) const; // Routines that actually do the enqueueing // and dequeueing and don’t hold locks. int enqueue_prio_i (ACE_Message_Block *); int enqueue_tail_i (ACE_Message_Block *); int dequeue_head_i (ACE_Message_Block *); int dequeue_tail_i (ACE_Message_Block *); // ... // Parameterized types for synchronization // primitives that control concurrent access. // Note use of C++ traits typename SYNCH_STRAT::MUTEX lock_; typename SYNCH_STRAT::COND not_empty_cond_; typename SYNCH_STRAT::COND not_full_cond_; size_t high_water_mark_; size_t low_water_mark_; size_t cur_bytes_; size_t cur_count_; }; Vanderbilt University
- 173. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Tips for Intra-class Locking Q: How should locking be performed in an OO class? A: Apply the Thread-Safe Interface pattern: – “Interface functions should lock and do no work – implementation functions should do the work and not lock ” This pattern helps to avoid intra-class method deadlock – This is actually a variant on a common OO pattern that “public functions should check, private functions should trust” Naturally, there are exceptions to this rule... – This pattern avoids the following surprises Unnecessary overhead from recursive mutexes Deadlock if recursive mutexes aren’t used www.cs.wustl.edu/˜schmidt/POSA/ Vanderbilt University 172
- 174. Advanced ACE Tutorial Do ACE_Message_Queue Class Implementation template class SYNCH_STRAT ACE_Message_QueueSYNCH_STRAT::ACE_Message_Queue (size_t hwm, size_t lwm) : not_empty_cond_ (lock_), not_full_cond_ (lock_), ... {} template class SYNCH_STRAT int ACE_Message_QueueSYNCH_STRAT::is_empty_i (void) const { return cur_bytes_ == 0 cur_count_ == 0; } template class SYNCH_STRAT int ACE_Message_QueueSYNCH_STRAT::is_full_i (void) const { return cur_bytes_ high_water_mark_; } template class SYNCH_STRAT int ACE_Message_QueueSYNCH_STRAT::is_empty (void) const { ACE_GUARD_RETURN (SYNCH_STRAT::MUTEX, g, lock_, -1); return is_empty_i (); } template class SYNCH_STRAT int ACE_Message_QueueSYNCH_STRAT::is_full (void) const { ACE_GUARD_RETURN (SYNCH_STRAT::MUTEX, g, lock_, -1); return is_full_i (); } Vanderbilt University
- 175. Advanced ACE Tutorial Douglas C. Schmidt ACE_Message_Queue Operations template class SYNCH_STRAT int template class SYNCH_STRAT int ACE_Message_QueueSYNCH_STRAT:: ACE_Message_QueueSYNCH_STRAT:: enqueue_tail (ACE_Message_Block *item, dequeue_head (ACE_Message_Block *item, ACE_Time_Value *tv) { ACE_Time_Value *tv) { ACE_GUARD_RETURN (SYNCH_STRAT::MUTEX, ACE_GUARD_RETURN (SYNCH_STRAT::MUTEX, guard, lock_, -1); guard, lock_, -1); // Wait while the queue is full. // Wait while the queue is empty. while (is_full_i ()) { while (is_empty_i ()) { // Release the lock_ and wait // Release lock_ and wait for timeout, // for timeout, signal, or space // signal, or a new message being // to become available in the list. // placed in the list. if (not_full_cond_.wait (tv) == -1) if (not_empty_cond_.wait (tv) == -1) return -1; return -1; } } // Actually enqueue the message at // Actually dequeue the first message. // the end of the list. dequeue_head_i (item); enqueue_tail_i (item); // Tell blocked threads that list // Tell blocked threads that // is no longer full. // list has a new item! if (cur_bytes_ = low_water_mark_) not_empty_cond_.signal (); not_full_cond_.signal (); } } Vanderbilt University 174
- 176. Advanced ACE Tutorial Douglas C. Schmidt Overcoming Algorithmic Decomposition Limitations Previous slides illustrate tactical techniques and patterns that: – Reduce accidental complexity e.g., Automate synchronization acquisition and release (Scoped Locking idiom) Improve synchronization mechanisms (Adapter, Wrapper Facade, Monitor Object, Thread-Safe Interface, Strategized Locking patterns) – Eliminate race conditions Next, we describe strategic patterns, frameworks, and components to: – Increase reuse and extensibility e.g., Decoupling service, IPC, and demultiplexing – Improve the flexibility of concurrency control Vanderbilt University 175
- 177. Advanced ACE Tutorial Douglas C. Schmidt Selecting the Server’s Concurrency Architecture Problem – A very strategic design decision for high-performance Web servers is selecting an efficient concurrency architecture Forces – No single concurrency architecture is optimal – Key factors include OS/hardware platform and workload Solution – Understand key alternative concurrency patterns Vanderbilt University 176
- 178. Advanced ACE Tutorial Douglas C. Schmidt Concurrency Patterns in the Web Server The following example illustrates the patterns and framework components in an OO implementation of a concurrent Web Server There are various architectural patterns for structuring concurrency in a Web Server – Reactive – Thread-per-request – Thread-per-connection – Synchronous Thread Pool Leader/Followers Thread Pool Half-Sync/Half-Async Thread Pool – Asynchronous Thread Pool Vanderbilt University 177
- 179. Advanced ACE Tutorial Do Reactive Web Server 2: HANDLE INPUT HTTP 3: CREATE HANDLER Handler 4: ACCEPT CONNECTION 5: ACTIVATE HANDLER HTTP HTTP Acceptor Handler Reactor 6: PROCESS HTTP REQUEST SERVER 1: CONNECT CLIENT CLIENT CLIENT Vanderbilt University
- 180. Advanced ACE Tutorial Do Thread-per-Request Web Server HTTP 2: HANDLE INPUT Handler 3: CREATE HANDLER 4: ACCEPT CONNECTION HTTP 5: SPAWN THREAD Handler HTTP HTTP Acceptor Handler Reactor 6: PROCESS HTTP REQUEST SERVER 1: CONNECT CLIENT CLIENT CLIENT Vanderbilt University
- 181. Advanced ACE Tutorial Do Thread-per-Connection Web Server 3: SPAWN THREAD 2: CREATE, ACCEPT, PER CONNECTION AND ACTIVATE HTTP_HANDLER HTTP HTTP Handler Handler HTTP HTTP Handler Acceptor Reactor 4: PROCESS HTTP REQUEST 1: HTTP REQUEST SERVER CLIENT CLIENT CLIENT Vanderbilt University
- 182. Advanced ACE Tutorial Do Leader/Followers Synchronous Thread Pool Web Server HTTP Event Dispatcher Handler 2: ACCEPT CONNECTION HTTP 3: MORPH INTO HANDLER Handler HTTP HTTP Acceptor HTTP Handler Acceptor 4: PROCESS HTTP REQUEST 1: HTTP REQUEST SERVER CLIENT CLIENT CLIENT Vanderbilt University
- 183. Advanced ACE Tutorial Do Half-Sync/Half-Async Synchronous Thread Pool Web Server 2: HANDLE INPUT Message 3: ENQUEUE REQUEST Queue Active HTTP Object Handler HTTP 4: DEQUEUE HTTP Handler PROCESS Active REQUEST Handler Object HTTP Acceptor Active Active Object Object Reactor 5: PROCESS HTTP REQUEST 1: HTTP REQUEST SERVER CLIENT CLIENT CLIENT Vanderbilt University
- 184. Advanced ACE Tutorial Do Asynchronous Thread Pool Web Server 1: INITIATE ASYNC ACCEPT I/O 2: RUN EVENT LOOP HTTP Completion 4: ACCEPT COMPLETES Handler Port 5: QUEUE COMPLETION 6: DEQUEUE COMPLETION Async PROCESS Async HTTP REQUEST Async ReadAccept Handler Read Async HTTP HTTP Accept Handler Handler Proactor 7: PROCESS HTTP REQUEST 3: HTTP REQUEST SERVER CLIENT CLIENT CLIENT Vanderbilt University
- 185. Advanced ACE Tutorial Do Web Server Software Architecture HTTP HTTP Handler Handler Sock Sock HTTP Stream Stream Handler HTTP Acceptor Sock Stream Event Sock Dispatcher Acceptor Event Dispatcher – Encapsulates Web server concurrency and dispatching strategies HTTP Handlers – Parses HTTP headers and processes requests HTTP Acceptor – Accepts connections and creates HTTP Handlers Vanderbilt University
- 186. Advanced ACE Tutorial Do Patterns in the Web Server Implementation Thread-per Thread Pool Acceptor Request Active Connector Thread-per Object Connection Component Half-Sync/ Configurator Half-Async Asynchronous Completion Reactor/ Double Token Proactor Checked Locking STRATEGIC PATTERNS TACTICAL PATTERNS Wrapper Abstract Facade Strategy Adapter Singleton Factory Vanderbilt University
- 187. Advanced ACE Tutorial Douglas C. Schmidt Patterns in the Web Client/Server (cont’d) The Web Client/Server uses same patterns as distributed logger – i.e., Reactor, Component Configurator, Active Object, and Acceptor It also contains patterns with the following intents: – Connector ! “Decouple the active connection and initialization of a peer service in a distributed system from the processing performed once the peer service is connected and initialized” – Double-Checked Locking Optimization ! “Allows atomic initialization, regardless of initialization order, and eliminates subsequent locking overhead” – Half-Sync/Half-Async ! “Decouples synchronous I/O from asynchronous I/O in a system to simplify concurrent programming effort without degrading execution efficiency” Vanderbilt University 186
- 188. Advanced ACE Tutorial Do Architecture of Our Web Server REQUEST PROCESSING LAYER svc_run svc_run svc_run svc_run QUEUEING HTTP Msg Queue Options LAYER Processor s s HTTP HTTP HTTP Handler Handler Handler I/O DEMUXING HTTP Reactor Acceptor LAYER www.cs.wustl.edu/˜schmidt/PDF/HPL.pdf Vanderbilt University
- 189. Advanced ACE Tutorial Do An Integrated Reactive/Active Web Server svc_run REGISTERED svc_run OBJECTS svc_run 4: getq(msg) 5:svc(msg) HTTP APPLICATION Handler HTTP LEVEL Handler HTTP Event Handler HTTP Handler Event Processor HTTP Handler Event Acceptor Handler 2: recv_request(msg) 3: putq(msg) Event Handler FRAMEWORK 1: handle_input() LEVEL : Handle Table : Reactor OS EVENT DEMULTIPLEXING INTERFACE KERNEL LEVEL We’re focusing on the Reactive layer here Vanderbilt University
- 190. Advanced ACE Tutorial Douglas C. Schmidt HTTP_Handler Public Interface template class ACCEPTOR The HTTP_Handler is class HTTP_Handler : public the Proxy for ACE_Svc_HandlerACCEPTOR::PEER_STREAM, communicating with ACE_NULL_SYNCH { public: clients (e.g., Web // Entry point into HTTP_Handler, browsers like Netscape // called by HTTP_Acceptor. or i.e.,) virtual int open (void *) { It implements the // Register with ACE_Reactor asynchronous // to handle input. reactor ()-register_handler portion of Half- (this, ACE_Event_Handler::READ_MASK); Sync/Half-Async // Register timeout in case client pattern // doesn’t send any HTTP requests. reactor ()-schedule_timer (this, 0, ACE_Time_Value (CLIENT_TIMEOUT)); } Vanderbilt University 189
- 191. Advanced ACE Tutorial Douglas C. Schmidt HTTP_Handler Protected Interface protected: These methods are // Reactor dispatches this invoked by callbacks // method when clients timeout. from ACE_Reactor virtual int handle_timeout (const ACE_Time_Value , const void *) HTTP REGISTERED { Handler OBJECTS // Remove from the Reactor. reactor ()-remove_handler Event 2: remove_handler(this) Handler (this, 1: handle_timeout() ACE_Event_Handler::READ_MASK); } : Timer Queue : Reactor // Reactor dispatches this method Reactor // when HTTP requests arrive. virtual int handle_input (ACE_HANDLE); // Receive/frame client HTTP // requests (e.g., GET). int recv_request (ACE_Message_Block *); }; Vanderbilt University 190
- 192. Advanced ACE Tutorial Douglas C. Schmidt Integrating Multi-threading Problem – Multi-threaded Web servers are needed since Reactive Web servers are often inefficient and non-robust Forces – Multi-threading can be very hard to program – No single multi-threading model is always optimal Solution – Use the Active Object pattern to allow multiple concurrent server operations in an OO-manner Vanderbilt University 191
- 193. Advanced ACE Tutorial Do Using the Active Object Pattern and ACE Task Framework in the Web Server svc_run REGISTERED svc_run OBJECTS svc_run 4: getq(msg) 5:svc(msg) HTTP APPLICATION Handler HTTP LEVEL Handler HTTP Event Handler HTTP Handler Event Processor HTTP HandlerEvent Acceptor Handler 2: recv_request(msg) 3: putq(msg) Event Handler FRAMEWORK 1: handle_input() LEVEL : Handle Table : Reactor OS EVENT DEMULTIPLEXING INTERFACE KERNEL LEVEL We’re focusing on the Active Object layer here Vanderbilt University
- 194. Advanced ACE Tutorial Douglas C. Schmidt The HTTP_Processor Class class HTTP_Processor Processes HTTP : public ACE_TaskACE_MT_SYNCH { requests using the private: HTTP_Processor (void); public: “Thread-Pool” // Singleton access point. concurrency model static HTTP_Processor *instance (void); // Pass a request to the thread pool. virtual int put (ACE_Message_Block *, This method ACE_Time_Value *); implements the // Entry point into a pool thread. virtual int svc (void) synchronous task { portion of the Half- ACE_Message_Block *mb = 0; Sync/Half-Async // Wait for messages to arrive. pattern for (;;) { getq (mb); // Inherited from ACE_Task // Identify and perform HTTP // Server request processing... Vanderbilt University 193
- 195. Advanced ACE Tutorial Do Using the Singleton Pattern // Singleton access point. HTTP_Processor * HTTP_Processor::instance (void) { // Beware of race conditions! if (instance_ == 0) // Create the Singleton on-demand. instance_ = new HTTP_Processor; return instance_; } // Constructor creates the thread pool. HTTP_Processor::HTTP_Processor (void) { // Inherited from class Task. activate (THR_BOUND, Options::instance ()-threads ()); } Vanderbilt University
- 196. Advanced ACE Tutorial Douglas C. Schmidt Subtle Concurrency Woes with the Singleton Pattern Problem – The canonical Singleton implementation has subtle “bugs” in multi-threaded applications Forces – Too much locking makes Singleton too slow... – Too little locking makes Singleton unsafe... Solution – Use the Double-Checked Locking optimization pattern to minimize locking and ensure atomic initialization Vanderbilt University 195
- 197. Advanced ACE Tutorial Douglas C. Schmidt The Double-Checked Locking Optimization Pattern if (instance_ == NULL) { Intent mutex_.acquire (); if (instance_ == NULL) Allows atomic initialization, instance_ = new HTTP_Processor; regardless of initialization order, mutex_.release (); } and eliminates subsequent return instance_; locking overhead HTTP Forces Resolved: Processor Ensures atomic object static instance() initialization static instance_ Mutex Minimizes locking overhead www.cs.wustl.edu/ Caveat! ˜schmidt/POSA/ This pattern assumes atomic memory access Vanderbilt University 196
- 198. Advanced ACE Tutorial Douglas C. Schmidt The ACE Singleton Template template class TYPE, class LOCK Features class ACE_Singleton : public ACE_Cleanup { public: Turns any class into static TYPE *instance (void) { // Memory barrier could go here... a singleton if (s_ == 0) { ACE_GUARD_RETURN (LOCK, g, Automates ACE_Object_Manager ::get_singleton_lock (), -1); Double-Checked if (s_ == 0) Locking Optimization s_ = new ACE_SingletonTYPE; // Memory barrier could go here. Ensures automatic ACE_Object_Manager::at_exit (s_); } cleanup when return s_-instance_; process exits } virtual void cleanup (void *param = 0); protected: www.cs.wustl.edu/ ACE_Singleton (void); ˜schmidt/PDF/ TYPE instance_; static ACE_SingletonTYPE, LOCK *s_; ObjMan.pdf }; Vanderbilt University 197
- 199. Advanced ACE Tutorial Douglas C. Schmidt Integrating Reactive and Multi-threaded Layers Problem – Justifying the hybrid design of our Web server can be tricky Forces – Engineers are never satisfied with the status quo ;-) – Substantial amount of time is spent re-discovering the intent of complex concurrent software design Solution – Use the Half-Sync/Half-Async pattern to explain and justify our Web server concurrency architecture Vanderbilt University 198
- 200. Advanced ACE Tutorial Douglas C. Schmidt The Half-Sync/Half-Async Pattern SYNCHRONOUS SYNC Intent TASK LAYER SYNC TASK 1 TASK 3 Decouples synchronous I/O SYNC TASK 2 from asynchronous I/O in a system to simplify concurrent programming effort without QUEUEING LAYER 1, 4: read(data) degrading execution efficiency MESSAGE QUEUES Forces Resolved: 3: enqueue(data) ASYNCHRONOUS Simplify programming TASK LAYER ASYNC TASK 2: interrupt Ensure efficient I/O EXTERNAL EVENT SOURCES www.cs.wustl.edu/ ˜schmidt/POSA/ Vanderbilt University 199
- 201. Advanced ACE Tutorial Do Using the Half-Sync/Half-Async Pattern in the Web Server svc_run SYNC TASK svc_run LEVEL svc_run 4: getq(msg) 5:svc(msg) QUEUEING LEVEL HTTP Processor HTTP Handler HTTP HTTP Handler HTTP Acceptor Event Handler Handler Event Event Handler Event Handler Handler ASYNC TASK 1: handle_input() : Handle LEVEL 2: recv_request(msg) 3: putq(msg) Table : Reactor Vanderbilt University
- 202. Advanced ACE Tutorial Do Joining Async and Sync Tasks in the Web Server // The following methods form the boundary // between the Async and Sync layers. template class PA int HTTP_HandlerPA::handle_input (ACE_HANDLE h) { ACE_Message_Block *mb = 0; // Try to receive and frame message. if (recv_request (mb) == HTTP_REQUEST_COMPLETE) { reactor ()-remove_handler (this, ACE_Event_Handler::READ_MASK); reactor ()-cancel_timer (this); // Insert message into the Queue. HTTP_ProcessorPA::instance ()-put (mb); } } int HTTP_Processor::put (ACE_Message_Block *msg, ACE_Time_Value *timeout) { // Insert the message on the Message_Queue // (inherited from class Task). putq (msg, timeout); } Vanderbilt University
- 203. Advanced ACE Tutorial Douglas C. Schmidt Optimizing Our Web Server for Asynchronous Operating Systems Problem – Synchronous multi-threaded solutions are not always the most efficient Forces – Purely asynchronous I/O is quite powerful on some OS platforms e.g., Windows NT 4.x or UNIX with aio_()* calls – Good designs should be adaptable to new contexts Solution – Use the Proactor pattern to maximize performance on Asynchronous OS platforms Vanderbilt University 202
- 204. Advanced ACE Tutorial Douglas C. Schmidt The Proactor Pattern APPLICATION- Intent INDEPENDENT APPLICATION- DEPENDENT 1 overlapped_result = HTTP Async GetQueuedCompleteStatus(); Demultiplexes and overlapped_result-complete() Handler Write dispatches service Completion HTTP Handler Acceptor requests that are handle_accept() handle_read_file() triggered by the handle_write_file() Async Async completion of handle_timeout() Op Accept get_handle() A open() asynchronous n cancel() operations Proactor Timer_Queue Handles 1 schedule_timer(h) handle_events() cancel_timer(h) Resolves same forces as Reactor www.cs.wustl.edu/˜schmidt/POSA/ Vanderbilt University 203
- 205. Advanced ACE Tutorial Do Structure of the ACE Proactor Framework ACE_Timer_Queue ACE_Service_Handler ACE_Proactor ACE_Asynch_Acceptor ACE_Asynch_Connector ACE_Handler ACE_Asynch_Read_Stream ACE_Asynch_Write_Stream ACE_Asynch_Result Framework characteristics Similar to the ACE Reactor framework, except behavior is “inverse” Portable to Windows and various UNIX platforms that support aio_*() family of methods Vanderbilt University
- 206. Advanced ACE Tutorial Do Using the ACE Proactor Framework for the Web Server Connection Setup Phase 1: accept Web Server connections 7: create HTTP Acceptor Handler 4: connect 6: 2: accept 8: read (Handler, Web accept (Acceptor, Dispatcher) Browser complete Dispatcher) Completion Operating Dispatcher System 3: handle 5: accept events complete Data Transfer Phase 1: GET /etc/passwd Web Server Web HTTP 4: parse request Browser Handler 6: write (File, Conn., 8: write Handler, Dispatcher) 5: read (File) 3: read complete complete 7: write complete File Completion Operating System Dispatcher System 2: read complete Vanderbilt University
- 207. Advanced ACE Tutorial Douglas C. Schmidt Structuring Service Initialization Problem – The communication protocol used between clients and the Web server is often orthogonal to the initialization protocol Forces – Low-level connection establishment APIs are tedious, error-prone, and non-portable – Separating initialization from use can increase software reuse substantially Solution – Use the Acceptor and Connector patterns to decouple passive service initialization from run-time protocol Vanderbilt University 206
- 208. Advanced ACE Tutorial Do Using the ACE_Acceptor in the Web Server : HTTP : HTTP Handler Handler : HTTP : HTTP Handler Acceptor : Svc : Svc Handler Handler : Svc : Acceptor Handler : HTTP PASSIVE ACTIVE Handler LISTENER CONNECTIONS 1: handle_input() : Svc 2: sh = make_svc_handler() Handler 3: accept_svc_handler(sh) : Reactor 4: activate_svc_handler(sh) The HTTP_Acceptor is a factory that creates, connects, and activates an HTTP_Handler Vanderbilt University
- 209. Advanced ACE Tutorial Douglas C. Schmidt HTTP_Acceptor Class Interface template class ACCEPTOR The HTTP_Acceptor class HTTP_Acceptor : class implements the public ACE_AcceptorHTTP_Handler Acceptor role ACCEPTOR::PEER_STREAM, // Note use of a trait. i.e., it accepts ACCEPTOR { connections/initializes public: HTTP_Handlers // Called when HTTP_Acceptor is // dynamically linked. virtual int init (int argc, char *argv[]); // Called when HTTP_Acceptor is // dynamically unlinked. virtual int fini (void); // ... }; Vanderbilt University 208
- 210. Advanced ACE Tutorial Do HTTP_Acceptor Class Implementation // Initialize service when dynamically linked. template class PA int HTTP_AcceptorPA::init (int argc, char *argv[]) { Options::instance ()-parse_args (argc, argv); // Initialize the communication endpoint and // register to accept connections. peer_acceptor ().open (typename PA::PEER_ADDR (Options::instance ()-port ()), Reactor::instance ()); } // Terminate service when dynamically unlinked. template class PA int HTTP_AcceptorPA::fini (void) { // Shutdown threads in the pool. HTTP_ProcessorPA::instance ()- msg_queue ()-deactivate (); // Wait for all threads to exit. HTTP_ProcessorPA::instance ()- thr_mgr ()-wait (); } Vanderbilt University
- 211. Advanced ACE Tutorial Douglas C. Schmidt Using the ACE Service Configurator Framework in the Web Server SERVICE Reactive Web CONFIGURATOR Server RUNTIME TP Web Server Service Object Service Service DLLS Repository Object TPR Web Server Service Reactor Service Config Object svc.conf dynamic Web_Server Service_Object * FILE web_server:make_Web_Server() -port 80 Vanderbilt University 210
- 212. Advanced ACE Tutorial Douglas C. Schmidt Component Configurator Implementation in C++ The concurrent Web Server is Factory function that dynamically configured and initialized via a allocates a Half-Sync/Half-Async configuration script Web Server object % cat ./svc.conf extern C ACE_Service_Object * dynamic Web_Server make_Web_Server (void); Service_Object * web_server:_make_Web_Server() ACE_Service_Object * -p 80 -t $THREADS make_Web_Server (void) # .dll or .so suffix added to { # web_server automatically return new HTTP_AcceptorACE_SOCK_Acceptor; // ACE dynamically unlinks and // deallocates this object. } Vanderbilt University 211
- 213. Advanced ACE Tutorial Douglas C. Schmidt Main Program for the Web Server int main (int argc, char *argv[]) The main() { function is totally // Initialize the daemon and generic! // dynamically configure services. ACE_Service_Config::open (argc, Dynamically argv); configure execute // Loop forever, running services Web Server // and handling reconfigurations. ACE_Reactor::instance ()- Make any run_reactor_event_loop (); application /* NOTREACHED */ “Web-enabled” } Vanderbilt University 212
- 214. Advanced ACE Tutorial Douglas C. Schmidt Optimizing the JAWS Framework Reactor/Proactor State Singleton Use lightweight Strategy I/O Strategy Cached Virtual concurrency Framework Filesystem Minimize locking Apply file caching Asynchronous Completion Token Tilde Expander ~ and memory /home/... mapping Protocol Acceptor Strategy Handler Event Dispatcher Use “gather-write” Protocol mechanisms Filter Minimize logging Adapter Pre-compute HTTP Active Object responses Concurrency Avoid excessive Protocol Pipeline Strategy Framework Framework time() calls Streams Service Configurator Optimize the www.cs.wustl.edu/˜jxh/research/ transport interface Vanderbilt University 213
- 215. Advanced ACE Tutorial Douglas C. Schmidt Application-level Telecom Gateway Example SATELLITES TRACKING This example explores the STATION PEERS patterns and reusable framework components for an application-level STATUS INFO Gateway WIDE AREA NETWORK The Gateway routes COMMANDS BULK DATA messages between Peers TRANSFER Gateway and Peers are GATEWAY connected via TCP/IP LOCAL AREA NETWORK GROUND STATION PEERS Vanderbilt University 214
- 216. Advanced ACE Tutorial Do OO Software Architecture of the Gateway Routing Consumer Table Supplier Handler Handler Reactor Consumer Supplier Handler Handler Connector Acceptor INCOMING OUTGOING MESSAGES GATEWAY MESSAGES CONNECTION CONNECTION REQUEST REQUEST www.cs.wustl.edu/˜schmidt/PDF/ TAPOS-00.pdf All components in this architecture are based on patterns from ACE Vanderbilt University
- 217. Advanced ACE Tutorial Douglas C. Schmidt Gateway Behavior Components in the Gateway behave as follows: 1. Gateway parses configuration files that specify which Peers to connect with and which routes to use 2. Proxy_Handler_Connector connects to Peers, then creates and activates Proxy_Handler subclasses (Supplier_Handler or Consumer_Handler) 3. Once connected, Peers send messages to the Gateway – Messages are handled by an Supplier_Handler – Supplier_Handlers work as follows: Receive and validate messages Consult a Routing_Table Forward messages to the appropriate Peer(s) via Consumer_Handlers Vanderbilt University 216
- 218. Advanced ACE Tutorial Do Patterns in the Gateway Active Object Non-blocking Acceptor- Buffered I/O Connector Half-Sync/ Component Half-Async Configurator STRATEGIC PATTERNS Reactor TACTICAL PATTERNS Iterator Template Factory Proxy Wrapper Method Method Facade The Gateway components are based upon a common pattern language Vanderbilt University
- 219. Advanced ACE Tutorial Do Class Diagram for Single-Threaded Gateway Proxy_Handler COMPONENTS APPLICATION- SOCK_Connector SOCK_Stream SPECIFIC Null_Synch 1 Proxy Handler Supplier/Consumer Connector n Handler activates COMPONENTS CONNECTION- SVC_HANDLER PEER_STREAM ORIENTED PEER_CONNECTOR SYNCH Connector Svc Handler PEER PEER CONNECTOR STREAM COMPONENTS FRAMEWORK Stream Connection ACE IPC_SAP Service Configurator Concurrency Reactor global Vanderbilt University
- 220. Advanced ACE Tutorial Douglas C. Schmidt OO Gateway Architecture Application-specific components – Proxy_Handlers route messages among Peers Connection-oriented application components – ACE_Svc_Handler Performs I/O-related tasks with connected clients – ACE_Connector factory Establishes new connections with clients Dynamically creates an ACE_Svc_Handler object for each client and “activates” it Application-independent ACE framework components – Perform IPC, explicit dynamic linking, event demultiplexing, event handler dispatching, multi-threading, etc. Vanderbilt University 219
- 221. Advanced ACE Tutorial Do Using the ACE Reactor Framework for the Gateway CONCRETE EVENT APPLICATION Consumer HANDLERS LEVEL Consumer Handler Handler 4: send(msg) Supplier Handler Event 2: recv(msg) Event Handler 3: route(msg) Handler Event Handler 1: handle_input() FRAMEWORK LEVEL Timer Handle Queue Table : Reactor OS EVENT DEMULTIPLEXING INTERFACE KERNEL LEVEL Benefits Liabilities Straightforward to Design is “brittle” program Can’t leverage Concurrency control multi-processors is trivial Vanderbilt University
- 222. Advanced ACE Tutorial Douglas C. Schmidt Addressing Active Endpoint Connection and Initialization Challenges Problem – Application communication protocols are often orthogonal to their connection establishment and service initialization protocols Forces – Low-level connection APIs are error-prone and non-portable – Separating initialization from processing increases software reuse – Asynchronous connections are important over long-delay paths Solution – Use the Acceptor-Connector pattern to decouple connection and initialization protocols from the Gateway routing protocol Vanderbilt University 221
- 223. Advanced ACE Tutorial Do The Acceptor-Connector Pattern (Connector Role) Svc Svc Handler Handler Connector peer_stream_ ACTIVATES connect(sh, addr) open() complete() APPLICATION- HANDLE ASYNC DEFINED CONNECTION COMPLETION APPLICATION- Reactor INDEPENDENT www.cs.wustl.edu/˜schmidt/POSA/ Intent of Connector Role Forces Resolved: Decouple the active Reuse connection connection and code initialization of a peer Efficiently setup service in a distributed connections with system from the many peers or over processing performed long delay paths once the peer service is connected and initialized Vanderbilt University
- 224. Advanced ACE Tutorial Douglas C. Schmidt Structure of the Acceptor-Connector Pattern in ACE Additional features of the ACE_Event_Handler SYNCH_STRATEGY SVC_HANDLER, PEER_CONNECTOR ACE_Task ACE_Connector ACE_Connector PEER_STREAM, Uses C++ parameterized SYNCH_STRATEGY SVC_HANDLER, PEER_ACCEPTOR ACE_Svc_Handler types to strategize IPC and ACE_Acceptor «bind» service aspects Application Service Uses Template Method pattern to strategize creation, connection establishment, and concurrency policies Vanderbilt University 223
- 225. Advanced ACE Tutorial Do Using the ACE_Connector in the Gateway Svc Svc Svc Svc Handler Handler Handler Handler Svc Handler ACTIVE Hash_Map CONNECTIONS Svc Connector Handler PENDING CONNECTIONS Reactor The ACE_Connector is a factory – i.e., it connects and activates an ACE_Svc_Handler There’s typically 1 ACE_Connector per-service Vanderbilt University
- 226. Advanced ACE Tutorial Do ACE_Connector Class Public Interface A reusable template factory class that establishes connections with clients template class SVC_HANDLER, // Type of service class PEER_CONNECTOR // Connection factory class ACE_Connector : public ACE_Service_Object { public: // Initiate connection to Peer. virtual int connect (SVC_HANDLER *svc_handler, typename const PEER_CONNECTOR::PEER_ADDR ra, ACE_Synch_Options synch_options); // Cancel a svc_handler that was // started asynchronously. virtual int cancel (SVC_HANDLER *svc_handler); Vanderbilt University
- 227. Advanced ACE Tutorial Do Design Interlude: Motivation for the ACE_Synch_Options Class Q: What is the ACE_Synch_Options class? A: This allows callers to define the synchrony/asynchrony policies, e.g., class ACE_Synch_Options { // Options flags for controlling // synchronization. enum { USE_REACTOR = 1, USE_TIMEOUT = 2 }; ACE_Synch_Options (u_long options = 0, const ACE_Time_Value timeout = ACE_Time_Value::zero, const void *act = 0); // This is the default synchronous setting. static ACE_Synch_Options synch; // This is the default asynchronous setting. static ACE_Synch_Options asynch; }; Vanderbilt University
- 228. Advanced ACE Tutorial Do ACE_Synch_Options and ACE_Connector Semantics Reactor Timeout Behavior Yes 0,0 Return , 1 with errno EWOULDBLOCK; service handler is closed via reactor event loop. Yes time Return , 1 with errno EWOULDBLOCK; wait up to specified amount of time for completion using the reactor. Yes NULL Return , 1 with errno EWOULDBLOCK; wait for completion indefinitely using the reactor. No 0,0 Close service handler directly; return , 1 with errno EWOULDBLOCK. No time Block in connect_svc_handler() up to specified amount of time for completion; if still not completed, , return 1 with errno ETIME. No NULL Block in connect_svc_handler() indefinitely for completion. Vanderbilt University
- 229. Advanced ACE Tutorial Do ACE_Connector Class Protected Interface protected: // Make a new connection. virtual SVC_HANDLER *make_svc_handler (void); // Accept a new connection. virtual int connect_svc_handler (SVC_HANDLER *sh, typename const PEER_CONNECTOR::PEER_ADDR addr, ACE_Time_Value *timeout); // Activate a service handler. virtual int activate_svc_handler (SVC_HANDLER *); // Demultiplexing hooks. virtual int handle_output (ACE_HANDLE);// Success. virtual int handle_input (ACE_HANDLE); // Failure. virtual int handle_timeout (ACE_Time_Value , const void *); // Table maps I/O handle to an ACE_Svc_Tuple *. Hash_Map_ManagerACE_HANDLE, ACE_Svc_Tuple *, ACE_Null_Mutex handler_map_; // Factory that establishes connections actively. PEER_CONNECTOR connector_; }; Vanderbilt University
- 230. Advanced ACE Tutorial Do ACE_Connector Class Implementation // Initiate connection using specified // blocking semantics. template class SH, class PC int ACE_ConnectorSH, PC::connect (SH *sh, const PC::PEER_ADDR r_addr, ACE_Synch_Options options) { ACE_Time_Value *timeout = 0; int use_reactor = options[ACE_Synch_Options::USE_REACTOR]; if (use_reactor) timeout = ACE_Time_Value::zero; else timeout = options[ACE_Synch_Options::USE_TIMEOUT] ? (Time_Value *) options.timeout () : 0; // Hook methods. if (sh == 0) sh = make_svc_handler (); if (connect_svc_handler (sh, raddr, timeout) != -1) activate_svc_handler (sh); } Vanderbilt University
- 231. Advanced ACE Tutorial Do ACE_Connector Hook Method Implementations template class SH, class PC SH * ACE_ConnectorSH, PC::make_svc_handler (void) { return new SH; } template class SH, class PC int ACE_ConnectorSH, PC::connect_svc_handler (SH *sh, typename const PEER_CONNECTOR::PEER_ADDR addr, ACE_Time_Value *timeout) { // Peer_Connector factory initiates connection. if (connector_.connect (sh, addr, timeout) == -1) // If the connection hasn’t completed, then // register with the Reactor to call us back. if (use_reactor errno == EWOULDBLOCK) // Create ACE_Svc_Tuple for sh return -1 } else // Activate immediately if we’re connected. activate_svc_handler (sh); } template class SH, class PC int ACE_ConnectorSH, PC::activate_svc_handler (SH *sh) { if (sh-open ((void *)this) == -1) sh-close (); } Vanderbilt University
- 232. Advanced ACE Tutorial Do Specializing ACE_Connector and ACE_Svc_Handler APPLICATION- INDEPENDENT Svc Handler APPLICATION- SPECIFIC Proxy Handler Supplier Consumer Message Queue Handler Handler Producing an application that meets Gateway requirements involves specializing ACE components – ACE_Connector ! ACE_Proxy_Handler_Connector – ACE_Svc_Handler ! ACE_Proxy_Handler ! ACE_Supplier_Handler and ACE_Consumer_Handler Vanderbilt University
- 233. Advanced ACE Tutorial Do ACE_Proxy_Handler Class Public Interface // Determine the type of threading mechanism. #if defined (ACE_USE_MT) typedef ACE_MT_SYNCH SYNCH; #else typedef ACE_NULL_SYNCH SYNCH; #endif /* ACE_USE_MT */ // Unique connection id that denotes Proxy_Handler. typedef short CONN_ID; // This is the type of the Routing_Table. typedef ACE_Hash_Map_Manager Peer_Addr, Routing_Entry, SYNCH::MUTEX ROUTING_TABLE; class Proxy_Handler : public ACE_Svc_HandlerACE_SOCK_Stream, SYNCH { public: // Initialize the handler (called by the // ACE_Connector or ACE_Acceptor). virtual int open (void * = 0); // Bind addressing info to Router. virtual int bind (const ACE_INET_Addr , CONN_ID); Vanderbilt University
- 234. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Parameterizing Synchronization into the ACE_Hash_Map_Manager Q: What’s a good technique to implement a Routing Table? A: Use a ACE_Hash_Map_Manager container – ACE provides a ACE_Hash_Map_Manager container that associates external ids with internal ids, e.g., External ids (keys) ! URI Internal ids (values) ! pointer to memory-mapped file Hashing provides O1 performance in the average-case Vanderbilt University 233
- 235. Advanced ACE Tutorial Douglas C. Schmidt Applying the Strategized Locking pattern to the ACE_Hash_Map_Manager Class template class EXT_ID, class INT_ID, ACE_Hash_Map_Manager class LOCK class ACE_Hash_Map_Manager { public: uses the template-based bool bind (EXT_ID, INT_ID *); Strategized Locking pattern bool unbind (EXT_ID); bool find (EXT_ID ex, INT_ID in) to { // Exception-safe code... ACE_READ_GUARD (LOCK, g, Enhance reuse lock_, false); // lock_.read_acquire (); Parameterize different if (find_i (ex, in)) return true; else return false; synchronization // lock_.release (); strategies, e.g.: } private: – ACE_Null_Mutex, LOCK lock_; bool find_i (EXT_ID, INT_ID ); ACE_Thread_Mutex, // ... ACE_RW_Mutex, etc. }; Vanderbilt University 234
- 236. Advanced ACE Tutorial Do Detailed OO Architecture of the Gateway Consumer Routing Supplier Table Handler Handler Hash Map Manager SOCK Message SOCK Stream Queue Stream Reactor Supplier Handler Consumer Handler Connector SOCK Stream Acceptor SOCK Message SOCK Hash Map SOCK Stream Queue Connector Manager Acceptor INCOMING OUTGOING MESSAGES GATEWAY MESSAGES CONNECTION CONNECTION REQUEST REQUEST Note the use of other ACE components, such as the socket wrapper facades and the ACE_Hash_Map_Manager Vanderbilt University
- 237. Advanced ACE Tutorial Do ACE_Supplier_Handler Interface class Supplier_Handler : public Proxy_Handler { public: Supplier_Handler (void); protected: // Receive and process Peer messages. virtual int handle_input (ACE_HANDLE); // Receive a message from a Peer. virtual int recv_peer (ACE_Message_Block *); // Action that routes a message from a Peer. int route_message (ACE_Message_Block *); // Keep track of message fragment. ACE_Message_Block *msg_frag_; }; Vanderbilt University
- 238. Advanced ACE Tutorial Do ACE_Consumer_Handler Interface class Consumer_Handler : public Proxy_Handler { public: Consumer_Handler (void); // Send a message to a Gateway // (may be queued). virtual int put (ACE_Message_Block *, ACE_Time_Value * = 0); protected: // Perform a non-blocking put(). int nonblk_put (ACE_Message_Block *mb); // Finish sending a message when // flow control abates. virtual int handle_output (ACE_HANDLE); // Send a message to a Peer. virtual int send_peer (ACE_Message_Block *); }; Vanderbilt University
- 239. Advanced ACE Tutorial Douglas C. Schmidt ACE_Proxy_Handler_Connector Class Interface class Proxy_Handler_Connector : ACE_Proxy_Handler_ public ACE_Connector Connector is a concrete Proxy_Handler, factory class that: // Type of Svc Handler ACE_SOCK_Connector – Establishes connections with // Connection factory { Peers to produce public: ACE_Proxy_Handlers // Initiate (or reinitiate) – Activates // a connection on ACE_Proxy_Handlers, // the Proxy_Handler. int initiate_connection which then route messages (Proxy_Handler *); } ACE_Proxy_Handler_ Connector also ensures reliability by restarting failed connections Vanderbilt University 238
- 240. Advanced ACE Tutorial Do ACE_Proxy_Handler_Connector Implementation // (re)initiate a connection to a Proxy_Handler int Proxy_Handler_Connector::initiate_connection (Proxy_Handler *ph) { // Use asynchronous connections... if (connect (ph, ph-addr (), ACE_Synch_Options::asynch) == -1) { if (errno == EWOULDBLOCK) // No error, we’re connecting asynchronously. return -1; else // This is a real error, so reschedule // ourselves to reconnect. reactor ()-schedule_timer (ph, 0, ph-timeout ()); } else // We’re connected synchronously! return 0; } Vanderbilt University
- 241. Advanced ACE Tutorial Douglas C. Schmidt The Non-blocking Buffered I/O Pattern Intent Routing Table Decouple multiple input find() 1 Message Queue sources from multiple output ROUTING n sources to prevent blocking LAYER Supplier Consumer Handler Handler Forces Resolved: handle_input() handle_output() put() Keep misbehaving connections from disrupting REACTIVE LAYER Event the QoS for well-behaved Handler n 1 Reactor connections www.cs.wustl.edu/˜schmidt/PDF/ Different concurrency TAPOS-00.pdf strategies for Supplier_Handlers and Consumer_Handlers Vanderbilt University 240
- 242. Advanced ACE Tutorial Do Collaboration in Single-threaded Gateway Routing Routing Consumer Table Handler Message 5: send_peer(msg) Queue ROUTE ID Consumer Handlers Consumer Handler ) 3: find() sg m t( Message pu Queue 4: Supplier 6: put (msg) 7: send_peer(msg) Handler 8: enqueue(msg) 9: schedule_wakeup() --------------- 10: handle_output() 1: handle_input() 11: dequeue(msg) 2: recv_peer(msg) 12: send_peer(msg) Note the complex cooperative scheduling logic required to handle output flow control correctly Vanderbilt University
- 243. Advanced ACE Tutorial Do Supplier_Handler and Consumer_Handler Implementations int Supplier_Handler::handle_input (ACE_HANDLE) { ACE_Message_Block *route_addr = 0; int n = recv_peer (route_addr); // Try to get the next message. if (n = 0) { if (errno == EWOULDBLOCK) return 0; else return n; } else route_message (route_addr); } // Send a message to a Peer (queue if necessary). int Consumer_Handler::put (ACE_Message_Block *mb, ACE_Time_Value *) { if (msg_queue_-is_empty ()) // Try to send the message *without* blocking! nonblk_put (mb); else // Messages are queued due to flow control. msg_queue_-enqueue_tail (mb, ACE_Time_Value::zero); } Vanderbilt University
- 244. Advanced ACE Tutorial Do Supplier_Handler Message Routing // Route message from a Peer. int Supplier_Handler::route_messages (ACE_Message_Block *route_addr) { // Determine destination address. CONN_ID route_id = *(CONN_ID *) route_addr-rd_ptr (); const ACE_Message_Block *const data = route_addr-cont (); Routing_Entry *re = 0; // Determine route. Routing_Table::instance ()-find (route_id, re); // Initialize iterator over destination(s). Set_IteratorProxy_Handler * si (re-destinations ()); // Multicast message. for (Proxy_Handler *out_ph; si.next (out_ph) != -1; si.advance ()) { ACE_Message_Block *newmsg = data-duplicate (); if (out_ph-put (newmsg) == -1) // Drop message. newmsg-release (); // Decrement ref count. } delete route_addr; } Vanderbilt University
- 245. Advanced ACE Tutorial Do Peer_Message Schema // Peer address is used to identify the // source/destination of a Peer message. class Peer_Addr { public: CONN_ID conn_id_; // Unique connection id. u_char logical_id_; // Logical ID. u_char payload_; // Payload type. }; // Fixed sized header. class Peer_Header { public: /* ... */ }; // Variable-sized message (sdu_ may be // between 0 and MAX_MSG_SIZE). class Peer_Message { public: // The maximum size of a message. enum { MAX_PAYLOAD_SIZE = 1024 }; Peer_Header header_; // Fixed-sized header. char sdu_[MAX_PAYLOAD_SIZE]; // Message payload. }; Vanderbilt University
- 246. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Tips on Handling Flow Control Q: What should happen if put() fails? – e.g., if a queue becomes full? A: The answer depends on whether the error handling policy is different for each router object or the same... – Strategy pattern: give reasonable default, but allow substitution A related design issue deals with avoiding output blocking if a Peer connection becomes flow controlled Vanderbilt University 245
- 247. Advanced ACE Tutorial Do Supplier Handler Message Reception // Pseudo-code for recv’ing msg via non-blocking I/O int Supplier_Handler::recv_peer (ACE_Message_Block *route_addr) { if (msg_frag_ is empty) { msg_frag_ = new ACE_Message_Block; receive fixed-sized header into msg_frag_ if (errors occur) cleanup else determine size of variable-sized msg_frag_ } else determine how much of msg_frag_ to skip non-blocking recv of payload into msg_frag_ if (entire message is now received) { route_addr = new Message_Block (sizeof (Peer_Addr), msg_frag_) Peer_Addr addr (id (), msg_frag_-routing_id_, 0); route_addr-copy (addr, sizeof (Peer_Addr)); return to caller and reset msg_frag_ } else if (only part of message is received) return errno = EWOULDBLOCK else if (fatal error occurs) cleanup } Vanderbilt University
- 248. Advanced ACE Tutorial Douglas C. Schmidt Design Interlude: Using the ACE_Reactor to Handle Flow Control Q: How can a flow controlled Consumer_Handler know when to proceed again without polling or blocking? A: Use the ACE_Event_Handler::handle_output() notification scheme of the Reactor – i.e., via the ACE_Reactor’s methods schedule_wakeup() and cancel_wakeup() This provides cooperative multi-tasking within a single thread of control – The ACE_Reactor calls back to the handle_output() hook method when the Proxy_Handler is able to transmit again Vanderbilt University 247
- 249. Advanced ACE Tutorial Douglas C. Schmidt Performing a Non-blocking put() of a Message int Consumer_Handler::nonblk_put This method is called (ACE_Message_Block *mb) { in two situations: // Try sending message // via non-blocking I/O 1. When first trying if (send_peer (mb) != -1 to send over a errno == EWOULDBLOCK) { connection // Queue in *front* of the // list to preserve order. 2. When flow control msg_queue_-enqueue_head abates (mb, ACE_Time_Value::zero); // Tell Reactor to call us // back it’s ok to send again. reactor ()-schedule_wakeup (this, ACE_Event_Handler::WRITE_MASK); } } Vanderbilt University 248
- 250. Advanced ACE Tutorial Do Sending a Message to a Consumer int Consumer_Handler::send_peer (ACE_Message_Block *mb) { ssize_t n; size_t len = mb-length (); // Try to send the message. n = peer ().send (mb-rd_ptr (), len); if (n = 0) return errno == EWOULDBLOCK ? 0 : n; else if (n len) // Skip over the part we did send. mb-rd_ptr (n); else /* if (n == length) */ { // Decrement reference count. mb-release (); errno = 0; } return n; } Vanderbilt University
- 251. Advanced ACE Tutorial Do Finish Sending when Flow Control Abates // Finish sending a message when flow control // conditions abate. This method is automatically // called by the Reactor. int Consumer_Handler::handle_output (ACE_HANDLE) { ACE_Message_Block *mb = 0; // Take the first message off the queue. msg_queue_-dequeue_head (mb, ACE_Time_Value::zero); if (nonblk_put (mb) != -1 || errno != EWOULDBLOCK) { // If we succeed in writing msg out completely // (and as a result there are no more msgs // on the ACE_Message_Queue), then tell the // ACE_Reactor not to notify us anymore. if (msg_queue_-is_empty () reactor ()-cancel_wakeup (this, ACE_Event_Handler::WRITE_MASK); } } Vanderbilt University
- 252. Advanced ACE Tutorial Do The Gateway Class Connector Service Hash Map Object Manager APPLICATION- INDEPENDENT APPLICATION- Proxy Config SPECIFIC Handler Connector Table SUPPLIER HANDLER Routing CONSUMER HANDLER Table Gateway This class integrates other application-specific and application-independent components Vanderbilt University
- 253. Advanced ACE Tutorial Douglas C. Schmidt Dynamically Configuring Gateway into an Application Parameterized by proxy handler Example of the Component template Configurator pattern class SUPPLIER_HANDLER, int main (int argc, char *argv[]) class CONSUMER_HANDLER { class Gateway // Initialize the daemon and : public Service_Object // dynamically configure services. { ACE_Service_Config::open (argc, public: argv); // Perform initialization. // Run forever, performing the virtual int init // configured services. (int argc, char *argv[]); ACE_Reactor::instance ()- run_reactor_event_loop (); // Perform termination. /* NOTREACHED */ virtual int fini (void); } Vanderbilt University 252
- 254. Advanced ACE Tutorial Do Using the ACE Service Configurator Framework for the Gateway SERVICE Thread-per Connection CONFIGURATOR Gateway RUNTIME Reactive Gateway Service Object Service Service DLLS Repository Object Thread Pool Gateway Service Reactor Service Config Object svc.conf dynamic Gateway Service_Object * FILE gateway:make_Gateway() -p 2001 We can replace the single-threaded Gateway with a multi-threaded Gateway Vanderbilt University
- 255. Advanced ACE Tutorial Douglas C. Schmidt Dynamic Linking a Gateway Service The Gateway service is Dynamically linked factory function configured via scripting in a that allocates a new svc.conf file: single-threaded Gateway extern C % cat ./svc.conf static Svc_Manager ACE_Service_Object *make_Gateway (void); -p 5150 ACE_Service_Object *make_Gateway (void) dynamic Gateway { Service_Object * return new gateway:_make_Gateway() -d -p $PORT GatewaySupplier_Handler, Consumer_Handler; # .dll or .so suffix // ACE automatically deletes memory. # added to gateway } # automatically Vanderbilt University 254
- 256. Advanced ACE Tutorial Douglas C. Schmidt Concurrency Strategies for Patterns The Acceptor-Connector pattern does not constrain the concurrency strategies of a ACE_Svc_Handler There are three common choices: 1. Run service in same thread of control 2. Run service in a separate thread 3. Run service in a separate process Observe how our patterns and ACE framework push this decision to the “edges” of the design – This greatly increases reuse, flexibility, and performance tuning Vanderbilt University 255
- 257. Advanced ACE Tutorial Do Using the Active Object Pattern for the Gateway CONCRETE Supplier APPLICATION Consumer EVENT Supplier Handler Consumer Handler Supplier HANDLERS Handler LEVEL Handler Handler Event Event 4: send(msg) Event Handler Event Handler Event Handler Handler Handler 2: recv(msg) 3: route(msg) 1: handle_input() FRAMEWORK LEVEL Timer Handle Signal Queue Table Handlers Reactor OS EVENT DEMULTIPLEXING INTERFACE KERNEL LEVEL Each Consumer_Handler is implemented as an Active Object Vanderbilt University
- 258. Advanced ACE Tutorial Do Collaboration in Multi-threaded Gateway Routing Routing Consumer Table Handler Message 5: send_peer(msg) Queue ROUTE ID Consumer Handlers Consumer Handler ) 3: find() sg m t( Message pu Queue 4: Supplier 6: put (msg) 5: send_peer(msg) Handler 1: handle_input() 2: recv_peer(msg) Note that this design is much simpler since the OS thread scheduler handles blocking Vanderbilt University
- 259. Advanced ACE Tutorial Douglas C. Schmidt Using the Half-Sync/Half-Async Pattern in the Gateway Consumer Consumer ACE_Reactor plays the SYNCHRONOUS Consumer TASK LAYER Handler Handler Handler role of “async” layer ACE_Task active object 1: dequeue(msg) plays the role of “sync” layer 2: send(msg) This particular configuration QUEUEING LAYER is a common variant of the MESSAGE QUEUES Half-Sync/Half-Async 2: recv(msg) pattern, as described in 3: get_route(msg) POSA2 ASYNCHRONOUS TASK LAYER Supplier 4: enqueue(msg) Handler Supplier Supplier 1: dispatch() Handler Handler Reactor Vanderbilt University 258
- 260. Advanced ACE Tutorial Do Class Diagram for Multi-Threaded Gateway Proxy_Handler COMPONENTS APPLICATION- SOCK_Connector SOCK_Stream SPECIFIC MT_Synch 1 Proxy Handler Supplier/Thr_Consumer Connector n Handler activates COMPONENTS CONNECTION- SVC_HANDLER PEER_STREAM ORIENTED PEER_CONNECTOR SYNCH Connector Svc Handler PEER PEER CONNECTOR STREAM COMPONENTS FRAMEWORK Stream Connection ACE IPC_SAP Service Configurator Concurrency Reactor global Vanderbilt University
- 261. Advanced ACE Tutorial Douglas C. Schmidt Thr_Consumer_Handler Class Interface #define ACE_USE_MT New subclass of #include Proxy_Handler.h Proxy_Handler uses the Active Object pattern for the class Thr_Consumer_Handler : public Consumer_Handler Consumer_Handler { public: Uses multi-threading and // Initialize the object and synchronous I/O (rather than // spawn new thread. non-blocking I/O) to transmit virtual int open (void *); message to Peers // Send a message to a peer. virtual int put Transparently improve (ACE_Message_Block *, ACE_Time_Value *); performance on a // Transmit peer messages multi-processor platform and // in separate thread. simplify design virtual int svc (void); }; Vanderbilt University 260
- 262. Advanced ACE Tutorial Douglas C. Schmidt Thr_Consumer_Handler Class Implementation Override definition in the The multi-threaded version Consumer_Handler class of open() is slightly int different since it spawns a Thr_Consumer_Handler::open (void *) new thread to become an { // Become an active object by active object! // spawning a new thread to // transmit messages to Peers. activate() is a pre-defined method on activate (THR_DETACHED); ACE_Task } Vanderbilt University 261
- 263. Advanced ACE Tutorial Do Thr_Consumer_Handler Class Implementation // Queue up a message for transmission. int Thr_Consumer_Handler::put (ACE_Message_Block *mb, ACE_Time_Value *) { // Perform non-blocking enqueue. msg_queue_-enqueue_tail (mb, ACE_Time_Value::zero); } // Transmit messages to the peer (note // simplification resulting from threads...) int Thr_Consumer_Handler::svc (void) { ACE_Message_Block *mb = 0; // Since this method runs in its own thread it // is OK to block on output. while (msg_queue_-dequeue_head (mb) != -1) send_peer (mb); } Vanderbilt University
- 264. Advanced ACE Tutorial Douglas C. Schmidt Dynamic Linking a Threaded Gateway Service % cat ./svc.conf Dynamically linked factory remove Gateway function that allocates a dynamic Gateway multi-threaded Gateway object Service_Object * extern C thr_gateway:_make_Gateway() ACE_Service_Object *make_Gateway (void); -d # .dll or .so suffix added ACE_Service_Object *make_Gateway (void) # to thr_Gateway { # automatically return new GatewaySupplier_Handler, Thr_Consumer_Handler; // ACE automatically deletes memory. } Vanderbilt University 263
- 265. Advanced ACE Tutorial Do Call Center Manager (CCM) Event Server Example SUPER SUPER SUPER VISOR VISOR VISOR CCM Session Router Stream Module Event Filter Module MIB Event Analyzer Module ACE Switch Adapter RUN-TIME Module TELECOM SWITCHES EVENT SERVER Vanderbilt University
- 266. Advanced ACE Tutorial Do Patterns in the CCM Event Server Publisher Subscriber Pipes Layers Filters Acceptor- Component Connector Configurator STRATEGIC PATTERNS Reactor TACTICAL Factory PATTERNS Iterator Composite Method Proxy Wrapper Facade The Event Server components are based upon a common pattern language www.cs.wustl.edu/˜schmidt/PDF/ DSEJ-94.pdf Vanderbilt University
- 267. Advanced ACE Tutorial Douglas C. Schmidt Overview of the ACE Streams Framework An ACE_Stream allows flexible configuration of layered processing modules It is an implementation of the Pipes and Filters architecture pattern – This pattern provides a structure for systems that process a stream of data – Each processing step is encapsulated in a filter ACE_Module component – Data is passed through pipes between adjacent filters, which can be re-combined The CCM Event Server was design and implemented using ACE Streams Vanderbilt University 266
- 268. Advanced ACE Tutorial Do Structure of the ACE Streams Framework SYNCH SYNCH SYNCH ACE_Task ACE_Module ACE_Stream 2 2..* Framework characteristics An ACE_Stream contains a stack of ACE_Modules Each ACE_Module contains two ACE_Tasks – i.e., a read task and a write task Each ACE_Task contains an ACE_Message_Queue and a pointer to an ACE_Thread_Manager Vanderbilt University
- 269. Advanced ACE Tutorial Do Implementing a Stream in ACE APPLICATION APPLICATION Stream Stream STREAM Head DOWNSTREAM UPSTREAM Multiplexor open()=0 STREAM close()=0 Tail put()=0 svc()=0 NETWORK INTERFACE OR PSEUDO-DEVICES MESSAGE MODULE WRITE READ TASK TASK Note similarities to System V STREAMS Vanderbilt University
- 270. Advanced ACE Tutorial Do Alternative Concurrency Models for Message Processing ACTIVE ACTIVE ACTIVE Module ACTIVE A Module 4: svc() A 3: put() 2: put() Module ACTIVE Module B 2: svc() B 1: put() 1: put() Module Module ACTIVE C C TASK-BASED MESSAGE-BASED PROCESS ARCHITECTURE PROCESS ARCHITECTURE MESSAGE MODULE WRITE TASK READ TASK PROCESS OR OBJECT OBJECT OBJECT OBJECT THREAD Task-based models are more intuitive but less efficient than Message-based models Vanderbilt University
- 271. Advanced ACE Tutorial Do Using the ACE Streams Framework for the CCM Event Server SUPER VISORS Session Router Module SUPER VISORS Presentation Module SUPER VISORS Event Filter Session IO Module Reactor Event Analysis Module Switch IO MD110 ERICSSON Presentation Module MD110 ERICSSON Switch Adapter MD110 ERICSSON Module TELECOM SWITCHES www.cs.wustl.edu/˜schmidt/PDF/ DSEJ-94.pdf Vanderbilt University
- 272. Advanced ACE Tutorial Douglas C. Schmidt Broader Context: External OS for Telecom Switches CLIENT CLIENT Features CLIENT CLIENT Allow clients to manage NETWORK various aspects of telecom SWITCHES switches without modifying the switch software DIRECTORY MANAGER CALL CENTER EXTENSION MANAGER Support reuse of existing MANAGER components based on a APPLICATION common architectural FRAMEWORK framework SERVER Vanderbilt University 271
- 273. Advanced ACE Tutorial Douglas C. Schmidt Applying ACE Streams to External OS CLIENT DIRECTORY CLIENT CALL CENTER MANAGER MANAGER CLIENT Event NETWORK Analyzer DATABASE SWITCHES Switch Session Adapter Router ACE Reactor FRAMEWORK EXTENSION MANAGER SERVER Vanderbilt University 272
- 274. Advanced ACE Tutorial Douglas C. Schmidt ACE Stream Example: Parallel I/O Copy 5: write() Program copies stdin to stdout via the use of a multi-threaded Consumer active ACE_Stream Module 4: svc() Stream implements a “bounded buffer” 3: put() Since the data flow is Producer uni-directional the Module active 2: svc() “read” ACE_Task is always ignored 1: read() Vanderbilt University 273
- 275. Advanced ACE Tutorial Do Producer Class Interface typedef short-hands for templates typedef ACE_StreamACE_MT_SYNCH MT_Stream; typedef ACE_ModuleACE_MT_SYNCH MT_Module; typedef ACE_TaskACE_MT_SYNCH MT_Task; Define the Producer interface class Producer : public MT_Task { public: // Initialize Producer. virtual int open (void *) { // activate() is inherited from class Task. activate (THR_BOUND); } // Read data from stdin and pass to consumer. virtual int svc (void); // ... }; Vanderbilt University
- 276. Advanced ACE Tutorial Do Producer Class Implementation Runs as an active object in a separate thread int Producer::svc (void) { for (;;) { ACE_Message_Block *mb; // Allocate a new message. ACE_NEW_RETURN (mb, ACE_Message_Block (BUFSIZ), -1); // Keep reading stdin, until we reach EOF. ssize_t n = ACE_OS::read (ACE_STDIN, mb-wr_ptr (), mb-size ()); if (n = 0) { // Send shutdown message to other // thread and exit. mb-length (0); this-put_next (mb); break; } else { mb-wr_ptr (n); // Adjust write pointer. // Send the message to the other thread. this-put_next (mb); } } } Vanderbilt University
- 277. Advanced ACE Tutorial Do Consumer Class Interface Define the Consumer interface class Consumer : public MT_Task { public: // Initialize Consumer. virtual int open (void *) { // activate is inherited from class Task. activate (THR_BOUND); } // Enqueue the message on the Message_Queue // for subsequent processing in svc. virtual int put (ACE_Message_Block *, ACE_Time_Value * = 0) { // putq is inherited from class Task. return putq (mb, tv); } // Receive message from producer // and print to stdout. virtual int svc (void); }; Vanderbilt University
- 278. Advanced ACE Tutorial Do Consumer Class Implementation Consumer dequeues a message from the ACE_Message_Queue, writes the message to the stderr stream, and deletes the message int Consumer::svc (void) { ACE_Message_Block *mb = 0; // Keep looping, reading a message from the queue, // until we get a 0 length message, then quit. for (;;) { int result = getq (mb); if (result == -1) break; int length = mb-length (); if (length 0) ACE_OS::write (ACE_STDOUT, mb-rd_ptr (), length); mb-release (); if (length == 0) break; } } The Producer sends a 0-sized message to inform the Consumer to stop reading and exit Vanderbilt University
- 279. Advanced ACE Tutorial Do Main Driver Function for the Stream Create Producer and Consumer Modules and push them onto the Stream int main (int argc, char *argv[]) { // Control hierarchically-related // active objects. MT_Stream stream; // All processing is performed in the // Stream after push’s complete. stream.push (new MT_Module (Consumer, new Consumer); stream.push (new MT_Module (Producer, new Producer)); // Barrier synchronization: wait for // the threads, to exit, then exit // the main thread. ACE_Thread_Manager::instance ()-wait (); } Vanderbilt University
- 280. Advanced ACE Tutorial Douglas C. Schmidt Evaluation of the ACE Stream Framework Structuring active objects via an ACE_Stream allows “interpositioning” – i.e., similar to adding a filter in a UNIX pipeline New functionality may be added by “pushing” a new processing ACE_Module onto an ACE_Stream, e.g.: stream.push (new MT_Module (Consumer, new Consumer)) stream.push (new MT_Module (Filter, new Filter)); stream.push (new MT_Module (Producer, new Producer)); Communication between ACE_Modules is typically anonymous Vanderbilt University 279
- 281. Advanced ACE Tutorial Douglas C. Schmidt Concurrency Strategies Developing correct, efficient, and robust concurrent applications is challenging Below, we examine a number of strategies that addresses challenges related to the following: – Concurrency control – Library design – Thread creation – Deadlock and starvation avoidance Vanderbilt University 280
- 282. Advanced ACE Tutorial Douglas C. Schmidt General Threading Guidelines A threaded program should not arbitrarily enter non-threaded (i.e., “unsafe”) code Threaded code may refer to unsafe code only from the main thread – e.g., beware of errno problems Use reentrant OS library routines (‘ r’) rather than non-reentrant routines Beware of thread global process operations, such as file I/O Make sure that main() terminates cleanly – e.g., beware of pthread_exit(), exit(), and “falling off the end” Vanderbilt University 281
- 283. Advanced ACE Tutorial Douglas C. Schmidt Thread Creation Strategies Use threads for independent jobs that must maintain state for the life of the job Don’t spawn new threads for very short jobs Use threads to take advantage of CPU concurrency Only use “bound” threads when absolutely necessary If possible, tell the threads library how many threads are expected to be active simultaneously – e.g., use thr_setconcurrency() Vanderbilt University 282
- 284. Advanced ACE Tutorial Douglas C. Schmidt General Locking Guidelines Don’t hold locks across long duration operations (e.g., I/O) that can impact performance – Use ACE_Token instead... Beware of holding non-recursive mutexes when calling a method outside a class – The method may reenter the module and deadlock Don’t lock at too small of a level of granularity Make sure that threads obey the global lock hierarchy – But this is easier said than done... Vanderbilt University 283
- 285. Advanced ACE Tutorial Douglas C. Schmidt Locking Alternatives Code locking – Associate locks with body of functions Typically performed using bracketed mutex locks – Often called a Monitor Object Data locking – Associate locks with data structures and/or objects – Permits a more fine-grained style of locking Data locking allows more concurrency than code locking, but may incur higher overhead Vanderbilt University 284
- 286. Advanced ACE Tutorial Douglas C. Schmidt Single-lock Strategy One way to simplify locking is use a single, application-wide mutex lock Each thread must acquire the lock before running and release it upon completion The advantage is that most legacy code doesn’t require changes The disadvantage is that parallelism is eliminated – Moreover, interactive response time may degrade if the lock isn’t released periodically Vanderbilt University 285
- 287. Advanced ACE Tutorial Douglas C. Schmidt Monitor Object Strategy A more OO locking strategy is to use a Monitor Object – www.cs.wustl.edu/˜schmidt/POSA/ Monitor Object synchronization mechanisms allow concurrent method invocations – Either eliminate access to shared data or use synchronization objects – Hide locking mechanisms behind method interfaces Therefore, modules should not export data directly Advantage is transparency Disadvantages are increased overhead from excessive locking and lack of control over method invocation order Vanderbilt University 286
- 288. Advanced ACE Tutorial Douglas C. Schmidt Active Object Strategy Each task is modeled as an active object that maintains its own thread of control Messages sent to an object are queued up and processed asynchronously with respect to the caller – i.e., the order of execution may differ from the order of invocation This approach is more suitable to message passing-based concurrency The ACE_Task class can be used to implement active objects – www.cs.wustl.edu/˜schmidt/POSA/ Vanderbilt University 287
- 289. Advanced ACE Tutorial Douglas C. Schmidt Invariants In general, an invariant is a condition that is always true For concurrent programs, an invariant is a condition that is always true when an associated lock is not held – However, when the lock is held the invariant may be false – When the code releases the lock, the invariant must be re-established e.g., enqueueing and dequeueing messages in the ACE_Message_Queue class Vanderbilt University 288
- 290. Advanced ACE Tutorial Douglas C. Schmidt Run-time Stack Problems Most threads libraries contain restrictions on stack usage – The initial thread gets the “real” process stack, whose size is only limited by the stacksize limit – All other threads get a fixed-size stack Each thread stack is allocated off the heap and its size is fixed at startup time Therefore, be aware of “stack smashes” when debugging multi-threaded code – Overly small stacks lead to bizarre bugs, e.g., Functions that weren’t called appear in backtraces Functions have strange arguments Vanderbilt University 289
- 291. Advanced ACE Tutorial Douglas C. Schmidt Deadlock Permanent blocking by a set of threads that are competing for a set of resources Caused by “circular waiting,” e.g., – A thread trying to reacquire a lock it already holds – Two threads trying to acquire resources held by the other e.g., T1 and T2 acquire locks L1 and L2 in opposite order One solution is to establish a global ordering of lock acquisition (i.e., a lock hierarchy) – May be at odds with encapsulation... Vanderbilt University 290
- 292. Advanced ACE Tutorial Douglas C. Schmidt Avoiding Deadlock in OO Frameworks Deadlock can occur due to properties of OO frameworks, e.g., – Callbacks – Inter-class method calls There are several solutions – Release locks before performing callbacks Every time locks are reacquired it may be necessary to reevaluate the state of the object – Make private “helper” methods that assume locks are held when called by methods at higher levels – Use an ACE_Token or ACE_Recursive_Thread_Mutex Vanderbilt University 291
- 293. Advanced ACE Tutorial Do ACE_Recursive_Thread_Mutex Implementation Here is portable implementation of recursive thread mutexes available in ACE: class ACE_Recursive_Thread_Mutex { public: // Initialize a recursive mutex. ACE_Recursive_Thread_Mutex (void); // Implicitly release a recursive mutex. ˜ACE_Recursive_Thread_Mutex (void); // Acquire a recursive mutex. int acquire (void); // Conditionally acquire a recursive mutex. int tryacquire (void); // Releases a recursive mutex. int release (void); private: ACE_Thread_Mutex nesting_mutex_; ACE_Condition_Thread_Mutex mutex_available_; ACE_thread_t owner_; int nesting_level_; }; Vanderbilt University
- 294. Advanced ACE Tutorial Do Acquiring an ACE_Recursive_Thread_Mutex int ACE_Recursive_Thread_Mutex::acquire (void) { ACE_thread_t t_id = ACE_Thread::self (); ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, nesting_mutex_, -1); // If there’s no contention, grab mutex. if (nesting_level_ == 0) { owner_ = t_id; nesting_level_ = 1; } else if (t_id == owner_) // If we already own the mutex, then // increment nesting level and proceed. nesting_level_++; else { // Wait until nesting level drops // to zero, then acquire the mutex. while (nesting_level_ 0) mutex_available_.wait (); // Note that at this point // the nesting_mutex_ is held... owner_ = t_id; nesting_level_ = 1; } return 0; Vanderbilt University
- 295. Advanced ACE Tutorial Do Releasing and Initializing an ACE_Recursive_Thread_Mutex int ACE_Recursive_Thread_Mutex::release (void) { ACE_thread_t t_id = ACE_Thread::self (); // Automatically acquire mutex. ACE_GUARD_RETURN (ACE_Thread_Mutex, guard, nesting_mutex_, -1); nesting_level_--; if (nesting_level_ == 0) { // Put the mutex into a known state. owner_ = ACE_OS::NULL_thread; // Inform waiters that the mutex is free. mutex_available_.signal (); } return 0; } ACE_Recursive_Thread_Mutex:: ACE_Recursive_Thread_Mutex (void) : nesting_level_ (0), owner_ (ACE_OS::NULL_thread), mutex_available_ (nesting_mutex_){} Vanderbilt University
- 296. Advanced ACE Tutorial Douglas C. Schmidt Avoiding Starvation Starvation occurs when a thread never acquires a mutex even though another thread periodically releases it The order of scheduling is often undefined This problem may be solved via: – Use of “voluntary pre-emption” mechanisms e.g., thr_yield() or Sleep() – Using an ACE “Token” that strictly orders acquisition and release Vanderbilt University 295
- 297. Advanced ACE Tutorial Douglas C. Schmidt Drawbacks to Multi-threading Performance overhead – Some applications do not benefit directly from threads – Synchronization is not free – Threads should be created for processing that lasts at least several 1,000 instructions Correctness – Threads are not well protected against interference – Concurrency control issues are often tricky – Many legacy libraries are not thread-safe Development effort – Developers often lack experience – Debugging is complicated (lack of tools) Vanderbilt University 296
- 298. Advanced ACE Tutorial Douglas C. Schmidt Lessons Learned using OO Patterns Benefits of patterns – Enable large-scale reuse of software architectures – Improve development team communication – Help transcend language-centric viewpoints Drawbacks of patterns – Do not lead to direct code reuse – Can be deceptively simple – Teams may suffer from pattern overload Vanderbilt University 297
- 299. Advanced ACE Tutorial Douglas C. Schmidt Lessons Learned using OO Frameworks Benefits of frameworks – Enable direct reuse of code (cf patterns) – Facilitate larger amounts of reuse than stand-alone functions or individual classes Drawbacks of frameworks – High initial learning curve Many classes, many levels of abstraction – The flow of control for reactive dispatching is non-intuitive – Verification and validation of generic components is hard Vanderbilt University 298
- 300. Advanced ACE Tutorial Douglas C. Schmidt Lessons Learned using C++ Benefits of C++ – Classes and namespaces modularize the system architecture – Inheritance and dynamic binding decouple application policies from reusable mechanisms – Parameterized types decouple the reliance on particular types of synchronization methods or network IPC interfaces Drawbacks of C++ – Some language features are not implemented – Some development environments are primitive – Language has many dark corners and sharp edges Purify helps alleviate many problems... Vanderbilt University 299
- 301. Advanced ACE Tutorial Douglas C. Schmidt Lessons Learned using OOD Good designs can be boiled down to a few key principles: – Separate interface from implementation – Determine what is common and what is variable with an interface and an implementation – Allow substitution of variable implementations via a common interface i.e., the “open/closed” principle Aspect-Oriented Programming (AOP) – Dividing commonality from variability should be goal-oriented rather than exhaustive Design is not simply drawing a picture using a CASE tool, using graphical UML notation, or applying patterns – Design is a fundamentally creative activity Vanderbilt University 300
- 302. Advanced ACE Tutorial Douglas C. Schmidt Software Principles for Distributed Applications Use patterns/frameworks to decouple policies/mechanisms – Enhance reuse of common concurrent programming components Decouple service functionality from configuration – Improve flexibility and performance Use classes, inheritance, dynamic binding, and parameterized types – Improve extensibility and modularity Enhance performance/functionality with OS features – e.g., implicit and explicit dynamic linking and multi-threading Perform commonality/variability analysis – Identify uniform interfaces for variable components and support pluggability of variation Vanderbilt University 301
- 303. Advanced ACE Tutorial Douglas C. Schmidt Conferences and Workshops on Patterns Pattern Language of Programs Conferences – PLoP, September, 2002, Monticello, Illinois, USA – OOPSLA, November, 2002, Seattle, USA – hillside.net/patterns/conferences/ Distributed Objects and Applications Conference – Oct/Nov, 2002, UC Irvine – www.cs.wustl.edu/˜schmidt/activities-chair.html Vanderbilt University 302
- 304. Advanced ACE Tutorial Douglas C. Schmidt Patterns, Frameworks, and ACE Literature Books – Gamma et al., Design Patterns: Elements of Reusable Object-Oriented Software AW, ’94 – Pattern Languages of Program Design series by AW, ’95-’99. – Siemens Schmidt, Pattern-Oriented Software Architecture, Wiley, volumes ’96 ’00 (www.posa.uci.edu) – Schmidt Huston, C++ Network Programming: Mastering Complexity with ACE and Patterns, AW, ’02 (www.cs.wustl.edu/˜schmidt/ACE/book1/) – Schmidt Huston, C++ Network Programming: Systematic Reuse with ACE and Frameworks, AW, ’03 (www.cs.wustl.edu/˜schmidt/ACE/book2/) Vanderbilt University 303
- 305. Advanced ACE Tutorial Do How to Obtain ACE Software and Technical Support All source code for ACE is freely available – www.cs.wustl.edu/˜schmidt/ACE. html Mailing lists – [email protected] – [email protected] – [email protected] – [email protected] Newsgroup – comp.soft-sys.ace Commercial support from Riverace and OCI – www.riverace.com – www.theaceorb.com Vanderbilt University
- 306. Advanced ACE Tutorial Douglas C. Schmidt Concluding Remarks Developers of networked application software confront recurring challenges that are largely application-independent – e.g., service configuration and initialization, distribution, error handling, flow control, event demultiplexing, concurrency, synchronization, persistence, etc. Successful developers resolve these challenges by applying appropriate patterns to create communication frameworks containing components Frameworks and components are an effective way to achieve systematic reuse of software Vanderbilt University 305