DISTRIBUTED COMPUTTING (snapshot).pptx

DISTRIBUTED COMPUTING
Chandy-Lamport Snapshot algorithm
for FIFO Channel

❖ In a Distributed system, more processes may be
running on different physical servers. These processes
communicate with each other via communication channels
using text messaging.
❖ These processes neither have a shared memory nor a
common physical clock, so determining the global state of a
distributed system is very difficult.
❖ But a process could record its own local state at a given
time but a message that is in transit (on its way to be
delivered) will not be included in the recorded state and hence
the actual state of the system/process is incorrect after the
transit message is delivered.

As shown in the below diagram, if we record only the local states of the
process S1 and S2 and omit the communication channels, then Rs.500will not
be tallied in both accounts A and B as that deliveringmessage is in transit
between both accounts while taking snapshot
FOR EXAMPLE

GLOBAL SNAPSHOT:
Global Snapshot = Global State = Collection of
individual states of each process in the distributed
system
+
individual state of each communication channel
in the distributed system
SNAPSHOT:
❖ Is a photograph of a process taken or record it
quickly.

What is the need for taking snapshots or recording
the global state of the system?
CHECK POINTING:
❖ Snapshot will use as a checkpoint, to restart the
application in case of failure.
COLLECTING GARBAGE:
❖ Use to remove objects that don't have any
references
DETECTING DEADLOCKS:
❖ Use to examine the current application state

VARIOUS SNAPSHOT ALGORITHMS
1) FIFO Channel
❖ Chandy_lamport algorithm
2) Non-FIFO Channel
❖ Lai-Yang algorithm Matterrn's algorithm
3)Casual ordering channel
❖ Acharya badrinath & Alagar venkatesan
algorithm

CHANDY LAMPORT ALGORITHM
❖ This algorithm always captures the consistent
global state of a distributed system
❖ The main idea behind the proposed algorithm is
that if we know that all messages that have been sent
by one process and they have been received by
another process, then we can record the global state
of the system.

❖ Any process in the distributed system can
initiate this global state recording algorithm using a
special message called MARKER.
❖ This marker traverse the distributed system
across all communication channel and cause each
process to record its own state.
❖ In the end, the state of entire system (Global state)
is recorded.
❖ This algorithm does not interfere with normal
execution of processes.

How it works
❖ This algorithm can be initiated by any process by
executing the "Marker Sending Rule" records its local
state and sends a marker on each outgoing channel.
❖ The receiving process executes the "Marker
Receiving Rule" once received the marker. If incase,
the process has not yet recorded its local state, it sets
the state of the incoming channel on which the marker
has been received as EMPTY or NULL

❖ The incoming channel on which the marker has
been received is set as empty in order to block any
other messages coming on this channel while
recording is being done.
❖ Then the receiving process executes the
"Marker Sending Rule" to record its own local state
+ record the state of other incoming channels i.e
except the channel on which the MARKER is
received.

Let us assume,
Two processes P1 & P2 are running two channels
are C12, C21

Initial global state (snapshot 1) of the processes and
channels

❖Now, P1 tells P2 to change its state variable x2 to 4.
❖This is an another global snapshot of this distributed
system - No: 2

❖ Now P2 receives the message sent by p1 through
the channel C12. So the channel C12 becomes empty
now.
❖ Global snapshot of this distributed system No: 3

❖ P2 changes its state variable x2 to 4.
❖ Global snapshot of the distributed system - No: 4

❖ Now, the Process P1 initiates the recording process.
i.e P1 is the initiator.
❖ So, P1 sends a MARKER message to P2 and begins recording
its local state and recording all messages on inbound
channels.
❖ Meanwhile, P2 also sent a message to P1

❖ P2 receives the MARKER message for the first
time, so records its local state.
❖ Meanwhile P1 also receives the message M1, sent by p2
❖ Now, P2 sends a MARKER message to P1

Now the statue is, P1 has already sent a
MARKER message to P2, so it records all
messages it received on inbound channels to the
appropriate channel's state

Now, Both processes have recorded
their states and also the states of their all
incoming channels

❖Each process has received a MARKER
on all of its incoming channels. i.e both
processes have recorded their local states.
❖Since only two processes running in this
distributed system, the algorithm terminates
here.
❖The collection of these two local snapshots,
determine the global state of the distributed
system.

DISTRIBUTED COMPUTTING (snapshot).pptx

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