DD-Vote is a simulated voting system designed to demonstrate fundamental concepts in distributed computing. It features multiple server instances communicating with each other for fault tolerance, and a client application for users to register, log in, and vote.

Key Distributed Computing Concepts Illustrated:

• Multi-Node Execution: The system can run across multiple machines (or multiple JVMs on the same machine), forming a cluster.
• Primary-Backup Replication: Implements a simple Primary-Backup data replication model for increased availability. The Primary handles write requests (registering, voting) and replicates the changes to Backup servers.
• Heartbeating & Failure Detection: Servers monitor each other's liveness using periodic heartbeats and timeouts.
• Logical Time (Lamport Timestamps): Uses Lamport clocks to establish a partial ordering of events across the servers.
• State Transfer: New or recovering Backup servers synchronize their state with the Primary.
• Eventual Consistency: The system is eventually consistent, meaning that backups might be temporarily out of sync with the primary.

Important Limitations:

• Simplified Replication: The replication mechanism is asynchronous, so there's a potential for data loss if the Primary server fails before propagating updates.
• No Strong Consensus: The system does not implement a true consensus algorithm (like Paxos or Raft) to guarantee agreement across all servers before committing a write operation.
• Rudimentary Leader Determination: The Primary is determined simply as the server with the lowest lexicographical ID among the UP nodes. There's no explicit leader election protocol.
• No Automatic Client Failover: If a client's connected server fails, the client does not automatically redirect to another server.
• Potential Split-Brain: Without a consensus protocol, network partitions can lead to a "split-brain" scenario where multiple servers incorrectly believe they are the Primary, resulting in data inconsistencies.

• Java Development Kit (JDK): You need a JDK installed (version 8 or later). Ensure that the javac and java commands are in your system's PATH environment variable. You can download a JDK from https://www.oracle.com/java/technologies/javase-downloads.html or use an open-source distribution like OpenJDK.

1. Download/Clone the Project: Download the source code as a ZIP file or clone it from a Git repository to a directory (e.g., DD-Vote).

2. Open a Terminal/Command Prompt: Navigate to the root directory of the project (the DD-Vote folder containing the src folder).

3. Compile the Code: Use the javac command to compile all .java files and create the output directory.

   mkdir bin  # Create the output directory (if it doesn't exist)
   javac -d bin src/ddvote/shared/*.java src/ddvote/server/*.java src/ddvote/client/*.java

   • Windows Note: If you're using the Windows Command Prompt and the / character causes problems, use backslashes \ instead.

1. Start the Servers: You need to open a separate terminal window for each server instance. Use the following command format:

   java -cp bin ddvote.server.VotingServer <port> <uniqueServerId> [peer1_host:port] [peer2_host:port] ...

   • <port>: The port number this server will listen on (e.g., 10001).
   • <uniqueServerId>: A unique identifier for this server (e.g., localhost:10001). It's common (and recommended) to use the host:port combination as the ID. This ID must be unique among all servers.
   • [peer1_host:port] [peer2_host:port] ...: A space-separated list of the addresses (host:port) of the other server instances in the cluster. This allows the servers to discover and communicate with each other for heartbeating and data replication.

   Example (Three Servers):

   • Terminal 1 (Server A):

     java -cp bin ddvote.server.VotingServer 10001 localhost:10001 localhost:10002 localhost:10003

   • Terminal 2 (Server B):

     java -cp bin ddvote.server.VotingServer 10002 localhost:10002 localhost:10001 localhost:10003

   • Terminal 3 (Server C):

     java -cp bin ddvote.server.VotingServer 10003 localhost:10003 localhost:10001 localhost:10002

2. Start the Client:

   • Open yet another terminal window.
   • Run the client application:

     java -cp bin ddvote.client.VotingClient

   • A GUI window should appear.

1. Client Connection: In the VotingClient GUI, enter the Host (e.g., localhost) and Port of one of the running servers (it's best to connect to the Primary server to ensure writes are processed) and click "Connect."
2. Register/Login: Use the Register and Login buttons to create a new voter account or log in to an existing one.
3. Vote: Select a candidate from the list and click the "Submit Vote" button.
4. Administrative Commands: Use the terminal window running the Primary server to enter administrative commands:
   • help: Shows a list of available commands.
   • status: Shows the current status of the server, including its role (Primary/Backup), the current Lamport clock, and the election state.
   • peers: Shows the list of known peers and their status (UP/DOWN).
   • start: Starts the election.
   • stop or finish: Stops the election and displays the final results.
   • reset_votes: Resets the vote count.

Here are some scenarios you can use to demonstrate the distributed computing features of the application:

• Normal Voting: Start all servers, connect the client to the Primary, register, log in, vote, and verify the vote is counted on the Primary.
• Replication: Register a user on the Primary server, then check the list_voters command on one of the Backup servers to confirm that the voter has been replicated.
• Simulate Primary Failure: During voting, close the terminal running the Primary server. Observe that the remaining servers detect the failure and a new Primary is established. Client action is temporarily disrupted until the new primary processes.
• State Transfer (New Server): Run one server, then later add another by starting the second server, observing the process where it takes over and becomes the new primary.
• Adding a peer while an election is active: Add a peer while an election is active to see the automatic new state being transferred.
• Adding Candidates: Add new candidates to the candidate listing in between election periods (using the force or in the not started phase).

• Logging Level: The servers are configured to log a lot of information for debugging. To reduce clutter, you can change the logging level by editing src/ddvote/server/VotingServer.java and modifying the rootLogger.setLevel() line in the setupBasicLogger() method (e.g., set it to Level.INFO or Level.WARNING).
• Firewall/Network Configuration: Ensure that your firewall or network configuration allows the servers to communicate with each other on the specified ports.