Code Methods
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Dec 6, 2023
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getStateRPC Method:
The getStateRPC method is a remote method that retrieves the current state of the server's state machine. It acquires a lock on the state machine's current state, checks the state, and returns an integer value representing the state.
The returned integer values seem to represent different states (0 for OFFLINE, 1 for FOLLOWER, 2 for CANDIDATE, 3 for LEADER).
Logging:
The code includes logging statements using a SysLog class. Logging is done at various levels, including
FINER and FINEST, providing detailed information about the execution flow.
The submitCommandRPC method is responsible for handling the submission of commands in a distributed system using the Raft consensus algorithm. Functionality:
1.
Lock Acquisition:
The method begins by acquiring several locks related to the server's state machine. These locks are crucial for ensuring proper synchronization in a concurrent and distributed environment.
2.
Command Handling:
The method first checks whether the server is in the LEADER state or if the command is of type SHOW. If either condition is true, it proceeds to process the command; otherwise, it forwards the command to the leader.
3.
Command-Specific Handling:
Depending on the command type (BUY, SHOW, CHANGE), specific actions are taken. For instance, for a BUY command, it checks the ticket pool and updates it if there are enough tickets.
4.
Local and Remote Log Updates:
If the command is processed successfully, the local log and state machine are updated. It appends a new log entry and sends an AppendEntriesRPC to other servers to replicate the log entry.
5.
Handling Responses from Other Servers:
The method waits for responses from other servers and ensures that a majority of servers acknowledge the log entry. It considers responses with different terms and handles various conditions, updating the server's state accordingly.
6.
Leader Communication:
If the server is not the leader, it forwards the command to the leader. It handles the case where there is no leader or if the leader is unreachable.
7.
Lock Release:
Before returning, the method releases all acquired locks, ensuring that other threads can access the relevant data structures.
8.
Logging:
Throughout the method, there are log statements using SysLog to provide information about the execution flow and the outcome of different actions. Logging is done at various levels to capture different levels of detail.
9.
Return Value:
The method returns a ReturnValueRPC object containing information about the success or failure of the command execution, the current ticket pool, and additional data for the SHOW command.
10.
Exception Handling:
The method handles potential exceptions like InterruptedException and ExecutionException during the processing of future tasks. Exception handling is done with log statements to capture any unexpected issues.
The `requestVoteRPC` and `appendEntriesRPC` methods are part of the Raft consensus algorithm implementation and are responsible for handling RPC (Remote Procedure Call) requests related to the election process and log replication, respectively.
requestVoteRPC` Method:
This method is used during the election process when a candidate requests votes from other nodes.
- Parameters:
- `candidateId`: ID of the candidate requesting the vote.
- `candidateTerm`: Term of the candidate requesting the vote.
- `lastLogIndex`: Index of the candidate's last log entry.
- `lastLogTerm`: Term of the candidate's last log entry.
- Functionality:
1. Lock Acquisition:
- Acquires locks related to the server's current state, term, voted-for status, and waiting state.
2. Check for Lower Term:
- If the server's current term is greater than the candidate's term, the server signals the candidate to step down.
3. Check for Less Complete Log:
- If the candidate's log is less complete than the server's log, the server signals the candidate to step down.
4. Check for Valid Vote:
- If the candidate's term is greater than or equal to the server's term, and the candidate's log is at least as up-to-date as the server's log, the server votes for the candidate.
5. Check for Higher Term:
- If the candidate's term is greater than the server's term, the server updates its term and signals a
step-down if it was a candidate or leader.
6. Check for Self-Voting:
- If the candidate has the same term as the server, and the server has already voted for itself, the server signals the candidate to step down.
7. Signal Waiting Condition:
- If the server is in the FOLLOWER state and waiting, it signals the timeout condition.
8. Update State to Follower:
- If a step-down occurred, the server updates its state to FOLLOWER.
9. Lock Release:
- Releases acquired locks.
- Return:
- Returns a `ReturnValueRPC` object indicating the server's decision and term.
`appendEntriesRPC` Method:
This method is used for log replication and handling heartbeat signals from the leader.
- Parameters:
- `leaderId`: ID of the leader sending the RPC.
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- `leaderTerm`: Term of the leader sending the RPC.
- `prevLogIndex`: Index of the log entry preceding the entries to be appended.
- `prevLogTerm`: Term of the log entry preceding the entries to be appended.
- `log`: List of log entries to be appended (null for heartbeat).
- `commitIndex`: Index of the last log entry known to be committed.
- `nextIndexes`: List of the next log entry indexes for each server.
- Functionality:
1. Lock Acquisition:
- Acquires locks related to the server's current state, term, voted-for status, commit index, leader ID, waiting state, and next indexes.
2. Heartbeat Handling:
- If the log is null, it is a heartbeat signal. The server resets its voted-for status and updates next indexes. It checks for a higher leader term and steps down if necessary.
3. Handle Lower Term Leader:
- If the leader's term is lower than the server's term, the server signals the leader to step down.
4. Handle Higher Term Leader:
- If the leader's term is higher than the server's term, the server steps down if necessary and updates its term.
5. Handle Matching Terms:
- If the terms match, the server compares logs. It appends entries if the previous log entry matches; otherwise, it signals a step-down.
6. Handle Commit Index Update:
- Updates the commit index if necessary and updates the ticket pool based on committed BUY commands.
7. Signal Waiting Condition:
- If the server is in the FOLLOWER state and waiting, it signals the timeout condition.
8. Update State to Follower:
- If a step-down occurred, the server updates its state to FOLLOWER.
9. Lock Release:
- Releases acquired locks.
- Return:
- Returns a `ReturnValueRPC` object indicating the server's decision, term, and next index.
Notes:
- Both methods use a combination of locks to ensure thread safety and consistency.
- They handle various conditions based on the Raft algorithm's rules to maintain the integrity of the distributed system.
- Log entries are appended and committed, and the system progresses in terms of leadership and state transitions.
These methods are crucial for the correct functioning of the Raft consensus algorithm, ensuring that the distributed nodes agree on the state of the system and replicate logs accurately.
`setupConnection` Method:
This method is responsible for setting up the RMI (Remote Method Invocation) connection for the server.
- Functionality:
1. Registry Creation:
- Creates an RMI registry on the server's designated port (`SysFiles.getBaseServerPort() + this.server.getServerId()`).
2. Bind Server to Registry:
- Binds the `RPCInterface` object (implementing the remote interface) to the registry with the name "RPCInterface."
3. Log and Return:
- Logs the successful completion of server setup and readiness for connections.
- Returns `true` to indicate a successful setup.
`call` Method (Callable Interface Implementation):
This method is the entry point for a `HandleRPC` thread, responsible for handling different types of RPCs.
- Functionality:
1. Switch on RPC Type:
- Determines the type of RPC (`SUBMITCOMMAND`, `REQUESTVOTE`, or `APPENDENTRIES`) and executes the corresponding action.
2. Logging and RPC Execution:
- Logs the initiation of the RPC.
- Calls the appropriate method (`submitCommandRPC`, `requestVoteRPC`, or `appendEntriesRPC`) on the remote server.
3. Invalid RPC Type Handling:
- Logs an error if an invalid `RPCType` is received.
4. Logging and Return:
- Logs the exit from the method.
- Returns the result of the RPC execution (`ReturnValueRPC`).
Notes:
- The `setupConnection` method sets up the RMI registry and binds the server object to it, making the server available for remote method calls.
- The `call` method serves as the entry point for a thread that handles RPCs. It switches based on the RPC type and calls the appropriate method on the remote server.
- The `RPCInterface` object implements the remote interface and provides methods that can be invoked remotely.
- Proper logging is implemented to record the server's activities and communication with other servers.
These methods play a crucial role in establishing the server's connectivity and enabling it to handle various types of RPCs as part of the Raft consensus algorithm. The `setupConnection` method ensures that the server is ready to receive remote calls, while the `call` method handles different RPC
types and invokes the corresponding remote methods on other servers.
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