AppendEntry

AppendEntry is a crucial part of the Raft consensus algorithm. It is a log replication RPC (Remote Procedure Call) invoked by a leader to replicate entries to a follower. The primary purpose of AppendEntry is to ensure that followers have the same log as the leader. It also serves as a heartbeat message to inform followers that the leader is still active and hasn't crashed.

The AppendEntry RPC contains several key pieces of information. It includes the leader's current term, the leader ID, the index of the log entry immediately preceding the new entries being sent, the term of that preceding log entry, a list of new log entries to store (if any), and the leader's commit index.

When a follower receives an AppendEntry RPC, it first checks for consistency. It verifies that its own log contains an entry at the specified previous log index with a matching term. If this check fails, it means the follower's log has diverged from the leader's, and the RPC is rejected. If the check succeeds, the follower appends any new log entries from the leader to its own log. It then updates its commit index to be the minimum of the leader's commit index and the index of the last new entry appended.

If the AppendEntry RPC includes no new log entries, it functions purely as a heartbeat. This mechanism is vital for maintaining leader stability, as followers will time out and initiate a new election if they don't receive heartbeats from the leader within a certain period. The success or failure of an AppendEntry RPC provides the leader with feedback on the state of its followers' logs, enabling it to determine when an entry can be safely committed.