Log Compaction and Client Interaction in Raft

Log compaction

In this lesson, we’ll elaborate on the need for log compaction and how Raft implements this in this algorithm.

The need for log compaction

As the Raft algorithm processes more client requests, its log expands to incorporate them. However, it’s not feasible for the log to keep growing indefinitely because it would consume more space and take longer to replay, potentially leading to availability issues. To prevent this, the Raft algorithm must save its progress in a compact form and discard obsolete information accumulated in the log.

Snapshotting

One way to do this is through snapshotting, where the system’s current state is saved to stable storage, and the entire log up to that point is deleted. ((It is important to note that log slots contain clients’ requests, which are applied to the state machine once slot entries have been committed. Once applied to the state machine, those log entries might not be needed. If we store the state machine’s state in the snapshot, we can discard that part of the log that has been applied to the state machine.)

The following illustration shows the basic idea of snapshotting in the Raft consensus algorithm:

The Raft consensus algorithm allows each server to take independent snapshots covering only the committed entries in its log. The state machine is responsible for writing its current state to the snapshot, which is ...