CODEWITHSUNDEEP
consensusraft
Ryan
Ryan

Reputation: 1242

When does the Raft consensus algorithm apply the commit log to the leader and followers?

In RAFT, I understand that a leader receives a request and federates it out to it's peer list to commit to their respective logs.

My question is, is there a distinction between committing the action to the log and actually applying the action? If the answer is yes, then at what point does the action get applied?

My understanding is once the leaders receives, from the majority - "hey I wrote this to my log", the leader applies the change then federates an "Apply" command to the peers that wrote the change to their respective logs and then the ack is sent to the client.

Upvotes: 1

Views: 1108

Answers (1)

kuujo
kuujo

Reputation: 8185

I would say there is a distinction between committing an entry and applying it to the state machine. Once an entry is committed (i.e. the commitIndex is >= the entry index) it can be applied at any time. In practice, you want the leader to apply committed entries as soon as possible to reduce latency, so entries will usually be applied to an in-memory state machine immediately.

In the case of in-memory state machines the distinction is not very obvious. But it’s the other use cases for Raft that do necessitate this distinction. For example, the distinction becomes particularly important with persistent state machines. If the state machine is persisting changes to e.g. an underlying database, it’s critical that each entry only be applied to the state machine once so that the underlying store does not go back in time when the node replays entries to the state machine when recovering from a failure. To make persistent state machines idempotent, the state machine on each node needs to persist the entries that have been applied on that node as part of the persistent state. In this case, the process of applying entries is indeed a distinction, and a critical one.

State machine replication is also only one use case for Raft. There are others as well. It’s perfectly valid, for example, to use the protocol for simple log replication, in which case entries wouldn’t be applied at all.

Upvotes: 2

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