Add an API that blocks until logs replay is done if save_committed() is implemented

[SteveLauC]

Is your feature request related to a problem? Please describe.

Hi, I would like to know is there any API that can be used to wait for a node to replay its logs upon restart. This is something a user can implement with committed_id (RaftState.committed) and applied_id (RaftStateMachine.applied), but maybe we can add it if it is a common request (Well, it does not seem to be as searching for "replay log" gives me nothing😁)

Describe the solution you'd like

We can:

1. Get the committed log index
2. Then wait for the applied log index to become greater than or equal to it

Describe alternatives you've considered

I have noticed the Raft::ensure_linearizable() method, which does something similar under the hood, but considering that it invokes Raft::get_read_log_id(), looks like it is only suitable for the leader node.

Additional context

No

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[drmingdrmer]

Raft::new() will block until all committed log entries applied, if committed_log_id is saved:
https://github.com/datafuselabs/openraft/blob/5d75cf21f71b083b2b8acd8ffdd8993c1bfce36f/openraft/src/storage/helper.rs#L97

[schreter]

We did it by simply awaiting until the node becomes the leader in a separate task and then await until applied index reaches the committed index (via the watch on Raft state). Only then it's clear that the (leader's) log is replayed and we can serve further requests.

This is basically the same that @SteveLauC describes above. Is there something wrong with this approach?

Also, if the restarted node became a follower, then no one really cares how far the apply went, since it can't really serve requests anyway. And, if the leader dies and the follower becomes leader, again, the leader awaiting task on the former follower now leader will get notified and the process repeats on the follower. But, we don't need to wait until Raft::new() applies all the entries - we shouldn't wait there either, IMHO.

[drmingdrmer]

• Get the committed log index
• Then wait for the applied log index to become greater than or equal to it

This approach aligns with my suggestion. Currently, only the caller of client_write() that proposes the log entry is notified of its completion. This notification occurs when client_write() returns, indicating the entry has been applied.

[SteveLauC]

Thanks for the replys, also thanks for the code link so that I can dig it further, close as the feature is not needed.