Why does the RocksDB example flush wal in apply_sm() vs append_entries()

[avantgardnerio]

I've observed in the RocksDB example code, that the WAL is not flushed during append_to_log().

It is, however, flushed during apply_to_state_machine(), which I found somewhat counter-intuitive.

Is it possible to explain (or point to docs) about what happens during a node crash & recovery? We are designing our persistent storage, and of course understanding these topics become very important to that design.

PS: I assume upon startup the first method called is last_applied_state() to see if the node is pristine or not? and if that state has an old raft log index as last_applied then the node will get all the intervening logs from it's peers and try to re-apply them? If that's true, why is it important to flush in apply_to_state_machine() if the logs will be re-applied anyway?

Sorry for all the questions, thanks for your help 🙌

[drmingdrmer]

This issue and the closing PR #619 should have answered part of your question:

• flush after write in rocksdb example #618

PS: I assume upon startup the first method called is last_applied_state() to see if the node is pristine or not?

Right.
The first step is to build a RaftState with the return value from last_applied_state() and other method:
https://github.com/datafuselabs/openraft/blob/cb2985ad6af8b0097e1842d4025f9532e1664fbd/openraft/src/raft_state.rs#L15

and if that state has an old raft log index as last_applied then the node will get all the intervening logs from it's peers and try to re-apply them?

Whether to apply these log has to be left to openraft to decide: If a leader has committed these log, the leader will send a committed-log-id to all nodes then every node calls apply_to_state_machien() with the logs that are smaller than the committed-log-id.

If that's true, why is it important to flush in apply_to_state_machine() if the logs will be re-applied anyway?

It does not have to:

• Question: There is no need to rebuild the state machine when a node restarts. #620