Allow entries to apply during snapshot #86
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That makes a lot of sense. For example, you could isolate a key value store FSM when it's being snapshotted by iterating over the keys and being aware of log entries setting values. However, I like that the default behaviour is so strict. It makes it easier to not break things. Therefore I think a good way to allow this is by adding flags to #define RAFT_SNAPSHOT_NONBLOCKING_APPLY 1
int raft_begin_snapshot(raft_server_t *me_, int flags); |
If specified when calling raft_begin_snapshot(), the library will not postpone applying entries during snapshots. This requires the FSM to provide full isolation between the snapshot and active state.
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Good optimisation for FSMs that can use this |
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Currently
raft_apply_entry()andraft_apply_all()will avoid applying entries if a snapshot is in progress. Assuming the FSM is able to provide the proper isolation, I don't see any real reason to prevent this in the library but @willemt perhaps you do?This requires care in maintaining the different indexes, but we anyway must assume the commit index can advance during snapshots (see #84 for example).