Accept any snapshot that allows replication #84
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This adds a test that documents the following behavior: It the leader is tracking a follower as StateSnapshot with PendingSnapshot equal to, say, 100, and the follower applies a snapshot at a lower index that reconnects the follower to the leader's log, then the leader will still ignore this snapshot.
Prior to this commit, the leader would not take into account snapshots reported by a follower unless they matched or exceeded the tracked PendingSnapshot index (which is the leader's last index at the time of requesting the snapshot). This is too inflexible: the leader should take into account any snapshot that reconnects the follower to its log. This PR makes that change. In doing so, it addresses long-standing problems that we've encountered in CockroachDB. Unless you create the snapshot immediately and locally when raft emits an MsgSnap, it's difficult/impossible to later synthesize a snapshot at the requested index. It is possible to get one above the requested index which raft always accepted, but CockroachDB delegates snapshots to followers who might be behind on applying the log, and it is awkward to have wait for log application to send the snapshot just to satisfy an overly strict condition in raft. Additionally, CockroachDB also sends snapshots preemptively when adding a new replica since there are qualitative differences between an initial snapshot and one needed to reconnect to the log and one does not want to wait for raft to round-trip to the follower to realize that a snapshot is needed. In this case, the sent snapshot is commonly behind the PendingSnapshot since the leader transitions the follower into StateProbe when a snapshot is already in flight. Touches cockroachdb/cockroach#84242. Touches cockroachdb/cockroach#87553. Touches cockroachdb/cockroach#87554. Touches cockroachdb/cockroach#97971. See also https://github.com/cockroachdb/cockroach/blob/2b91c3829270eb512c5380201c26a3d838fc567a/pkg/kv/kvserver/raft_snapshot_queue.go#L131-L143. Signed-off-by: Tobias Grieger <[email protected]>
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This was referenced Jul 18, 2023
This was referenced Nov 16, 2023
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Replaced by #110. |
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Prior to this commit, the leader would not take into account snapshots reported
by a follower unless they matched or exceeded the tracked PendingSnapshot index
(which is the leader's last index at the time of requesting the snapshot). This
is too inflexible: the leader should take into account any snapshot that
reconnects the follower to its log. This PR makes that change.
In doing so, it addresses long-standing problems that we've encountered in
CockroachDB. Unless you create the snapshot immediately and locally when raft
emits an MsgSnap, it's difficult/impossible to later synthesize a snapshot at
the requested index. It is possible to get one above the requested index which
raft always accepted, but CockroachDB delegates snapshots to followers who
might be behind on applying the log, and it is awkward to have wait for log
application to send the snapshot just to satisfy an overly strict condition in
raft. Additionally, CockroachDB also sends snapshots preemptively when adding a
new replica since there are qualitative differences between an initial snapshot
and one needed to reconnect to the log and one does not want to wait for raft
to round-trip to the follower to realize that a snapshot is needed. In this
case, the sent snapshot is commonly behind the PendingSnapshot since the leader
transitions the follower into StateProbe when a snapshot is already in flight.
Touches cockroachdb/cockroach#84242.
Touches cockroachdb/cockroach#87553.
Touches cockroachdb/cockroach#87554.
Touches cockroachdb/cockroach#97971.
See also https://github.com/cockroachdb/cockroach/blob/2b91c3829270eb512c5380201c26a3d838fc567a/pkg/kv/kvserver/raft_snapshot_queue.go#L131-L143.