raft: send up-to-date commit index in heartbeats #140
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This change makes the commit index advancement in handleHeartbeat safe. Previously, a follower would attempt to update the commit index to whichever was sent in the MsgHeartbeat message. Out-of-bound indices would crash the node. It is always safe to advance a commit index if the follower's log is "in sync" with the leader, i.e. when its log is guaranteed to be a prefix of the leader's log. This becomes true when the first MsgApp append message succeeds. At the moment, the leader will never send a commit index that exceeds the follower's log size. However, this may change in future. This change is a defence-in-depth. Signed-off-by: Pavel Kalinnikov <[email protected]>
This commit adds a test demonstrating the effect of delayed commit on a follower node after a network hiccup between the leader and this follower. In the described scenario, after the moment of committing an entry on the leader, it takes HeartbeatInterval + 3/2 * RTT until the follower learns this entry is committed. This is suboptimal, and could take HeartbeatInverval + 1/2 * RTT if the leader didn't cut the commit index at Progress.Match before sending it to the follower. Signed-off-by: Pavel Kalinnikov <[email protected]>
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TODO: describe why it is now safe Signed-off-by: Pavel Kalinnikov <[email protected]>
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Fixes #138