Bug in FastLeaderElection
-------------------------
Key: ZOOKEEPER-275
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-275
Project: Zookeeper
Issue Type: Bug
Components: leaderElection
Reporter: Flavio Paiva Junqueira
Assignee: Flavio Paiva Junqueira
I found an execution in which leader election does not make progress. Here is
the problematic scenario:
- We have an ensemble of 3 servers, and we start only 2;
- We let them elect a leader, and then crash the one with lowest id, say S_1
(call the other S_2);
- We restart the crashed server.
Upon restarting S_1, S_2 has its logical clock more advanced, and S_1 has its
logical clock set to 1. Once S_1 receives a notification from S_2, it notices
that it is in the wrong round and it advances its logical clock to the same
value as S_1. Now, the problem comes exactly in this point because in the
current code S_1 resets its vote to its initial vote (its own id and zxid).
Since S_2 has already notified S_1, it won't do it again, and we are stuck. The
patch I'm submitting fixes this problem by setting the vote of S_1 to the one
received if it satisfies the total order predicate ("received zxid" is higher
or "received zxid is the same and received id is higher").
Related to this problem, I noticed that by trying to avoid unnecessary
notification duplicates, there could be scenarios in which a server fails
before electing a leader and restarts before leader election succeeds. This
could happen, for example, when there isn't enough servers available and one
available crashes and restarts. I fixed this problem in the attached patch by
allowing a server to send a new batch of notifications if there is at least one
outgoing queue of pending notifications empty. This is ok because we space out
consecutive batches of notifications.
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