Flavio Paiva Junqueira updated ZOOKEEPER-275:

    Attachment: ZOOKEEPER-275.patch

> Bug in FastLeaderElection
> -------------------------
>                 Key: ZOOKEEPER-275
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-275
>             Project: Zookeeper
>          Issue Type: Bug
>          Components: leaderElection
>    Affects Versions: 3.0.0, 3.0.1
>            Reporter: Flavio Paiva Junqueira
>            Assignee: Flavio Paiva Junqueira
>             Fix For: 3.1.0
>         Attachments: ZOOKEEPER-275.patch, ZOOKEEPER-275.patch
> 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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