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https://issues.apache.org/jira/browse/KAFKA-1451?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14068193#comment-14068193
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Jun Rao commented on KAFKA-1451:
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Neha, I am not sure if #1 is need. We can get into elect from two paths (1)
from startup or (2) from handleDeleted. If it's from startup, we already
register the watcher before calling elect. If it's from handleDeleted, it means
that the watcher must have already been registered. So, once in elect, we know
the watcher is already registered. So if after we check the existence of the
controller node and the controller node goes away immediately afterward, the
watcher is guaranteed to be triggered.
> Broker stuck due to leader election race
> -----------------------------------------
>
> Key: KAFKA-1451
> URL: https://issues.apache.org/jira/browse/KAFKA-1451
> Project: Kafka
> Issue Type: Bug
> Components: core
> Affects Versions: 0.8.1.1
> Reporter: Maciek Makowski
> Priority: Minor
> Labels: newbie
>
> h3. Symptoms
> The broker does not become available due to being stuck in an infinite loop
> while electing leader. This can be recognised by the following line being
> repeatedly written to server.log:
> {code}
> [2014-05-14 04:35:09,187] INFO I wrote this conflicted ephemeral node
> [{"version":1,"brokerid":1,"timestamp":"1400060079108"}] at /controller a
> while back in a different session, hence I will backoff for this node to be
> deleted by Zookeeper and retry (kafka.utils.ZkUtils$)
> {code}
> h3. Steps to Reproduce
> In a single kafka 0.8.1.1 node, single zookeeper 3.4.6 (but will likely
> behave the same with the ZK version included in Kafka distribution) node
> setup:
> # start both zookeeper and kafka (in any order)
> # stop zookeeper
> # stop kafka
> # start kafka
> # start zookeeper
> h3. Likely Cause
> {{ZookeeperLeaderElector}} subscribes to data changes on startup, and then
> triggers an election. if the deletion of ephemeral {{/controller}} node
> associated with previous zookeeper session of the broker happens after
> subscription to changes in new session, election will be invoked twice, once
> from {{startup}} and once from {{handleDataDeleted}}:
> * {{startup}}: acquire {{controllerLock}}
> * {{startup}}: subscribe to data changes
> * zookeeper: delete {{/controller}} since the session that created it timed
> out
> * {{handleDataDeleted}}: {{/controller}} was deleted
> * {{handleDataDeleted}}: wait on {{controllerLock}}
> * {{startup}}: elect -- writes {{/controller}}
> * {{startup}}: release {{controllerLock}}
> * {{handleDataDeleted}}: acquire {{controllerLock}}
> * {{handleDataDeleted}}: elect -- attempts to write {{/controller}} and then
> gets into infinite loop as a result of conflict
> {{createEphemeralPathExpectConflictHandleZKBug}} assumes that the existing
> znode was written from different session, which is not true in this case; it
> was written from the same session. That adds to the confusion.
> h3. Suggested Fix
> In {{ZookeeperLeaderElector.startup}} first run {{elect}} and then subscribe
> to data changes.
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