Description
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:
[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$)
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
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.
Suggested Fix
In ZookeeperLeaderElector.startup first run elect and then subscribe to data changes.
Attachments
Attachments
Issue Links
- relates to
-
KAFKA-1387 Kafka getting stuck creating ephemeral node it has already created when two zookeeper sessions are established in a very short period of time
- Resolved
-
KAFKA-1585 Client: Infinite "conflict in /consumers/"
- Resolved