Maciek Makowski created KAFKA-1451:
--------------------------------------
Summary: 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
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 sinle 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
* {{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.
--
This message was sent by Atlassian JIRA
(v6.2#6252)
