[KAFKA-1451] Broker stuck due to leader election race

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 0.8.1.1
• Fix Version/s: 0.8.2.0
• Component/s: core
• Labels: newbie
• Assignee: Manikumar
• Reporter: Maciek Makowski
• Created: 14/May/14 10:52
• Updated: 19/Nov/15 17:43
• Resolved: 30/Jul/14 15:15

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:

1. start both zookeeper and kafka (in any order)
2. stop zookeeper
3. stop kafka
4. start kafka
5. 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.

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
• relates to: KAFKA-1585 Client: Infinite "conflict in /consumers/"