[jira] [Created] (ZOOKEEPER-2985) Expired session may unexpired after leader failover

["Chris Thunes (JIRA)" <ji...@apache.org>]

Chris Thunes created ZOOKEEPER-2985:
---------------------------------------

             Summary: Expired session may unexpired after leader failover
                 Key: ZOOKEEPER-2985
                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2985
             Project: ZooKeeper
          Issue Type: Bug
    Affects Versions: 3.4.11, 3.5.3
            Reporter: Chris Thunes


We recently observed an inconsistency in our Kafka cluster which we tracked down to ZooKeeper sessions expiring and then re-appearing after a ZooKeeper leadership failover. The Kafka nodes received session "Expired" events, leading to them starting new sessions and attempting to re-create some ephemeral nodes (broker ID nodes in kafka/brokers/ids specifically). However, between receiving the session Expired event and establishing a new session a leadership failover occurred within the ZooKeeper cluster which resulted in the expired session re-appearing. When Kafka attempted to re-create the ephemeral nodes mentioned above it (unexpectedly) received NODEEXISTS errors.

This behavior is a result of how session expiration is handled by the leader. Specifically, the expired session is marked as "closing" immediately upon expiration (in SessionTrackerImpl) and _before_ the corresponding "closeSession" entry is committed. A client can therefore receive a session Expired event before its session is fully closed. A leadership failover which results in the loss of the (uncommitted) closeSession entry thus leads to the sessions' ephemeral nodes "re-appearing" until another expiration of the old session on the new leader takes place.

I'm not certain if this should be considered a bug or an edge case that client are expected to handle. If it is the latter then I think it would be good to include this in the Programmer's Guide in the documentation.

If it's helpful I have code to reproduce this on an in-process cluster running 3.4.11 or 3.5.3-beta.



--
This message was sent by Atlassian JIRA
(v7.6.3#76005)