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From "Chris Thunes (JIRA)" <j...@apache.org>
Subject [jira] [Created] (ZOOKEEPER-2985) Expired session may unexpired after leader failover
Date Thu, 22 Feb 2018 17:00:00 GMT
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.



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