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https://issues.apache.org/jira/browse/KAFKA-992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13742356#comment-13742356
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Guozhang Wang commented on KAFKA-992:
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Thanks for the comments Jun.
114. Agreed, deleted the Controller.scala and moved logic to KafkaController
object.
120. I thought ZookeeperLeaderElector.resign() is a public function that can be
called by the parent process of the Elector. Currently ZookeeperLeaderElector
is dependent on KafkaController (it takes controllerContext as its parameters),
but I think it would be refactored in the future as an independent election
module?
121. In this case what really happens is that another broker has elected as the
leader but somehow gets "resigned". This will trigger another election round.
So instead of log an error we would better log it as warn and set leaderId to
-1?
> Double Check on Broker Registration to Avoid False NodeExist Exception
> ----------------------------------------------------------------------
>
> Key: KAFKA-992
> URL: https://issues.apache.org/jira/browse/KAFKA-992
> Project: Kafka
> Issue Type: Bug
> Reporter: Neha Narkhede
> Assignee: Guozhang Wang
> Attachments: KAFKA-992.v10.patch, KAFKA-992.v11.patch,
> KAFKA-992.v12.patch, KAFKA-992.v13.patch, KAFKA-992.v1.patch,
> KAFKA-992.v2.patch, KAFKA-992.v3.patch, KAFKA-992.v4.patch,
> KAFKA-992.v5.patch, KAFKA-992.v6.patch, KAFKA-992.v7.patch,
> KAFKA-992.v8.patch, KAFKA-992.v9.patch
>
>
> The current behavior of zookeeper for ephemeral nodes is that session
> expiration and ephemeral node deletion is not an atomic operation.
> The side-effect of the above zookeeper behavior in Kafka, for certain corner
> cases, is that ephemeral nodes can be lost even if the session is not
> expired. The sequence of events that can lead to lossy ephemeral nodes is as
> follows -
> 1. The session expires on the client, it assumes the ephemeral nodes are
> deleted, so it establishes a new session with zookeeper and tries to
> re-create the ephemeral nodes.
> 2. However, when it tries to re-create the ephemeral node,zookeeper throws
> back a NodeExists error code. Now this is legitimate during a session
> disconnect event (since zkclient automatically retries the
> operation and raises a NodeExists error). Also by design, Kafka server
> doesn't have multiple zookeeper clients create the same ephemeral node, so
> Kafka server assumes the NodeExists is normal.
> 3. However, after a few seconds zookeeper deletes that ephemeral node. So
> from the client's perspective, even though the client has a new valid
> session, its ephemeral node is gone.
> This behavior is triggered due to very long fsync operations on the zookeeper
> leader. When the leader wakes up from such a long fsync operation, it has
> several sessions to expire. And the time between the session expiration and
> the ephemeral node deletion is magnified. Between these 2 operations, a
> zookeeper client can issue a ephemeral node creation operation, that could've
> appeared to have succeeded, but the leader later deletes the ephemeral node
> leading to permanent ephemeral node loss from the client's perspective.
> Thread from zookeeper mailing list:
> http://zookeeper.markmail.org/search/?q=Zookeeper+3.3.4#query:Zookeeper%203.3.4%20date%3A201307%20+page:1+mid:zma242a2qgp6gxvx+state:results
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