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https://issues.apache.org/jira/browse/KAFKA-992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13741179#comment-13741179
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Neha Narkhede commented on KAFKA-992:
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Overall, v11 is a good refactor. Few minor formatting comments -
1. Broker
I think getZkString can be removed. This is a nice to have clean up item, not
introduced in your patch
2. ZkUtils
2.1 Can we break the long log line that says "A broker is already registered..."
2.2 Typo in the comments above createEphemeralPathExpectConflictHandleZKBug()
=> NodeExistEception
3. KafkaController
Typo => zkSessionTimout
4. Controller
Can we add back the following statement in warn. It was helpful for me to know
this while testing a cluster upgrade with this patch -
warn("Failed to parse the controller info as json. " +
"Probably this controller is still using the old format [%s] of
storing the broker id in the zookeeper path".format(controller))
> 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.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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