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https://issues.apache.org/jira/browse/KAFKA-992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13732132#comment-13732132
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Neha Narkhede edited comment on KAFKA-992 at 8/7/13 4:26 PM:
-------------------------------------------------------------
Thanks for the follow up patch. The changes to consumer look good. I have a few
concerns about the changes to controller -
1. ZookeeperLeaderElector
1.1 This change is backwards incompatible. Unfortunately, when we versioned the
zookeeper data, we left out the controller path. So we have to handle both the
previous format and the new format in the code until the old format can be
phased out. This will be hacky but we cannot accept this patch without handling
this correctly, since that would require downtime at relase
1.2 We have moved to using json for zookeeper data. It will be good if we can
follow that while making this change to the controller path
1.3 The while loop has a lot of return statements. How about refactoring it to
have while(!writeSucceeded) {} and keep the return amILeader at the very end ?
was (Author: nehanarkhede):
Thanks for the follow up patch. The changes to consumer look good. I have a
few concerns about the changes to controller -
1. ZookeeperLeaderElector
1.1 This change is backwards incompatible. Unfortunately, when we versioned the
zookeeper data, we left out the controller path. So we have to handle both the
previous format and the new format in the code until the old format can be
phased out. This will be hacky but we cannot accept the change since that would
require downtime at relase
1.2 We have moved to using json for zookeeper data. It will be good if we can
follow that while making this change to the controller path
1.3 The while loop has a lot of return statements. How about refactoring it to
have while(!writeSucceeded) {} and keep the return amILeader at the very end ?
> 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.v1.patch, KAFKA-992.v2.patch,
> KAFKA-992.v3.patch, KAFKA-992.v4.patch, KAFKA-992.v5.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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