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https://issues.apache.org/jira/browse/KAFKA-992?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13734994#comment-13734994
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Guozhang Wang commented on KAFKA-992:
-------------------------------------
Thanks for the comments Jun. I think the re-registering logic is slightly
different for broker, controller and consumer:
Broker: need to check hostname + port
Controller: only need to check brokerId
Consumer: need not check anything since the consumer info like hostname and
port is encoded in the ZkPath.
So I think it is hard to unify consumer's logic with broker and controller's
logic; it is possible to unify the broker and controller's logic though, by
passing the list of json fields that we need to check. But I am not sure it
worth the effort.
> 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,
> KAFKA-992.v6.patch, KAFKA-992.v7.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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