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From "Ramnatthan Alagappan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (KAFKA-1120) Controller could miss a broker state change
Date Mon, 04 Dec 2017 19:22:00 GMT

    [ https://issues.apache.org/jira/browse/KAFKA-1120?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16277309#comment-16277309

Ramnatthan Alagappan commented on KAFKA-1120:

My way to constantly reproduce this issue might be not so correct or desired. However, here
are the "steps". The key problem is with the broker registering with ZK before two events
on the controller: 1. ZKClient re-registering callback for /brokers/ids (the callback is initially
issued for the deletion of the broker), 2. ZKClient getting the data of /brokers/ids to check
what has changed. This code is done in the fireChildChangedEvents function in the ZKClient
library. To consistently trigger this issue, I patched ZkClient with a sleep inside this function:
Thread.sleep(5000);  exists(path);  List<String> children = getChildren(path). With
this added delay, the deletion handler would sleep before re-registering the callback. If
I restart the shutdown broker during this sleep, I see that the restarted broker would never
appear in newBrokerIds. 

> Controller could miss a broker state change 
> --------------------------------------------
>                 Key: KAFKA-1120
>                 URL: https://issues.apache.org/jira/browse/KAFKA-1120
>             Project: Kafka
>          Issue Type: Sub-task
>          Components: core
>    Affects Versions: 0.8.1
>            Reporter: Jun Rao
>            Assignee: Mickael Maison
>              Labels: reliability
>             Fix For: 1.1.0
> When the controller is in the middle of processing a task (e.g., preferred leader election,
broker change), it holds a controller lock. During this time, a broker could have de-registered
and re-registered itself in ZK. After the controller finishes processing the current task,
it will start processing the logic in the broker change listener. However, it will see no
broker change and therefore won't do anything to the restarted broker. This broker will be
in a weird state since the controller doesn't inform it to become the leader of any partition.
Yet, the cached metadata in other brokers could still list that broker as the leader for some
partitions. Client requests routed to that broker will then get a TopicOrPartitionNotExistException.
This broker will continue to be in this bad state until it's restarted again.

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