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https://issues.apache.org/jira/browse/HELIX-548?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14323117#comment-14323117
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Zhen Zhang commented on HELIX-548:
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There is a timing issue in ZkHelixParticipant#setupMsgHandler(). We should hook
up ZK callback (line 347 in
https://github.com/apache/helix/blob/master/helix-core/src/main/java/org/apache/helix/manager/zk/ZkHelixParticipant.java)
after all message handler registrations are done (line 354 in
https://github.com/apache/helix/blob/master/helix-core/src/main/java/org/apache/helix/manager/zk/ZkHelixParticipant.java).
Fix is to move adding ZK callback to the end. Will add a test case that can
reliably reproduce this issue.
> race condition in Helix message handler factory registration
> ------------------------------------------------------------
>
> Key: HELIX-548
> URL: https://issues.apache.org/jira/browse/HELIX-548
> Project: Apache Helix
> Issue Type: Bug
> Reporter: Zhen Zhang
>
> -- copied from email --
> I have a small issue when running a Helix cluster in Docker (all on the same
> machine, very small RTT):
> I register two message handler factories, similarly to the way it is done in
> BootstrapProcess.java example. The manager is not yet connected. My
> understanding is that this places the handlers in a hashmap from which they
> will be read and registered later, upon connect.
> Upon connect, I see that the DefaultMessagingService.onConnected function is
> called and Helix starts registering the message handlers. However, messages
> start arriving already and HelixTaskExecutor.createMessageHandler is called,
> before all handlers have been registered. The result is that I get
> NullPointerExceptions for the missing handlers in the following line of
> HelixTaskExecutor.createMessageHandler:
> MessageHandlerFactory handlerFactory = item.factory();
> due to the fact that there is no item (MessageHandlerFactoryRegistryItem) for
> that message type registered yet.
> I came upon this bug after updating the git code from an old snapshot of
> 0.7.1. I saw that the relevant code changes are from around the summer.
> Regards,
> Vlad
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