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https://issues.apache.org/jira/browse/KAFKA-8304?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16829648#comment-16829648
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Chris Egerton commented on KAFKA-8304:
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Looks like performing REST extension initialization in a separate thread is not
the right approach. If an extension aims to restrict the REST API for security
reasons, it's possible that a blocking call to
{{ConnectClusterState.connectors()}} would prevent the extension from
registering any relevant filters between the time that the REST server starts
and when the CLI (standalone or distributed) invokes
{{HerderProvider.setHerder(...)}}.
It also appears that, due to the nature of the code changes to support the new
integration testing harness, providing a {{Herder}} instance to
{{ConnectClusterStateImpl}} instances before they're passed to REST may not be
possible at all, or at least not without considerable refactoring.
> Connect susceptible to deadlock while registering REST extensions
> -----------------------------------------------------------------
>
> Key: KAFKA-8304
> URL: https://issues.apache.org/jira/browse/KAFKA-8304
> Project: Kafka
> Issue Type: Bug
> Components: KafkaConnect
> Affects Versions: 2.2.0, 2.1.1, 2.0.2, 2.3.0, 2.1.2, 2.2.1
> Reporter: Chris Egerton
> Assignee: Chris Egerton
> Priority: Blocker
>
> As part of KAFKA-7503, the {{ConnectClusterStateImpl}} class was altered to
> use a {{HerderProvider}} instance instead of a {{Herder}}. However, the
> Connect {{RestServer}} registers REST extensions before a herder is given to
> that {{HerderProvider}}, so any extensions that invoke, e.g.,
> {{ConnectClusterState.connector()}} in their {{register(...)}} method end up
> in a deadlock that eventually causes Connect startup to fail with the error
> message "Timed out waiting for herder to be initialized."
> If possible, the {{HerderProvider}} used for {{ConnectClusterStateImpl}}
> instances given to REST extensions should be supplied with a {{Herder}}
> before those extensions are registered. If that isn't feasible, another
> option could be to install Connect REST extensions on a separate thread so
> that they don't block the Connect startup process and eventual call of
> {{HerderProvider.setHerder(...)}}.
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