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Status

Current state: Under Discussion

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The admin client allows users to instruct the controller to attempt to elect the preferred replica as leader on a given set of topic partitions. In addition to preferred replica leader elections the controller also supports three other types of elections. One of those other election types is what we call unclean leader election. For the user to enable and trigger unclean leader election they need to either modify  the topic configuration or the broker configuration. We believe that this is potentially a dangerous configuration that could lead to data loss. Specially if the user forgets to disable unclean leader election after it completes. Being able to trigger unclean leader election once for a given partition is a much safer mechanism. 

In this proposal we modify the “PreferredLeaderElection” RPC to also support unclean leader election in addition to preferred leader election. The new RPC definition also makes it possible to easily add new type of elections in the future.

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In addition to the protocol and client changes enumerated above, the Controller will be extended to allow unclean leader election requests to come from the admin client. Currently the controller only supports preferred leader election from the admin client. Unclean leader election can only be enabled through either a topic configuration change or a broker configuration change.

When performing unclean leader election on a topic partition through the admin client the "unclean leader election" topic configuration and broker configuration will be ignored. In the code this will result is code similar to the one below:

uncleanElectionFromAdminClient || logConfigs(partition.topic).uncleanLeaderElectionEnable.booleanValue()

Compatibility, Deprecation, and Migration Plan

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