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In the post-KIP-500 world, controller shutdown is handled by the broker heartbeat system instead.  In its periodic heartbeats, the broker asks the controller if it can transition into the controlled shutdown state.  It does this by setting the ShouldShutdown boolean.  This motivates the controller to move all of the leaders off of that broker.  Once they are all moved, the controller responds to the heartbeat with a nextState of SHUTDOWNControlledShutdownOk = true.  At that point, the broker knows it's safe to begin the shutdown process proper.

The Broker State Machine

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