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Solution: This window is very small (milli seconds), and typically rare – it can only happen in a rare case where an incompatible broker comes up in the cluster around the time that a feature version upgrade is finalized. Here is how we will handle the race condition: In the controller, the thread that handles the ApiKeys.UPDATE_FEATURES request will be the ControllerEventThread. This is also the same thread that updates the controller's cache of Broker info whenever a new broker joins the cluster. In this setup, if an ApiKeys.UPDATE_FEATURES request (E1) is processed ahead of a notification from ZK about an incompatible broker joining the cluster (E2), then the controller can certainly detect the incompatibility when it processes E2 after E1. The controller could stop block the remaining of the new broker startup sequence by refusing to send an UpdateMetadataRequest to bootstrap the new broker. Then, it is only a matter of time (milli seconds) before the new broker receives a ZK notification (E3) about a change to '/features' node, then automatically shuts itself down due to the incompatibility.

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