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Today, the client relies on the ApiVersions{Request|Response} to determine at a per-request level which version of request should be used to send to the brokers. This is sufficient for client-side logic that only relies on the specific versions of request, however for other logic that requires client-side behavior to be different as well, such request-level ApiVersions check would not be sufficient. Detailed examples can be found below.

Feature Gating

Development in Kafka (from a high level) is organized into features, each of which is tracked by a separate version number. Feature gating tries to answer the question, How does the broker decide what features to enable?  For example, When is it safe for the brokers to begin handling EOS traffic? Should the controller send some additional information when it sends out LeaderAndIsrRequest? Should the broker write its log data in the "new" message format or one of the "old" ones? 

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