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Now that the RPCs in-use by a broker or controller can change at runtime (due to changing metadata.version), we will need a way to inform a node's remote clients that new RPCs are available. Brokers will be able to observe changes to metadata.version by observing as they replicate records from the metadata log, and could then submit a new ApiVersionsRequest to the other Kafka nodes. In order for clients to learn about new broker RPCs, closing the connection would trigger a new ApiVersionsRequest to be sent by the client. If a new metadata.version is seen, brokers will renegotiate compatible RPCs with other brokers through the the ApiVersions workflow. This will allow for new RPCs to be put into effect without restarting the brokers.

Since clients have no visibility to changes in metadata.version, the only mechanism we have for updating the negotiated ApiVersions is connection establishment. By closing the broker side of the connection, clients would be forced to reconnect and receive an updated set of ApiVersions. We may want to investigate alternative approaches here in a future KIP.

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