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The FetchRequest schema has field for the replica id. Consumers typically use the sentinel -1, which indicates that fetching is only allowed from the leader. A lesser known sentinel is -2, which was originally intended to be used for debugging and allows fetching from followers. We propose to let the consumer use this to indicate the intent to allow fetching from a follower. Similarly, when we need to send a ListOffsets request to a follower in order to find the log start offset, we will follow a similar approachuse the same sentinel for the replica id field.

It is still necessary, however, to bump the version of the Fetch request because of the new error codes. The request and response schemas will be unchanged, but instead of returning OFFSET_OUT_OF_RANGE following an out of range error, the broker will return the more specific FETCH_OFFSET_TOO_LARGE and FETCH_OFFSET_TOO_SMALL error codes.

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