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The current implementation always includes the HWM in the FETCH responseFETCH response, this will stay true in this KIP. What this KIP changes is when the FETCH request is FETCH request may be parked and when the FETCH request is FETCH request may be completed when parked.

If the response is empty and the remote replica's HWM is greater than or equal to the leader's HWM, the FETCH request is parked.

When the leader's HWM changes, the leader will may complete all parked FETCH requestsFETCH requests.

Replica manager

The current implementation parks the FETCH request if all of these conditions are true: fetch request has a wait time, fetch request requires data, fetch request doesn't have enough data to respond, no errors happened while reading data, no diverging epoch and no preferred read replica.

In addition to these conditions, the replica manager will may also park fetch requests if the remote replica's HWM is greater than or equal to the local replica's HWM.

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