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FetchResponsePartitionHeader => partition error_code high_watermark low_watermark
  partition => int32
  error_code => int16
  high_watermark => int64
  low_watermark => int64  <-- NEW. This is the low_watermark of this partition on the leader.

3) Checkpoint file

We create one more checkpoint file, named "replication-low-watermark-checkpoint", in every log directory. The checkpoint file will have the same format as existing checkpoint files (e.g. replication-offset-checkpoint) which map TopicPartition to Long.

Proposed Changes

The idea is to add new APIs in Admin Client (see KIP-4) that can be called by user to purge data that is no longer needed. New request and response needs to be added to communicate this request between client and broker. Given the impact of this API on the data, the API should be protected by Kafka’s authorization mechanism described in KIP-11 to prevent malicious or unintended data deletion. Furthermore, we adopt the soft delete approach because it is expensive to purge data in the middle of a segment. Those segments whose maximum offset < offset-to-purge can be deleted safely. Brokers can increment low_watermark of a partition above offset-to-purge so that data with offset < offset-to-purge will not be exposed to consumer even if it is still on the disk. And the low_watermark will be checkpointed periodically similar to high_watermark to be persistent. 

...

- Broker will checkpoint low_watermark for all replicas periodically in the file "replication-low-watermark-checkpoint", in the same way it checkpoints high_watermark of replicas. The checkpiont file will have the same format as existing checkpoint files which map TopicPartition to Long.

3) API Authorization

Given the potential damage that can be caused if this API is used by mistake, it is important that we limit its usage to only authorized users. For this matter, we can take advantage of the existing authorization framework implemented in KIP-11. purgeDataBefore() will have the same authorization setting as deleteTopic(). Its operation type is be DELETE and its resource type is TOPIC.

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