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PurgeResponse => topics
  topics => [PurgeResponseTopic]
 
PurgeResponseTopic => topic partitions
  topic => str
  partitions => [PurgeResponsePartition]
 
PurgeResponsePartition => partition low_watermark error_code
  partition => int32
  low_watermark => int64
  error_code => int16

Add a log_beginstart_offset field to FetchRequestPartition

FetchRequestPartition => partition fetch_offset low_watermark max_bytes
  partition => int32
  fetch_offset => int64
  log_beginstart_offset => int64  <-- NEW. If it is issued from consumer, the value is -1. Otherwise, this is the log_beginstart_offset of this partition on the follower.
  max_bytes => int32

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

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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 log_beginstart_offset of a partition to 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 log_beginstart_offset will be checkpointed periodically similar to high_watermark to be persistent. 

Note that the way broker handles PurgeRequest is similar to how it handles ProduceRequest with ack = all and isr=all_live_replicas, e.g. the leader waits for all followers to catch up with its log_beginstart_offset, doesn't expose message below log_beginstart_offset, and checkpoints log_beginstart_offset periodically. The low_watermark of a partition will be the minimum log_beginstart_offset of all replicas of this partition and this value will be returned to user in PurgeResponse.

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3) After receiving the PurgeRequest, for each partition in the PurgeRequest, the leader first sets offsetToPurge to high_watermark if offsetToPurge is -1L. It then sets log_beginstart_offset of leader replica to max(log_beginstart_offset, offsetToPurge) if offsetToPurge <= high_watermark. Those segments whose largest offset < log_beginstart_offset will be deleted by the leader.

4) The leader puts the PurgeRequest into a DelayedOperationPurgatory. The PurgeRequest can be completed when results for all partitions specified in the PurgeRequest are available. The result of a partition will be available within RequestTimeoutMs and it is determined using the following logic:

• If log_beginstart_offset of this partition on all live followers is larger than or equal to the offsetToPurge, the result of this partition will be its low_watermark, which is the minimum log_beginstart_offset of all its live replicas.
• If high_watermark of this partition is smaller than the offsetToPurge, the result of this partition will be OffsetOutOfRangeException.
  
• If the leadership of this partition moves to another broker, the result of this partition will be NotLeaderException
• If the result of this partition is not available after RequestTimeoutMs, the result of this partition will be TimeoutException

5) The leader sends FetchResponse with its log_beginstart_offset to followers.

6) Follower sets replica's log_beginstart_offset to the max(log_beginstart_offset of leader, log_beginstart_offset of local replica). It also deletes those segments whose largest offset < log_beginstart_offset.

7) Follower sends FetchRequest with replica's log_beginstart_offset to the leader.

8) The leader updates log_beginstart_offset of each follower. If the PurgeRequest can be completed, the leader removes the PurgeRequest from DelayedOperationPurgatory and sends PurgeResponse with the results (i.e. low_watermark or error) for the specified set of partitions.

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- Broker will delete those segments whose largest offset < log_beginstart_offset.

- Only message with offset >= log_beginstart_offset can be sent to consumer.

- When a segment is deleted due to log retention, broker updates log_beginstart_offset to max(log_beginstart_offset, smallest offset in the replica's log)

- Broker will checkpoint log_beginstart_offset for all replicas periodically in the file "log-begin-offset-checkpoint", in the same way it checkpoints high_watermark of replicas. The checkpoint file will have the same format as existing checkpoint files which map TopicPartition to Long.

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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.

4) ListOffsetRequest

log_beginstart_offset of a partition will be used to decide the smallest offset of the partition that will be exposed to consumer. It will be returned when smallest_offset option is used in the ListOffsetRequest.

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