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1. 1. Force a roll of non-empty active segment if the first record is older than "max.compaction.lag.ms"  so that compaction can be done on that segment.  The time to roll an active segments is controlled by "segment.ms" today.  However, to ensure messages currently in the active segment can be compacted in time, we need to roll the active segment when either "max.compaction.lag.ms" or "segment.ms" is reached.  
   We define: 
   

   Info
   icon false title maximum time to roll an active segment:
   maxSegmentMs =  if (the log has "compact" enabled) {min(“segment.ms”, “max.compaction.lag.ms")}                                 else {“segment.ms” }

   
   
   

2. Estimate the earliest message timestamp of an un-compacted log segment. we only need to estimate earliest message timestamp for un-compacted log segments to ensure timely compaction because the deletion requests that belong to compacted segments have already been processed.

   1. for the first (earliest) log segment:  The estimated earliest timestamp is set to the timestamp of the first message if timestamp is present in the message. Otherwise, the estimated earliest timestamp is set to "segment.largestTimestamp - maxSegmentMs”  (segment.largestTimestamp is lastModified time of the log segment or max timestamp we see for the log segment.). In the later case, the actual timestamp of the first message might be later than the estimation, but it is safe to pick up the log for compaction earlier.  

   2. from the second log segment onwards:  there are two methods to estimate the earliest message timestamp of a log segment. First method is to use the largestTimestamp (lastmodified time) of previous segment as an estimation. Second method is to use the timestamp of the first message if timestamp is present in the message.  Since getting the timestamp of a message requires additional IOs, the first method of estimation is sufficient in practice.

3. Let log cleaner pick up logs that have reached max compaction lag for compaction.  
   The Rule is simple,  as long as the estimated earliest message timestamp of first un-compacted segment is earlier than "max.compaction.lag.ms", the log is picked up for compaction. Otherwise, Kafka uses "min.cleanable.dirty.ratio" and "min.compaction.lag.ms" to determine the log's eligibility for compaction as it does today.  
4. If both log compaction and log retention are enabled for the topic partition, Kafka also deletes records that have reached both retention time and max compaction lag. 
   If compaction and time based retention are both enabled on a topic, the compaction might prevent records from being deleted on time. The reason is when compacting multiple segments into one single segment, the newly created segment will have same lastmodified timestamp as latest original segment. We lose the timestamp of all original segments except the last one. As a result, records might not be deleted as it should be through time based retention.  Therefore,  we need to explicitly delete those expired records.  This deletion only applies to message records that have timestamp. 
5. Add two Metrics (as described in the next section)
   
   
   

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• Adding topic level configuration "max.compaction.lag.ms",  and corresponding broker configuration "log.cleaner.max.compaction.lag.ms", which is set to MAX_LONG by default.  If both "max.compaction.lag.ms" and "min.compaction.lag.ms" are provided in topic creation, Kafka enforces "max.compaction.lag.ms" is no less than "min.compaction.lag.ms".  This configuration only applies to topics that have compaction enabled. 

• Add two Metrics:  
  1) kafka.log:type=LogCleaner,name=num-logs-compacted-by-max-compaction-lag
  type: gauge
  value: the total number of  logs which needs to be immediately compacted as determined by max.compaction.lag; 

  2) kafka.log:type=LogCleaner,name=max-compaction-delay
  type: gauge
  value: the maximum value of "compaction_finish_time - earliest_timestamp_of_first_uncompacted_segment - max.compaction.lag.ms" among all logs that need to be compacted.
  This metric tells the max delay between the time when a log is required to be picked up for compaction and the time when the compaction is done for the log. 
  

  
  

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