Major Description
Currently compaction works based on null value, there are some use cases where this is in effective or causes unclean work arounds (as discussed in KIP-82 discussion) this is to add a tombstone flag to enable compaction to also work where value is not null, but a producer needs to signify this to the broker.
This JIRA is related to KIP found here:
https://cwiki.apache.org/confluence/display/KAFKA/KIP-87+-+Add+Compaction+Tombstone+Flag