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1. Reassignments especially for large topic/partition is costly.  In some case, the performance of the Kafka cluster can be severely impacted when reassignments are kicked off.   There should be a fast, clean, safe way to cancel and rollback the pending reassignments.   e.g.  original replicas [1,2,3],  new replicas [4,5,6],   causing performance impact on Leader 1,  the reassignment should be able to get cancelled immediately and reverted back to original replicas [1,2,3],  and dropping the new replicas. 
2. Each batch of reassignments takes as long as the slowest partition; this slowest partition prevents other reassignments from happening.   This can be happening even in the case submitting the reassignments by grouping similar size topic/partitions into each batch. How to optimally group reassignments into one batch for faster execution and less impact to the cluster is beyond the discussion in this KIP.   This is addressed in the Planned Future Changes section and may be implemented in another KIP. 
3. Currently, the reassignment operations are still communicated directly with the Zookeeper.   Other admin types of operation like create/delete topics, etc. are moving to the RPC based KIP-4 wire protocol.   By moving from interacting directly with Zookeeper to  RPC,  it offers the user the recommended path and discourages directly modifying the Zookeeper nodes.  This will pave the way to lock down Zookeeper security by ACLs,  that only brokers need to communicate with ZK.

This change would enable 

• Cancel all pending reassignments currently in /admin/reassign_partitions and revert them back to their original replicas.

• Development of an AdminClient API which supported the above features.  Change the current administrative APIs to go through  RPC instead of Zookeeper. 
  
  

Public Interfaces

Strictly speaking this is not a change that would affect any public interfaces (since ZooKeeper is not considered a public interface, and it can be made in a backward compatible way), however since some users are known to operate on the /admin/reassign_partitions znode directly,  this could break in future versions of Kafka  (e.g. as reported in KAFKA-7854),  and such operations should be discouraged.  

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