Current state: Under Discussion

Discussion thread: here

JIRA:

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Currently, when restoring a state store in a Kafka Streams application, we put one key-value at a time into the store.  This proposal aims to make this recovery more efficient by creating a new interface with "restoreAll" functionality allowing for bulk writes by the underlying state store implementation.  The proposal will also add "beginRestore" and "endRestore" callback methods potentially used for

1. Tracking when the bulk restoration process begins and ends.
2. Keeping track of the number of records and last offset restored.

NOTE: This work is building off ideas originally proposed in 

Public Interfaces

This KIP will introduce

• The BatchingStateRestoreCallback interface.  
• The StateRestoreContext interface.

The interfaces will look like:

public interface BatchingStateRestoreCallback extends StateRestoreCallback {

   void restoreAll(Collection<KeyValue<byte[], byte []>> records);

   void beginRestore(StateRestoreContext restoreContext);

   void endRestore();

}
 
 
public interface StateRestoreContext {

  void setOffsetRestored(long offset);
 
  void setNumberRestored(int restoreCount);

  long offsetRestored();
  
  int numberRestored();

}

The idea behind the StateRestoreContext is applications can query at the end of the restoration process to get the last offset restored and the total count of records restored.

Compatibility, Deprecation, and Migration Plan

• • Since the BatchingStateResoreCallback extends the StateRestoreCallback there should be no impact to classes already implementing this interface.
  • The StateRestoreContext is an addition to the code base so no impact is expected either.
    
    

Rejected Alternatives

N/A