Status

Current state: Under Discussion

Discussion thread: TBD

JIRA:here [Change the link from KAFKA-1 to your own ticket] 

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

Motivation

Rebalance during scaling up is always painful. Every newly joined member will keep the group at rebalancing stage until all of new instances finished bootstrapping. There could be multiple shuffling of active tasks around existing and new instances, thus decreasing the entire system availability. This negative impact has been mitigated after we introduced KIP-345. Under static membership, client user  user could provide a list of hard-coded `group.instance.id` so that the server id`s to pre-register their identities on broker if the new host info is known, so that broker coordinator could respond to scaling operations more intelligently. For example when we scale up the fleet by defining 4 new client instance ids, the server shall wait until all 4 new members to join the group before kicking out the only one rebalance, same with scale down. instead of four in the worst case.

Proposed Changes

This change requires us to change JoinGroup protocol to batch mode in order to enable admin request to easily scale multiple members at once.

Public Interfaces

We will bump JoinGroup request/response version to support batch adding members.

JoinGroupRequest => GroupId SessionTimeout RebalanceTimeout MemberId GroupInstanceId ProtocolType GroupProtocols
  GroupId             => String
  SessionTimeout      => int32  // removed
  RebalanceTimeout    => int32  // removed
  MemberId            => String // removed
  GroupInstanceId     => String // removed
  ProtocolType        => String 
  GroupProtocols      => [Protocol MemberMetadata]
  Protocol            => String // removed
  MemberMetadata      => bytes  // removed
  JoinGroupMembers    => []JoinGroupRequestMember // new
						   SessionTimeout      => int32  // new
						   RebalanceTimeout    => int32  // new
 						   MemberId            => String // new
						   GroupInstanceId     => String // new
						   Protocol            => String // new

JoinGroupResponse => ThrottleTime ErrorCode GenerationId ProtocolName LeaderId MemberId Members
  ThrottleTime           => int16
  ErrorCode              => int16  // removed
  GenerationId           => int32
  ProtocolName           => String
  LeaderId               => String
  MemberId               => String // removed
  Members                => []JoinGroupResponseMember 	
							  MemberId         => String
                              GroupInstanceId  => String
                              Metadata         => bytes
  MemberJoinResponseList => []JoinGroupResult  // new
							  MemberInfo 	 => JoinGroupResponseMember // new	
							  ErrorCode   => int16 // new

A new admin request shall be created to execute this request by supplying for user to supply a list of `group.instance.id` to batch join the group:

public static AddMemberResult addMembersToGroup(String groupId, list<String> groupInstanceIdsToAdd, AddMemberToGroupOptions options);

...

DescribeGroupRequest => ThrottleTime Groups
  ThrottleTime           => int16
  Groups                 => []DescribeGroups
							  ErrorCode        => int16
							  GroupId          => String
							  GroupState       => String
							  ProtocolType     => String
							  ProtocolData     => int16
							  Members          => []DescribedGroupMember
									    			MemberId   => String
										    		GroupInstanceId  => String // new
											    	ClientId         => String							
										    		ClientHost       => String
											    	MemberMetadata   => bytes
    												MemberAssignment => bytes

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Rejected Alternatives

• User needs to upgrade broker to latest version to be able to use this new feature.
• Since we are only introducing new admin API, the change should be backward compatible.

Rejected Alternatives

We could trigger multiple join group requests at the same time without changing JoinGroup protocol. However, considering our change in LeaveGroupRequest, it's hard to handle multiple responses within single admin client request. Changing the protocol to adapt to this change shall be more consistentIf there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.