Current state: Under Discussion
Discussion thread: TBD
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
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, user could provide a list of hard-coded `group.instance.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 only one rebalance, instead of four in the worst case.
This change requires us to change JoinGroup protocol to batch mode in order to easily scale multiple members at once.
We will bump JoinGroup request/response version to support batch adding members.
JoinGroupRequest => GroupId SessionTimeout RebalanceTimeout MemberId GroupInstanceId ProtocolType GroupProtocols GroupId => String SessionTimeout => int32 RebalanceTimeout => int32 MemberId => String // removed GroupInstanceId => String // removed ProtocolType => String GroupProtocols => [Protocol MemberMetadata] Protocol => String // removed MemberMetadata => bytes // removed JoinGroupMembers => JoinGroupRequestMember // 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 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);
In the meantime, for better visibility for static members, we are also going to bump DescribeGroup request/response protocol to include `group.instance.id`:
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
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 consistent.