Kafka Detailed Consumer Coordinator Design

/consumers/coordinator --> brokerId (ephemeral; created by coordinator)

coordinatorElector : ZkElection                     // A ZK based elector using the coordinator path mentioned above

groupsBeingRebalanced : Map[String, AtomicBoolean]  // For each group, a bit indicating if the group is under rebalancing

consumerGroupsPerTopic : Map[String, Set[String]]   // For each topic, the consumer groups that are interested in the topic

groupsWithWildcardTopics : Set[String]              // Groups that has wildcard interests for topics

rebalanceRequestQ : LinkedBlockingQueue[String]     // A blocking queue storing all the rebalance requests, the request just contain the group name

requestHandler : RebalanceRequestHandler            // A thread handling all the rebalance requests read from the rebalanceReques

coordinatorStartup :

1. Read all the topics from ZK and initialize consumerGroupsPerTopic

2. Read all the consumer groups from ZK

2.1 Get the current interested topics of each group, update consumerGroupsPerTopic by adding the group to each topic's interested group list

2.2 If the group has some consumer specifying wildcard topic-counts, then add the group to groupsWithWildcardTopics

2.3 Always try to rebalance every group by adding (group -> new AtomicBoolean(true)) to groupsBeingRebalanced and put group to rebalanceRequestQ

3. Register listeners for topics and their partition changes

3.1 Subscribe TopicChangeListener to /brokers/topics

3.2 Subscribe TopicPartitionChangeListener to each /brokers/topics/[topic]

4. Register listeners for consumer groups and their member changes

4.1 Subscribe registerGroupChangeListener to /consumers/groups/

4.2 Subscribe registerGroupMemeberChangeListener to each /consumers/groups/[groups]/ids

5. Register session expiration listener

6. Initialize and start the requestHandler thread

handleNewSession :

1. Reset its state by clearing consumerGroupsPerTopic, groupsWithWildcardTopics and rebalanceRequestQ, etc

2. Re-register the session expiration listener (this is because ZkClient does not re-register itself once fired)

3. Try to re-elect to be the coordinator by directly calling the elect function of its coordinatorElector.

GroupChangeListener.handleChildChange :

1. Get the newly added group (since /consumers/groups are persistent nodes, no groups should be deleted even if there is no consumers any more inside the group)

2. Subscribe the registerGroupMemeberChangeListener on /consumers/groups/group

3. Read all the topics this group is interested in, for each topic:

3.1 If the topic already exists in consumerGroupsPerTopic, update its list by adding this group

3.2 If the topic is not in consumerGroupsPerTopic yet, add the entry (topic -> Set(group))

4. If some of this group's consumers has wildcard interests, add that to groupsWithWildcardTopics

5. If the group already has some interested existed topics, put (group -> new AtomicBoolean(true)) to groupsUnderRebalance, and put the group to rebalanceRequestQ;

   Otherwise just put (group -> new AtomicBoolean(false)) to groupsUnderRebalance

GroupMemberChangeListener.handleChildChange :

1. If some topics are no longer interested due to the deletion of some consumer, update consumerGroupsPerTopic by removing the group from these topics' list

2. If the group no longer contain any consumer, do nothing;

   Otherwise if groupsBeingRebalanced(group).compareAndSet(false, true) succeeds, put the group to rebalanceRequestQ.

TopicChangeListener.handleChildChange :

1. Get the newly added topic (since /brokers/topics are persistent nodes, no topics should be deleted even if there is no consumers any more inside the group)

2. For each newly added topic:

2.1 Subscribe TopicPartitionChangeListener to /brokers/topics/topic

2.2 Get the set of groups that are interested in this topic from both consumerGroupsPerTopic(topic) and groupsWithWildcardTopics (filtered by wildcard pattern regex), and try to request rebalance for each group*

* By trying to request rebalance, we do the following:

if (groupsBeingRebalanced(group).compareAndSet(false, true)) rebalanceRequestQ.put(group)

TopicPartitionChangeListener.handleChildChange :

Get the set of groups that are interested in this topic from consumerGroupsPerTopic(topic) and groupsWithWildcardTopics (filtered by wildcard pattern regex), and try to request rebalance for each group

rebalance (group) :

1. Get the topics that are interested by the group.

2. Compute the new ownership assignment after reading from ZK the number of partitions and number of threads for each topic.

3. Check if a rebalance is necessary by trying to get the current ownership from ZK for each topic.

3.1 If there is no registered ownership info in ZK, rebalance is necessary

3.2 If some partitions are not owned by any threads, rebalance is necessary

3.3 If some partitions registered in the ownership map do not exist any longer, rebalance is necessary

3.4 If ownership map do not match with the new one, rebalance is necessary

3.5 Otherwise rebalance is not necessary

4. If rebalance is necessary, do the following

4.1 For each consumer in the group, send the "StopRequest" (details of communication is introduced later)

4.2 Then for each consumer in the group, send the "StartRequest" with part of the newly computed ownership specific to the consumer

4.3 Then wait until all the consumer has finished starting the fetcher (details of waiting is introduced later)

5. If waiting has timed out, return false; otherwise return true.

{brokerId : (host, port)}

consumerStartup (initBrokers : Map[Int, (String, String)]):

1. Randomly pick a broker in the initBrokers, create a socket channel with that broker

1.1. If the socket channel cannot be created, try another broker in the initBroker

1.2. If all the brokers in initBroker cannot be connected, throw a AllKnownBrokersNotAvailable exception

2. Send a consult-request request to the broker and get a consult-response from the broker.

3. From the consult-response update serverCluster and curCoordinator.

4. Set up a socket channel with the current coordinator.

5. Keep block-reading from the channel.

{
  size: int32                // the size of this request
  request_type_id: int16     // the request id
  version_id: int16          // the version of this request
  correlation_id: int32      // an id set by the client that will be returned untouched in the response
  client_id: string          // an optional non-machine-specific identifier for this client
  required_acks: int8        // the number of acknowledgements required from the brokers before a response can be made
  ack_timeout: int32         // the time in ms to wait for acknowledgement from replicas
  data: null                 // no need data for the consult request
}

{
  size: int32                                        // the size of this request
  correlation_id: int16                              // the request id
  version_id: int16                                  // the version of this request
  error: int16                                       // an id set by the client that will be returned untouched in the response
  data: {coordinatorId: int16, [<broker_struct>]}    // the actual data requested (in the same format as defined for the produce request)
}

broker_struct =>
{
  id:
  creatorId:
  host:
  port:
}

{

}

{

}

{

}

{

}

{

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{

}