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In a Kafka cluster, one of the brokers serves as the controller, which is responsible for managing the states of partitions and replicas and for performing administrative tasks like reassigning partitions. The following describes the states of partitions and replicas, and the kind of operations going through the controller.

PartitionStateChange: 

Valid states are:

...

  • NewPartition: After creation, the partition is in the NewPartition state. In this state, the partition should have replicas assigned to it, but no leader/isr yet.

  • OnlinePartition: Once a leader is elected for a partition, it is in the OnlinePartition state.

  • OfflinePartition: If, after successful leader election, the leader for partition dies, then the partition moves to the OfflinePartition state.

...

NewPartition,OnlinePartition -> OfflinePartition
  1. nothing other than marking partition state as Offline
OfflinePartition -> NonExistentPartition
  1. nothing other than marking the partition state as NonExistentPartition

ReplicaStateChange:

Valid states are:
  1. NewReplica: When replicas are created during topic creation or partition reassignment. In this state, a replica can only get become follower state change request. 
  2. OnlineReplica: Once a replica is started and part of the assigned replicas for its partition, it is in this state. In this state, it can get either become leader or become follower state change requests.
  3. OfflineReplica : If a replica dies, it moves to this state. This happens when the broker hosting the replica is down.
  4. NonExistentReplica: If a replica is deleted, it is moved to this state.

Valid state transitions are:

NonExistentReplica --> NewReplica
  1. send LeaderAndIsr request with current leader and isr to the new replica and UpdateMetadata request for the partition to every live broker
NewReplica-> OnlineReplica
  1. add the new replica to the assigned replica list if needed
OnlineReplica,OfflineReplica -> OnlineReplica
  1. send LeaderAndIsr request with current leader and isr to the new replica and UpdateMetadata request for the partition to every live broker
NewReplica,OnlineReplica -> OfflineReplica
  1. send StopReplicaRequest to the replica (w/o deletion)
  2. remove this replica from the isr and send LeaderAndIsr request (with new isr) to the leader replica and UpdateMetadata request for the partition to every live broker.
OfflineReplica -> NonExistentReplica
  1. send StopReplicaRequest to the replica (with deletion)

KafkaController Operations:

onNewTopicCreation:
  1. call onNewPartitionCreation

 

onNewPartitionCreation:
  1. new partitions -> NewPartition
  2. all replicas of new partitions -> NewReplica
  3. new partitions -> OnlinePartition
  4. all replicas of new partitions -> OnlineReplica


onBrokerFailure:
  1. partitions w/o leader -> OfflinePartition
  2. partitions in OfflinePartition and NewPartition -> OnlinePartition (with OfflinePartitionLeaderSelector)
  3. each replica on the failed broker -> OfflineReplica


onBrokerStartup:
  1. send UpdateMetadata requests for all partitions to newly started brokers
  2. replicas on the newly started broker -> OnlineReplica
  3. partitions in OfflinePartition and NewPartition -> OnlinePartition (with OfflinePartitionLeaderSelector)
  4. for partitions with replicas on newly started brokers, call onPartitionReassignment to complete any outstanding partition reassignment


onPartitionReassignment: (OAR: old assigned replicas; RAR: new re-assigned replicas when reassignment completes)

...

For example, if OAR = {1, 2, 3} and RAR = {4,5,6}, the values in the assigned replica (AR) and leader/isr path in ZK may go through the following transition.
AR                  leader/isr
{1,2,3}            1/{1,2,3}           (initial state)
{1,2,3,4,5,6}   1/{1,2,3}           (step 2)
{1,2,3,4,5,6}   1/{1,2,3,4,5,6}  (step 24)
{1,2,3,4,5,6}   4/{1,2,3,4,5,6}  (step 7)

...