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Status

Current state: Under Discussion

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

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

Motivation

When Kafka is used as messaging infrastructure of consumer business, we often need to adjust the capacity of certain topics to accommodate the bursty nature of the data. The adjustment involves both adding more partitions hence the throughput, and desirably reducing partitions so that they can be reclaimed. Operating clusters with ever growing partitions adds overhead to the operation. First, the performance degrades when a single disk needs to support large number partitions; and second larger cluster footprints makes it more vulnerable to disruption at infrastructure level such as machine or rack decommission (which is not uncommon in large enterprise). This motivates us to add an easy and transparent way to reduce partitions for topics, which is particularly convenient in the following situation.

  • The cluster has a large number of topics and total number of partitions is close to the limit.

  • Most of the dynamic topics do not contain keyed messages. (See limitation section)

  • The data retention period is relatively short.


We have implemented the described changes and deployed it in various setups.

Public Interfaces

Protocol API changes


A new API type DeletePartitions will be added: DeletePartitionsRequest and DeletePartitionsResponse

DeletePartitionsRequest Request (Version: 0) => timeout_ms 
immediate_delete [topics] TAG_BUFFER 
  timeout_ms => INT32
  immediate_delete => BOOLEAN
  topics => name [partitions] TAG_BUFFER 
    name => COMPACT_STRING
    partitions => INT32 TAG_BUFFER

DeletePartitionsResponse Response (Version: 0) => throttle_time_ms error_code error_message [responses] TAG_BUFFER 
	throttle_time_ms => INT32
	error_code => INT16
	error_message => COMPACT_NULLABLE_STRING
	responses => name [partitions] TAG_BUFFER 
		name => COMPACT_STRING
		partitions => partition_index status_code error_code error_message TAG_BUFFER 
			partition_index => INT32
			status_code => INT8
			error_code => INT16
			error_message => COMPACT_NULLABLE_STRING


MetaData API changes

  • Add status_code field in Metadata API that represents the partitions of current topic
Metadata Request (Version: 10) => [topics] allow_auto_topic_creation include_cluster_authorized_operations include_topic_authorized_operations TAG_BUFFER 
  topics => name TAG_BUFFER 
    name => COMPACT_STRING
  allow_auto_topic_creation => BOOLEAN
  include_cluster_authorized_operations => BOOLEAN
  include_topic_authorized_operations => BOOLEAN

Metadata Response (Version: 10) => throttle_time_ms [brokers] cluster_id controller_id [topics] cluster_authorized_operations TAG_BUFFER 
  throttle_time_ms => INT32
  brokers => node_id host port rack TAG_BUFFER 
    node_id => INT32
    host => COMPACT_STRING
    port => INT32
    rack => COMPACT_NULLABLE_STRING
  cluster_id => COMPACT_NULLABLE_STRING
  controller_id => INT32
  topics => error_code name is_internal [partitions] topic_authorized_operations TAG_BUFFER 
    error_code => INT16
    name => COMPACT_STRING
    is_internal => BOOLEAN
    partitions => error_code partition_index leader_id leader_epoch [replica_nodes] [isr_nodes] [offline_replicas] TAG_BUFFER 
      error_code => INT16
      partition_index => INT32
      leader_id => INT32
      leader_epoch => INT32
      replica_nodes => INT32
      isr_nodes => INT32
      offline_replicas => INT32
      status_code => INT8
    topic_authorized_operations => INT32
  cluster_authorized_operations => INT32


Client API changes


public class PartitionInfo {
    private final String topic;
    private final int partition;
    private final Node leader;
    private final Node[] replicas;
    private final Node[] inSyncReplicas;
    private final Node[] offlineReplicas;
    private int status;
}


Admin Client API changes

  • The AdminClient API will have new methods
deletePartitions(Map<String, DeletePartitions> partitions)

@InterfaceStability.Evolving
public class DeletePartitions {
    private int totalCount;
    private boolean immediateDelete;
｝

Kafka Topic Command changes

  • Support using --partitions options to specify a smaller number than current partitions
  • Add --delete-partitions-delay option (Long) to specify when the data should be deleted. The default value is 0 meaning the partition should be deleted right away.
./kafka-topics.sh --alter --topic ... --partitions ...　--delete-partitions-delay ...
  • The command might issue warning under certain situations, such as
    • "WARNING: If partitions are increased for a topic that has a key, the partition logic or ordering of the messages will be affected"
    • "WARNING: This feature is only enabled with Metadata version above 10 and delete.topic.partition.enable turned on"
    • "WARNING: The topic is currently under changes"

Proposed Changes

We propose to add new status for a partition that indicates if a partition is to be removed. Specifically at any time a partition can be 

  • online (code=0): it means the partition is normal

  • offline (code=1): it means the partition should be excluded by SDK client for both message production and consumption. When the data is purged this partition will be removed from the topics metadata.

  • readOnly (code=-1): it means the partition should be filtered and not written to, but consumption is not impacted. After the retention period has passed the status will transit to offline.

The status is stored in zookeeper:

{"controller_epoch":1,"leader":0,"version":1,"leader_epoch":0,"isr":[0], "status_code": 0}


The following change in Controller will be made:

  • In zookeeper a path /admin/delete_topic_partitions will be added. The format of the node is ()

  • In the controller monitoring path, register DeleteTopicPartitionHandler to monitor the zookeeper event; add a controller event TopicPartitionDeletion

  • Add a class TopicPartitionDeletionManager to handle TopicPartitionDeletion event

The workflow involving TopicPartitionDeletionManager class is summarized as below:

  • TopicCommand class executes the delete command and creates a node in zookeeper directory /admin/delete_topic_partitions with the node topic name and the list of partitions 

  • TopicPartitionDeletion class monitors change in the above directory, initiates the deletion process and adds it to the queue for status polling

  • TopicPartitionDeletionManager.onPartitionDeletion update the status of the partition to readOnly or offline according to the deleteNow flag; then notify all brokers through PartitionStateMachine

  • TopicPartitionDeletionManager checks the status for each partition and call onPartitionDeletion for all the offline partitions. Notify all the brokers through PartitionStateMachine

  • ReplicaStateMachine executes OfflineReplica and ReplicaDeletionStarted to stop synchronize data at all brokers

  • When Controller gets the successful stopReplica response from Broker, ReplicaStateMachine executes ReplicaDeletionSuccessful to update partition information. Then it cleans up zookeeper path as well. On the other hand if stopReplica fails, it will call ReplicaDeletionIneligible and wait for the Controller to retry

Compatibility, Deprecation, and Migration Plan

When will we remove the existing behavior?When a topic contains keyed messages, partition removal cannot be completely transparent to the client, as during the process (i.e. when the old partition is in readOnly state) there will be two partitions that contain messages of the same key. We The application needs to handle this temporary discrepancy until the old partition is offline. In the change we propose to issue a warning message in the kafka-topics command.

Rejected Alternatives

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