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• The AdminClient API currently lacks any functionality for reassigning partitions: Users have to use the kafka-reassign-partitions.sh tool (ReassignPartitionsCommand) which talks directly to ZooKeeper. This prevents the tool being used in deployments where only the brokers are exposed to clients (i.e. where the zookeeper servers are intentionally not exposed). In addition, there is a general push to refactor/rewrite/replace tools which need ZooKeeper access with equivalents which use the AdminClient API.
• ReassignPartitionsCommand currently has no proper facility to report progress of a reassignment;  While --verify can be used periodically to check whether the request assignments have been achieved the tool provides no means of knowing how quickly new replicas are catching up. It would be useful if the tool could report progress better.
• ReassignPartitionsCommand, when used with a replication throttle, requires a a --verify invocation when the reassignment has finished in order to remove the throttledthrottle. Therefor So there exists the possibility that throttles are not removed after reassignment, with negative consequences for the performance of the cluster. It would be better if throttles could be removed automatically.

Public Interfaces

The AdminClient API will have new methods added (plus overloads for options):

• reassignPartitions(Map<TopicPartition, List<Integer>>)

• alterInterbrokerThrottledRates(Map<Integer, ThrottledRate> throttledRates)

• alterInterbrokerThrottledReplicas(Map<TopicPartition, ThrottledReplicas> replicas)

The options for for reassignPartitions() will support setting a throttle, and a flag for its automatic removal at the end of the reassignment. Likewise the options for changing the throttled rates and replicas will include the ability to have the throttles automatically removed.

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• ReassignPartitionsRequest and ReassignPartitionsResponse

• AlterInterbrokerThrottledRatesRequest and and AlterInterbrokerThrottledRatesResponse
• AlterInterbrokerThrottledReplicasRequest and and AlterInterbrokerThrottledReplicasResponse

The options accepted by kafka-reassign-partitions.sh command will change:

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When run with --bootstrap-server it will no longer be necessary to run run kafka-reassign-partitions.sh --verify to remove a throttle: This will be done automatically.

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 kafka-reassign-partitions.sh and ReassignPartitionsCommand

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1. Perform the reassignment via the AdminClient API (described below) using the given broker(s) as bootstrap brokers.
2. When used with with --execute and --throttle, the throttle will be an auto-removed one.

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1. Changing the partition assignment, via kafka-reassign-partitions.sh
2. Changing the replication factor, via kafka-reassign-partitions.sh

Notes:

Notes:

• This API is asynchronous in the sense that the This API is asynchronous in the sense that the client cannot assume that the request is complete (or the request was rejected) once they have obtained the result for the topic from the ReassignPartitionsResult.
• The describeReplicaLogDirs() method from KIP-113 can be used to determine progress.

• A call to reassignPartitions() with the validateOnly option can be used to determine whether a reassignment is currently running, and therefore whether the last reassignment has finished.

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• The request must be sent to the controller.
• The request  requires the Alter operation on the CLUSTER resource, since it can require significant inter-broker communication.
• The request will be subject to a policy, as described in KIP-201.

ReassignPartitionsRequest => [topic_reassignments] timeout validate_only throttle remove_throttle
  topic_reassignments => topic [partition_reassignments]
    topic => STRING
    partition_reassignments => partition_id [broker]
      partition_id => INT32
      broker => INT32
  timeout => INT32
  validate_only => BOOLEAN
  throttle => INT64
  remove_throttle => BOOLEAN

...

Algorithm:

1. The controller validates the request request against configured authz, policy etc.
2. The controller computes set of topics in the request, and writes this as JSON to the new new /admin/throttled_replicas_removal znode
3. The controller then updates the existing existing leader.replication.throttled.replicas and and follower.replication.throttled.replicas properties of each topic config.
4. The controller computes the union of 1) The the brokers currently hosting replicas of the topic partitions in the request 2) the brokers assigned to host topic partitions in the request, and write this as JSON to the new new /admin/throttled_rates_removal znode.
5. The controller then updates the existing existing leader.replication.throttled.rates and and follower.replication.throttled.rates properties of each broker config.
6. The controller writes reassignment JSON to the /admin/reassign_partitions znode

The intent behind this algorithm is that should the controller crash during the update, the reassignment won't have started and the throttles will be removed on controller failover.

The broker will use the same algorithm for determing the values of the topic and broker configs as is currently used in the ReassignPartitionsCommand.

...

1. The controller validates the brokers and rates in the request .
    and that the principal has Alter operation on the CLUSTER resource.
2. The controller gets the current value of the /admin/throttled_rates_removal znode, forms the union of those brokers with those in the request and updates the /admin/throttled_rates_removal znode with JSON representation of this union
   
3. The controller then subtracts the brokers in the request from the current brokers and removes the leader.replication.throttled.rates and follower.replication.throttled.rates properties from each broker config
   
4. The controller then, for each broker in the request, adds the leader.replication.throttled.rates and follower.replication.throttled.rates properties to each broker config.
5. The controller then updates /admin/throttled_rates_removal znode with JSON representation of brokers in the request.

The intent behind this algorithm is that should the controller crash during the update, throttles will still be removed on completion of reassignment.

AlterInterbrokerThrottledRatesResponse 

AlterInterbrokerThrottledRatesResponse => [broker_error]
  broker_error => broker_id error_code error_messgae
      broker_id => INT32
      error_code => INT16
      error_message => NULLABLE_STRING

AdminClient: alterInterbrokerThrottledReplicas()

Anticipated Errors:

• NOT_CONTROLLER (41) if the request was sent to a broker that wasn't the controller.
• CLUSTER_AUTHORIZATION_FAILED (31) Authorization failed
• INVALID_THROTTLE (new) if the throttled rate is <= 0.

• UNKNOWN_MEMBER_ID(25) if the broker id in the request is not a broker in the cluster

AdminClient: alterInterbrokerThrottledReplicas()

/**
 * Set the partitions and brokers

/**
 * Set the partitions and brokers subject to the
 * {@linkplain #alterInterbrokerThrottledRate(Map)
 * interbroker throttled rate}.
 * The brokers specified as the {@link ThrottledReplicas#leaders()} corresponding to a
 * topic partition given in {@code replicas} will be subject to the
 leader* throttled rate{@linkplain #alterInterbrokerThrottledRate(Map)
 * wheninterbroker acting as the leader for that partitionthrottled rate}.
 * The brokers specified as the {@link ThrottledReplicas#followersThrottledReplicas#leaders()} corresponding to a
 * topic partition given in {@code repicasreplicas} will be subject to the followerleader throttled rate
 * when acting as the followerleader for that partition.
 * IfThe thebrokers givenspecified {@codeas replicas}the is {@link ThrottledReplicas#followers()} corresponding to a
 * topic partition given in {@code repicas} will be subject to the follower throttled rate
 * when acting as the follower for that partition.
 * If the given {@code replicas} is null then the throttle will apply to all topic partitions on all brokers.
 * TODO No! It should depend on the current reassignment
 *
 * The throttle will be automatically removed at the end of the current reassignment,
 * unless overridden in the given options.
 *
 * The current throttled replicas can be obtained via {@link #describeConfigs(Collection)} with a
 * ConfigResource with type {@link ConfigResource.Type#TOPIC TOPIC} and name "leader.replication.throttled.replicas"
 * or "follower.replication.throttled.replicas".
 */
public abstract AlterInterbrokerThrottledReplicasResult alterInterbrokerThrottledReplicas(
        Map<TopicPartition, ThrottledReplicas> replicas,
        AlterInterbrokerThrottledReplicasOptions options);

...

public class ThrottledReplicas {

    public ThrottledReplicas(Collection<Integer> leaders, Collection<Integer> followers) { ... }

    /**
     * The brokers which should be throttled when acting as leader
. A null value indicates all brokers in the cluster.
     */
    public Collection<Integer> leaders() { .. }

    /**
     * The brokers which should be throttled when acting as follower. A null value indicates all brokers in the cluster.
     */
    public Collection<Integer> followers() { ... }

}

public class AlterInterbrokerThrottledReplicasOptions extends AbstractOptions<AlterInterbrokerThrottledReplicasOptions> {

    public boolean autoRemoveThrottle() { ... }

    /**
     * True to automatically remove the throttle at the end of the current reassignment.
     */
    public AlterInterbrokerThrottledReplicasOptions autoRemoveThrottle(boolean autoRemoveThrottle) { ... }
}

public class AlterInterbrokerThrottledReplicasResult {
    // package-access ctor

    public Map<TopicPartition, KafkaFuture<Void>> values() { ... }

    public KafkaFuture<Void> all() { ... }
}

Network API: AlterInterbrokerThrottledReplicasRequest and AlterInterbrokerThrottledReplicasResponse

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1. The controller validates the partitions and brokers in the request and that the principal has Alter operation on the CLUSTER resource.
    
2. The controller gets the current value of the the /admin/throttled_replicas_removal znode, forms the union of those topics with those in the request and updates the the /admin/throttled_rates_removal znode with JSON representation of this union
   
3. The controller then subtracts the topics in the request from the current topics and removes the the leader.replication.throttled.replicas and and follower.replication.throttled.replicas properties from each topic config
   

4. The controller then, for each topic in the request, adds

   the

   the leader.replication.throttled.rates

   and

   and follower.replication.throttled.rates properties to each topic config.

5. The controller then updates /admin/throttled_replicas_removal znode with JSON representation of topics in the request.

The intent behind this algorithm is that should the controller crash during the update, throttles will still be removed on completion of reassignment.

AlterInterbrokerThrottledReplicasResponse => 

AlterInterbrokerThrottledReplicasResponse => [topic_errors]
  topic_errors => topic [partition_errors]
    topic => STRING
    partition_errors => partition_id error_code error_messgae
      partition_id error_code error_messgae
      partition_id => INT32
      error_code => INT16
      error_message => NULLABLE_STRING_id => INT32
      error_code => INT16
      error_message => NULLABLE_STRING

Anticipated errors:

• NOT_CONTROLLER (41) if the request was sent to a broker that wasn't the controller.
• CLUSTER_AUTHORIZATION_FAILED (31) Authorization failed
• UNKNOWN_TOPIC_OR_PARTITION (3) if a partition in the request is not known in the cluster.

• UNKNOWN_MEMBER_ID(25) if the broker id in the request is not a broker in the cluster

On Controller Failover

The algorithms presented above, using the new znodes, are constructed so that should the controller fail, on election of a new controller ZooKeeper is not left in an inconsistent state where throttles which should be removed automatically are not removed at the end of the reassignment. The recovery algorithm is as follows:

1. If the the /admin/reassign_partitions znode exists we assume a reassignment is on-going and do nothing.
2. Otherwise, if the the /admin/reassign_partitions znode does not exists, we proceed to remove the throttles, as detailed in "Throttle removal" section below.

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When reassignment is complete:

1. The The /admin/reassign_partitions znode gets removed.
2. We remove the throttles, as detailed in "Throttle removal" section below.

Throttle removal

The algorithm for removing the throttled replicas is:

1. If If /admin/remove_throttled_replicas is set:

   1. For each of the topics listed in that znode:

      1. Remove the the (leader|follower).replication.throttled.replicas propertes properties for that topic config.

   2. Remove the the /admin/remove_throttled_replicas znode

The symmetric algorithm is used for /admin/remove_throttled_rates, only with broker configs.

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