Note this was initially erroneously assigned as KIP-178, which was already taken, and has been reassigned KIP-179.

Status

Current state: Under Discussion [One of "Under Discussion", "Accepted", "Rejected"]

Discussion thread: here (when initially misnumbered as KIP-178) and here (when assigned KIP-179)

JIRA: here Unable to render Jira issues macro, execution error. Unable to render Jira issues macro, execution error.

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Describe the problems you are trying to solve.

Firstly, the ReassignPartitionsCommand (which is used by the kafka-reassign-partitions.sh tool) 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. Thus it is necessary to change the ReassignPartitionsCommand so that it no longer talks to ZooKeeper directly, but via an intermediating broker. Similar work is needed for the kafka-topics.sh tool (which can also change assignments and numbers of partitions and replicas), so common AdminClient and protocol APIs are desirable.

Secondly, ReassignPartitionsCommand currently has no proper facility to report progress of a reassignment; --verify can be used periodically to check whether the request assignments have been achieved. It would be useful if the tool could report progress better.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

A public interface is any change to the following:

• Binary log format

• The network protocol and api behavior

• Any class in the public packages under clientsConfiguration, especially client configuration

  • org/apache/kafka/common/serialization

  • org/apache/kafka/common

  • org/apache/kafka/common/errors

  • org/apache/kafka/clients/producer

  • org/apache/kafka/clients/consumer (eventually, once stable)

• Monitoring

• Command line tools and arguments

• Anything else that will likely break existing users in some way when they upgrade

New network protocol APIs will be added:

• IncreasePartitionCountsRequest and IncreasePartitionCountsResponse

• ReassignPartitionsRequest and ReassignPartitionsResponse

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

• increasePartitionCounts(Map<String, PartitionCount> partitionCounts)

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

Support for changing DynamicConfig broker configs via AdminClient.alterConfigs() will be added

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

• --zookeeper will be deprecated, with a warning message
• a new --bootstrap-server option will be added
• a new --progress action option will be added

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Summary of use cases

 kafka-reassign-partitions.sh and ReassignPartitionsCommand

The --zookeeper option will be retained and will:

1. Cause a deprecation warning to be printed to standard error. The message will say that the --zookeeper option will be removed in a future version and that --bootstrap-server is the replacement option.
2. Perform the reassignment via ZooKeeper, as currently.

A new --bootstrap-server option will be added and will:

1. Perform the reassignment via the given intermediating broker.

Using both --zookeeper and --bootstrap-server in the same command will produce an error message and the tool will exit without doing the intended operation.

It is anticipated that a future version of Kafka would remove support for the --zookeeper option.

A new --progress action option will be added. This will only be supported when used with --bootstrap-server. If used with --zookeeper the command will produce an error message and the tool will exit without doing the intended operation. --progress will report on the synchronisation of each of the partitions and brokers in the reassignment given via the --reassignment-json-file option

For example:

# If the following command is used to start a reassignment
bin/kafka-reassign-partitions.sh --bootstrap-server localhost:9878 \
  --reassignment-json-file expand-cluster-reassignment.json \
  --execute
       
# then the following command will print the progress of
# that reassignment, then exit immediately
bin/kafka-reassign-partitions.sh --bootstrap-server localhost:9878 \
  --reassignment-json-file expand-cluster-reassignment.json \
  --progress

That might print something like the following:

Topic      Partition  Broker  Status
-------------------------------------
my_topic   0          0       In sync
my_topic   0          1       Behind: 10456 messages behind
asdf       0          1       Unknown topic
my_topic   42         1       Unknown partition
my_topic   0          42      Unknown broker
my_topic   1          0       Broker does not host this partition

The implementation of --progress will make use of the describeReplicaLogDirs() method from KIP-113 to find the lag of the syncing replica.

Internally, the ReassignPartitionsCommand will be refactored to support the above changes to the options. An interface will abstract the commands currently issued directly to zookeeper.

There will be an implementation which makes the current calls to ZooKeeper, and another implementation which uses the AdminClient API described below.

In all other respects, the public API of ReassignPartitionsCommand will not be changed.

AdminClient: increasePartitionCount()

This API supports the use case of increasing the partition count  via kafka-topics.sh --alter --partitions ...

Notes:

• This API is synchronous in the sense that the client can assume that the partition count has been changed (or the request was rejected) once they have obtained the result for the topic from the AlterPartitionCountsResult.

/**
 * <p>Increase the partition count of the topics given as the keys of {@code counts}
 * according to the corresponding values.</p>
 */
public IncreasePartitionCountsResult increasePartitionCounts(Map<String, PartitionCount> counts,
                    IncreasePartitionCountsOptions options)
public IncreasePartitionCountsResult increasePartitionCounts(Map<String, PartitionCount> counts) 

Where:

/** Describes a change in a topic's partition count. */
public class PartitionCount {
    private int partitionCount;
    private List<List<Integer>> assignments;
    private PartitionCount(int partitionCount) { ... }

    /** 
     * Increase the partition count for a topic to the given {@code newCount}. 
     * The assignment of new replicas to brokers will be decided by the broker
     * but may subsequently be moved using {@link #reassignPartitions(Map)}.</p>
     */
    public static PartitionCount increasePartitionCount(int newCount) { ... }

    /** 
     * <p>Increase the partition count for a topic to the given {@code newCount} 
     * assigning the new partitions according to the given {@code newAssignments}.
     * The length of {@code newAssignments} should equal {@code newCount - oldCount}, since 
     * the assignment of existing partitions are not changed. 
     * Each inner list of {@code newAssignments} should have a length equal to 
     * the topic's replication factor. 
     * The first broker id in each inner list is the "preferred replica".</p>
     *
     * <p>For example, suppose a topic currently has a replication factor of 2, and 
     * has 3 partitions. The number of partitions can be increased to 4 
     * (with broker 1 being the preferred replica for the new partition) using a 
     * {@code PartitionCount} constructed like this:</p>
     *
     * <pre><code>PartitionCount.increasePartitionCount(4, Arrays.asList(Arrays.asList(1, 2))</code></pre>
     *
     */
    public static PartitionCount increasePartitionCount(int newCount, List<List<Integer>> newAssignments) { ... }
}
    

public class IncreasePartitionCountsOptions {
    public IncreasePartitionCountsOptions() { ... }
    public Integer timeoutMs() { ... }
    public IncreasePartitionCountsOptions timeoutMs(Integer timeoutMs) { ... }
    public boolean validateOnly() { ... }
    /**
     * Validate the request only: Do not actually change any partition counts.
     */
    public IncreasePartitionCountsOptions validateOnly() { ... }
}
 
public class IncreasePartitionCountsResult {
    // package access constructor
    Map<String, KafkaFuture<Void>> values() { ... }
    KafkaFuture<Void> all() { ... }
}

Network Protocol: IncreasePartitionCountsRequest and IncreasePartitionCountsResponse

The IncreasePartitionCountsRequest is used to increase the partition count for a batch of topics, and is the basis for the  AdminClient.increasePartitionCounts() method.

The request can be sent to any broker.

The request will require the ALTER operation on the Topic resource.

The request is subject to the CreateTopicPolicy of the broker as configured by the broker's create.topic.policy.class.name config property. This is to ensure that the policy applies to topics modified after creation.

After validating the request the broker calls AdminUtils.addPartitions() which ultimately updates the topic partition assignment znode (/brokers/topics/${topic}).

IncreasePartitionCountsRequest => [topic_partition_count] timeout
  topic_partition_count => topic partition_count
    topic => STRING
    partition_count => count [assignment]
      count => INT32
      assignment => [INT32]
  timeout => INT32

Where

assignment

The response provides an error code and message for each of the topics present in the request.

IncreasePartitionCountsResponse => throttle_time_ms [topic_partition_count_error]
  topic_partition_count_error => topic error_code error_message
    topic => STRING
    error_code => INT16
    error_message => NULLABLE_STRING

Where

Anticipated errors:

• TOPIC_AUTHORIZATION_FAILED (29) The user lacked Alter on the topic

• POLICY_VIOLATION(44) The request violated the configured policy

• INVALID_TOPIC_EXCEPTION (17) If the topic doesn't exist
• INVALID_PARTITIONS (37) If the partition count was <= the current partition count for the topic.
• INVALID_REQUEST (42) If duplicate topics appeared in the request, or the size of the partitions list did not equal the number of new partitions, or if the size of any of the lists contained in the partitions list was not equal to the topic replication factor
• NONE (0) The topic partition count was changed successfully.

AdminClient: reassignPartitions()

This API will support a number of use cases:

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

Notes:

• 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.

• The API doesn't directly support setting a throttle itself. A prior set of calls to alterInterBrokerThrottle() can be used to set a throttle.

/**
 * <p>Assign the partitions given as the key of the given <code>assignments</code> to the corresponding 
 * list of brokers. This can be used to change the replica assignment or change the topic's replication factor. 
 * The first broker in each list is the one which holds the "preferred replica".</p>
 *
 * <p>To change the replication factor for a topic there must be a key for each partition in 
 * the {@code assignment}s map and the corresponding list of brokers must each be of the same 
 * length which will become the new replication factor.</p>
 *
 * <p>If only a subset of the partitions of a particular topic are present in {@code assignments} 
 * the change is taken  to be a reassignment of replicas to brokers and each list must have 
 * the same length as the current topic replication factor.</p>
 *
 * <h3>Throttling</h3>
 * <p>Inter-broker reassignment and/or increasing the replication factor causes significant 
 * inter-broker traffic and can take a long time 
 * in order to copy the replica data to brokers. It may be necessary to impose a throttle on 
 * inter-broker traffic for the duration of the reassignment so that client-broker traffic is not
 * adversely affected.</p>
 *
 * <h3>Preferred replica</h3>
 * <p>When brokers are configured with <code>auto.leader.rebalance.enable=true</code>, the broker
 * with the preferred replica will be elected leader automatically. 
 * <code>kafka-preferred-replica-election.sh</code> provides a manual trigger for this 
 * election when <code>auto.leader.rebalance.enable=false</code>.</p>
 */
public ReassignPartitionsResult reassignPartitions(Map<TopicPartition, List<Integer>> assignments)
public ReassignPartitionsResult reassignPartitions(Map<TopicPartition, List<Integer>> assignments, 
                        ReassignPartitionsOptions options)

Where:

public class ReassignPartitionsOptions {

    public boolean validateOnly()

    /**
     * Validate the request only: Do not actually trigger replica reassignment.
     */
    public ReassignPartitionsOptions validateOnly(boolean validateOnly)

    public long timeoutMs()

    /**
     * Set a timeout for the starting of the reassignment. 
     * Note this timeout does not include the time take to actually
     * move replicas between brokers.
     */
    public ReassignPartitionsOptions timeoutMs(long timeoutMs)


 }
public class ReassignPartitionsResult {
    public Map<TopicPartition, KafkaFuture<Void>> values();
    public KafkaFuture<Void> all();
}

Network Protocol: ReassignPartitionsRequest and ReassignPartitionsResponse

A ReassignPartitionsRequest initiates the movement of replicas between brokers, and is the basis of the AdminClient.reassignPartitions() method

Notes:

• The request can be sent to any broker.
• The request  requires the Alter operation on the CLUSTER resource, since it can require significant inter-broker communication.
• The request is subject to the CreateTopicPolicy of the broker as configured by the broker's create.topic.policy.class.name config property. This is to ensure that the policy applies to topics modified after creation.
• After validating the request the broker writes reassignment JSON to the /admin/reassign_partitions znode

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

Where

A ReassignPartitionsResponse describes which partitions in the request will be moved, and what was wrong with the request for those partitions which will not be moved.

ReassignPartitionsResponse => throttle_time_ms [reassign_partition_result]
  throttle_time_ms => INT32
  reassign_partition_result => topic [partition_error]
    topic => STRING
    partition_error => partition_id error_code error_message
      partition_id => INT32
      error_code => INT16
      error_message => NULLABLE_STRING

Where

Anticipated errors:

• CLUSTER_AUTHORIZATION_FAILED (31) Authorization failed
• POLICY_VIOLATION(44) The request violated the configured policy
• INVALID_TOPIC_EXCEPTION (17) If the topic doesn't exist
• UNKNOWN_MEMBER_ID (25) If any broker ids in the partition_reassignments included an unknown broker id
• INVALID_REQUEST (42) If duplicate topics appeared in the request
• PARTITION_REASSIGNMENT_IN_PROGRESS (new) If the reassignment cannot be started because a reassignment is currently running (i.e. the /admin/reassign_partitions znode exists)
• INVALID_REPLICA_ASSIGNMENT (39) If a partition, replica or broker id in the partition_assignment doesn't exist or is incompatible with the requested num_partitions and /or replication_factor. The error_message would contain further information.
• NONE (0) reassignment has started

AdminClient: alterConfigs()

This API already exists, but doesn't currently support changing broker configs. Inter-broker throttling is implemented via DynamicConfigs on the broker:

• leader.replication.throttled.rate
• follower.replication.throttled.rate

DynamicConfigs are configs "which have no physical manifestation in the server.properties and can only be set dynamically".

And via topic configs:

• leader.replication.throttled.replicas
  

• follower.replication.throttled.replicas
  

The alterConfigs() API will be changed to allow altering DynamicConfigs in order to support  the following use cases:

1. A previous call to reassignPartitions()  has been made and the user wants to apply or change the throttle.
2. A previous call to reassignPartitions() is complete and the user wants to remove the throttle.

 This KIP does not propose to support altering broker configs that are not DynamicConfigs.

This will involve changing the API doc.

Network Protocol: AlterConfigsRequest and AlterConfigsResponse

This protocol already exists, but support will be added for altering broker DynamicConfigs. The network format will not need to change, but the AlterConfigsResponse will (when the request attempts to alter broker configs) be able to fail with a new error code: CLUSTER_AUTHORIZATION_FAILED.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Existing users of the kafka-reassign-partitions.sh will receive a deprecation warning when they use the --zookeeper option. The option will be removed in a future version of Kafka. If this KIP is introduced in version 1.0.0 the removal could happen in 2.0.0.

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.

One alternative is to do nothing: Let the ReassignPartitionsCommand continue to communicate with ZooKeeper directly.

Another alternative is to do exactly this KIP, but without the deprecation of --zookeeper. That would have a higher long term maintenance burden, and would prevent any future plans to, for example, provide alternative cluster technologies than ZooKeeper.

An alterTopics() AdminClient API, mirroring the existing createTopics() API, was considered, but:

• Some calls to alterTopics() (such as increasing the partition count) would have been synchronous, while others (such as moving replicas between brokers) would have been long running and thus asynchronous. This made for an API which synchronousness depended on the arguments.
• createTopics() allows to specify topic configs, whereas alterConfigs() is already provided to change topic configs, so it wasn't an exact mirror

Just providing reassignPartitions() was considered, with changes to partition count inferred from partitions present in the assignments argument. This would require the caller to provide an assignment of partitions to brokers, whereas currently it's possible to increase the partition count without specifying an assignment. It also suffered from the synchronous/asynchronous API problem.

Similarly a alterReplicationFactors() method, separate from reassignPartitions() was considered, but both require a partition to brokers assignment, and both are implemented in the same way (by writing to the /admin/reassign_partitions znode), so there didn't seem much point in making an API which distinguished them.