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Status

Current state: Accepted (2.2)Adopted

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

JIRA: KAFKA-5692

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.

Similarly to KIP-179, the kafka-preferred-replica-election.sh tool takes a --zookeeper option which means users of the tool must have access to the ZooKeeper cluster backing the Kafka cluster. There is no AdminClient API via which the preferred leader can be elected, so it is only this tool which can be used to do this job. This KIP will provide an AdminClient API for electing the preferred leader, add an option to the kafka-preferred-replica-election.sh tool to use this new API and deprecate the --zookeeper option.

Public

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Interface

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

The kafka-preferred-replica-election.sh tool will gain a --bootstrap-server option and the existing --zookeeper option will be deprecated.

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• ElectPreferredLeadersRequest and ElectPreferredLeadersResponse

Proposed Changes

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kafka-preferred-replica-election.sh

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

When the --bootstrap-server option is used new further new options will be available:

• admin.config — "Admin client config properties file to pass to the admin client when --bootstrap-server is given."

The --help output of the tool will be updated to explain what the preferred replica *is*, because this is currently not discoverable from the command line tool help, only from the documentation on the Kafka website.

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public class ElectPreferredLeadersOptions {
    public ElectPreferredLeadersOptions() { ... }
    /**
     * The request timeout in milliseconds for this operation or {@code null} if the default request timeout for the
     * AdminClient should be used.
     */
    public Integer timeoutMs() { ... }
    /**
     * Set the request timeout in milliseconds for this operation or {@code null} if the default request timeout for the
     * AdminClient should be used.
     */
    public ElectPreferredLeadersOptions timeoutMs(Integer timeoutMs) { ... }
}
public extends AbstractOptions<ElectPreferredLeadersOptions> {
}
public class ElectPreferredLeadersResult {
    // package access constructor

    /**
     * Get the result of the election for the given TopicPartition.
     * If there was not an election triggered for the given TopicPartition, the
     * returned future will complete with an error.
     */
    public KafkaFuture<Void> partitionResult(TopicPartition partition) { ... }

    /**
     * <p>Get the topic partitions for which a leader election was attempted.
     * The presence of a topic partition in the Collection obtained from 
     * the returned future does not indicate the election was successful: 
     * A partition will appear in this result if an election was attempted
     * even if the election was not successful.</p>
     *
     * <p>This method is provided to discover the partitions when
     * {@link AdminClient#electPreferredLeaders(Collection)} is called 
     * with a null {@code partitions} argument.</p>
     */
    public KafkaFuture<Collection<TopicPartition>>KafkaFuture<Set<TopicPartition>> partitions();

    /**
     * Return a future which succeeds if all the topic elections succeed.
     */
    KafkaFuture<Void> all() { ... }
 }

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ElectPreferredLeadersRequest => [topic_partitionsTopicPartitions] TimeoutMs
  topic_partitionsTopicPartitions => topic [partition_id]Topic PartitionId
    topicTopic => STRINGstring
    partition_idPartitionId => INT32
[int32]
  TimeoutMs => int32 

Where

The request will require AlterCluster on the Cluster resource, since it is a change that affects the whole cluster.

...

ElectPreferredLeadersResponse => throttle_time_msThrottleTimeMs [replica_election_resultReplicaElectionResult]
  throttle_time_msThrottleTimeMs => INT32int32
  replica_election_resultReplicaElectionResult => topicTopic [partition_resultPartitionResult]
    topicTopic => STRINGstring
    partition_resultPartitionResult => partition_id error_code error_messagePartitionId ErrorCode ErrorMessage
      partition_id PartitionId => INT32int32
      error_codeErrorCode => INT16int16
      error_messageErrorMessage => NULLABLE_STRINGstring

Where

Anticipated errors:

• UNKNOWN_TOPIC_OR_PARTITION (3) If the topic or partition doesn't exist on any broker in the cluster. Note that the use of this code is not precisely the same as it's usual meaning of "This server does not host this topic-partition".

• NOT_CONTROLLER (41) If the request is sent to a broker that is not the controller for the cluster.

• CLUSTER_AUTHORIZATION_FAILED (31) If the user didn't have Alter access to the topic.

• PREFERRED_LEADER_NOT_AVAILABLE (80) If the preferred lead could not be elected (for example because it is not currently in the ISR)

• NONE (0) The elections were successful.

Broker-side election algorithm

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1. On receipt of ElectPreferredLeadersRequest the controller will atomically check-and-set a flag (to prevent concurrent elections) then enqueue a PreferredReplicaLeaderElection with the ControllerManager
2. The controller will when await completion of the PreferredReplicaLeaderElection, with a timeout.

3. After the batch of elections has been started, a callback will either return the responses to the client (if they're available immediately, for example all the leaders were already the preferred ones), or use a purgatory to await the completion of the all of the elections.

4. Each UpdateMetadataRequest will try to complete the election purgatoryWhen processing the PreferredReplicaLeaderElection the controller will clear the flag.
5. Successful or timed-out completion of the PreferredReplicaLeaderElection will result in a ElectPreferredLeadersResponse being returned to the client

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This change means that the ElectPreferredLeadersResponse is sent when the election is actually complete, rather than when the /admin/preferred_replica_election znode has merely been updated. Thus if the election fails, the ElectPreferredLeadersResponse's error_code will provide a reason.

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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-preferred-replica-election.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

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

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