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Current state: Adopted
Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]
Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).
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
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/clients/consumer (eventually, once stable)
Command line tools and arguments
- Anything else that will likely break existing users in some way when they upgrade
kafka-preferred-replica-election.sh tool will gain a
--bootstrap-server option and the existing
--zookeeper option will be deprecated.
The AdminClient will gain a new method:
A new network protocol will be added:
ElectPreferredLeadersRequest and ElectPreferredLeadersResponse
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.
--zookeeper option will be retained and will:
- Cause a deprecation warning to be printed to standard error. The message will say that the
--zookeeperoption will be removed in a future version and that
--bootstrap-serveris the replacement option.
- Perform the election via ZooKeeper, as currently.
--bootstrap-server option will be added and will:
- Perform the election by calling AdminClient.electPreferredLeaders() on an AdminClient instance bootstrapped from the via the given
Using both options in the same command line 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
--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.
--help output for the tool will be updated to note that the command is not necessary if the broker is configured with
The following methods will be added to AdminClient:
A call to electPreferredLeaders() will send a ElectPreferredLeadersRequest to the controller broker.
NetworkProtocol: ElectPreferredLeadersRequest and ElectPreferredLeadersResponse
|a partition of the topic|
|the time to wait for the election to complete|
The request will require AlterCluster on the Cluster resource, since it is a change that affects the whole cluster.
Note: It is not an error if there is a duplicate (topic, partition)-pair in the request.
Note that a
ElectPreferredLeadersRequest must be sent to the controller of the cluster.
|duration in milliseconds for which the request was throttled|
|a topic name from the request|
|a partition id for the topic|
|an error code for that partition|
|The error message|
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.
NONE(0) The elections were successful.
Broker-side election algorithm
The broker-side handling of
ElectPreferredLeadersRequest will be somewhat different than currently:
- On receipt of
ElectPreferredLeadersRequestthe controller will atomically check-and-set a flag (to prevent concurrent elections) then enqueue a
- The controller will when await completion of the
PreferredReplicaLeaderElection, with a timeout.
- When processing the
PreferredReplicaLeaderElectionthe controller will clear the flag.
- Successful or timed-out completion of the
PreferredReplicaLeaderElectionwill result in a
ElectPreferredLeadersResponsebeing returned to the client
(The flag will also be checked-and-set when handling a change of the
/admin/preferred_replica_election znode, via the existing
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
error_code will provide a reason.
When support for the
--zookeeper option is eventually removed, the need for the
/admin/preferred_replica_election znode will disappear and consequently the code managing it will be removed.
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.
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 tool 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.