KRaft

KRaft

Configuration

Process Roles

In KRaft mode each Kafka server can be configured as a controller, a broker, or both using the process.roles property. This property can have the following values:

• If process.roles is set to broker, the server acts as a broker.
• If process.roles is set to controller, the server acts as a controller.
• If process.roles is set to broker,controller, the server acts as both a broker and a controller.
• If process.roles is not set at all, it is assumed to be in ZooKeeper mode.

Kafka servers that act as both brokers and controllers are referred to as “combined” servers. Combined servers are simpler to operate for small use cases like a development environment. The key disadvantage is that the controller will be less isolated from the rest of the system. For example, it is not possible to roll or scale the controllers separately from the brokers in combined mode. Combined mode is not recommended in critical deployment environments.

Controllers

In KRaft mode, specific Kafka servers are selected to be controllers (unlike the ZooKeeper-based mode, where any server can become the Controller). The servers selected to be controllers will participate in the metadata quorum. Each controller is either an active or a hot standby for the current active controller.

A Kafka admin will typically select 3 or 5 servers for this role, depending on factors like cost and the number of concurrent failures your system should withstand without availability impact. A majority of the controllers must be alive in order to maintain availability. With 3 controllers, the cluster can tolerate 1 controller failure; with 5 controllers, the cluster can tolerate 2 controller failures.

All of the servers in a Kafka cluster discover the quorum voters using the controller.quorum.voters property. This identifies the quorum controller servers that should be used. All the controllers must be enumerated. Each controller is identified with their id, host and port information. For example:

controller.quorum.voters=id1@host1:port1,id2@host2:port2,id3@host3:port3

If a Kafka cluster has 3 controllers named controller1, controller2 and controller3, then controller1 may have the following configuration:

process.roles=controller
node.id=1
listeners=CONTROLLER://controller1.example.com:9093
controller.quorum.voters=1@controller1.example.com:9093,2@controller2.example.com:9093,3@controller3.example.com:9093

Every broker and controller must set the controller.quorum.voters property. The node ID supplied in the controller.quorum.voters property must match the corresponding id on the controller servers. For example, on controller1, node.id must be set to 1, and so forth. Each node ID must be unique across all the servers in a particular cluster. No two servers can have the same node ID regardless of their process.roles values.

Storage Tool

The kafka-storage.sh random-uuid command can be used to generate a cluster ID for your new cluster. This cluster ID must be used when formatting each server in the cluster with the kafka-storage.sh format command.

This is different from how Kafka has operated in the past. Previously, Kafka would format blank storage directories automatically, and also generate a new cluster ID automatically. One reason for the change is that auto-formatting can sometimes obscure an error condition. This is particularly important for the metadata log maintained by the controller and broker servers. If a majority of the controllers were able to start with an empty log directory, a leader might be able to be elected with missing committed data.

Debugging

Metadata Quorum Tool

The kafka-metadata-quorum tool can be used to describe the runtime state of the cluster metadata partition. For example, the following command displays a summary of the metadata quorum:

$ bin/kafka-metadata-quorum.sh --bootstrap-server localhost:9092 describe --status
ClusterId:              fMCL8kv1SWm87L_Md-I2hg
LeaderId:               3002
LeaderEpoch:            2
HighWatermark:          10
MaxFollowerLag:         0
MaxFollowerLagTimeMs:   -1
CurrentVoters:          [3000,3001,3002]
CurrentObservers:       [0,1,2]

Dump Log Tool

The kafka-dump-log tool can be used to debug the log segments and snapshots for the cluster metadata directory. The tool will scan the provided files and decode the metadata records. For example, this command decodes and prints the records in the first log segment:

$ bin/kafka-dump-log.sh --cluster-metadata-decoder --files metadata_log_dir/__cluster_metadata-0/00000000000000000000.log

This command decodes and prints the records in the a cluster metadata snapshot:

$ bin/kafka-dump-log.sh --cluster-metadata-decoder --files metadata_log_dir/__cluster_metadata-0/00000000000000000100-0000000001.checkpoint

Metadata Shell

The kafka-metadata-shell tool can be used to interactively inspect the state of the cluster metadata partition:

$ bin/kafka-metadata-shell.sh --snapshot metadata_log_dir/__cluster_metadata-0/00000000000000000000.log
>> ls /
brokers  local  metadataQuorum  topicIds  topics
>> ls /topics
foo
>> cat /topics/foo/0/data
{
  "partitionId" : 0,
  "topicId" : "5zoAlv-xEh9xRANKXt1Lbg",
  "replicas" : [ 1 ],
  "isr" : [ 1 ],
  "removingReplicas" : null,
  "addingReplicas" : null,
  "leader" : 1,
  "leaderEpoch" : 0,
  "partitionEpoch" : 0
}
>> exit

Deploying Considerations

• Kafka server’s process.role should be set to either broker or controller but not both. Combined mode can be used in development environments, but it should be avoided in critical deployment environments.
• For redundancy, a Kafka cluster should use 3 controllers. More than 3 controllers is not recommended in critical environments. In the rare case of a partial network failure it is possible for the cluster metadata quorum to become unavailable. This limitation will be addressed in a future release of Kafka.
• The Kafka controllers store all the metadata for the cluster in memory and on disk. We believe that for a typical Kafka cluster 5GB of main memory and 5GB of disk space on the metadata log director is sufficient.

Missing Features

The following features are not fully implemented in KRaft mode:

• Supporting JBOD configurations with multiple storage directories. Note that an Early Access release is supported in 3.7 as per KIP-858. Note that it is not yet recommended for use in production environments. Please refer to the release notes to help us test it and provide feedback at KAFKA-16061.
• Modifying certain dynamic configurations on the standalone KRaft controller

ZooKeeper to KRaft Migration

Terminology

• Brokers that are in ZK mode store their metadata in Apache ZooKepeer. This is the old mode of handling metadata.
• Brokers that are in KRaft mode store their metadata in a KRaft quorum. This is the new and improved mode of handling metadata.
• Migration is the process of moving cluster metadata from ZooKeeper into a KRaft quorum.

Migration Phases

In general, the migration process passes through several phases.

• In the initial phase , all the brokers are in ZK mode, and there is a ZK-based controller.
• During the initial metadata load , a KRaft quorum loads the metadata from ZooKeeper,
• In hybrid phase , some brokers are in ZK mode, but there is a KRaft controller.
• In dual-write phase , all brokers are KRaft, but the KRaft controller is continuing to write to ZK.
• When the migration has been finalized , we no longer write metadata to ZooKeeper.

Limitations

• While a cluster is being migrated from ZK mode to KRaft mode, we do not support changing the metadata version (also known as the inter.broker.protocol.version.) Please do not attempt to do this during a migration, or you may break the cluster.
• After the migration has been finalized, it is not possible to revert back to ZooKeeper mode.
• During the migration, if a ZK broker is running with multiple log directories, any directory failure will cause the broker to shutdown. Brokers with broken log directories will only be able to migrate to KRaft once the directories are repaired. For further details refer to KAFKA-16431.
• As noted above, some features are not fully implemented in KRaft mode. If you are using one of those features, you will not be able to migrate to KRaft yet.

Preparing for migration

Before beginning the migration, the Kafka brokers must be upgraded to software version 3.8.1 and have the “inter.broker.protocol.version” configuration set to “3.8”.

It is recommended to enable TRACE level logging for the migration components while the migration is active. This can be done by adding the following log4j configuration to each KRaft controller’s “log4j.properties” file.

log4j.logger.org.apache.kafka.metadata.migration=TRACE

It is generally useful to enable DEBUG logging on the KRaft controllers and the ZK brokers during the migration.

Provisioning the KRaft controller quorum

Two things are needed before the migration can begin. First, the brokers must be configured to support the migration and second, a KRaft controller quorum must be deployed. The KRaft controllers should be provisioned with the same cluster ID as the existing Kafka cluster. This can be found by examining one of the “meta.properties” files in the data directories of the brokers, or by running the following command.

$ bin/zookeeper-shell.sh localhost:2181 get /cluster/id

The KRaft controller quorum should also be provisioned with the latest metadata.version. This is done automatically when you format the node with the kafka-storage.sh tool. For further instructions on KRaft deployment, please refer to the above documentation.

In addition to the standard KRaft configuration, the KRaft controllers will need to enable support for the migration as well as provide ZooKeeper connection configuration.

Here is a sample config for a KRaft controller that is ready for migration:

# Sample KRaft cluster controller.properties listening on 9093
process.roles=controller
node.id=3000
controller.quorum.voters=3000@localhost:9093
controller.listener.names=CONTROLLER
listeners=CONTROLLER://:9093

# Enable the migration
zookeeper.metadata.migration.enable=true

# ZooKeeper client configuration
zookeeper.connect=localhost:2181

# The inter broker listener in brokers to allow KRaft controller send RPCs to brokers
inter.broker.listener.name=PLAINTEXT

# Other configs ...

Note: The KRaft clusternode.id values must be different from any existing ZK broker broker.id. In KRaft-mode, the brokers and controllers share the same Node ID namespace.

Enter Migration Mode on the Brokers

Once the KRaft controller quorum has been started, the brokers will need to be reconfigured and restarted. Brokers may be restarted in a rolling fashion to avoid impacting cluster availability. Each broker requires the following configuration to communicate with the KRaft controllers and to enable the migration.

• controller.quorum.voters
• controller.listener.names
• The controller.listener.name should also be added to listener.security.property.map
• zookeeper.metadata.migration.enable

Here is a sample config for a broker that is ready for migration:

# Sample ZK broker server.properties listening on 9092
broker.id=0
listeners=PLAINTEXT://:9092
advertised.listeners=PLAINTEXT://localhost:9092
listener.security.protocol.map=PLAINTEXT:PLAINTEXT,CONTROLLER:PLAINTEXT

# Set the IBP
inter.broker.protocol.version=3.8

# Enable the migration
zookeeper.metadata.migration.enable=true

# ZooKeeper client configuration
zookeeper.connect=localhost:2181

# KRaft controller quorum configuration
controller.quorum.voters=3000@localhost:9093
controller.listener.names=CONTROLLER

Note: Once the final ZK broker has been restarted with the necessary configuration, the migration will automatically begin. When the migration is complete, an INFO level log can be observed on the active controller:

Completed migration of metadata from Zookeeper to KRaft

Migrating brokers to KRaft

Once the KRaft controller completes the metadata migration, the brokers will still be running in ZooKeeper mode. While the KRaft controller is in migration mode, it will continue sending controller RPCs to the ZooKeeper mode brokers. This includes RPCs like UpdateMetadata and LeaderAndIsr.

To migrate the brokers to KRaft, they simply need to be reconfigured as KRaft brokers and restarted. Using the above broker configuration as an example, we would replace the broker.id with node.id and add process.roles=broker. It is important that the broker maintain the same Broker/Node ID when it is restarted. The zookeeper configurations should be removed at this point. Finally, if you have set control.plane.listener.name. please remove it before restarting in KRaft mode.

If your broker has authorization configured via the authorizer.class.name property using kafka.security.authorizer.AclAuthorizer, this is also the time to change it to use org.apache.kafka.metadata.authorizer.StandardAuthorizer instead.

# Sample KRaft broker server.properties listening on 9092
process.roles=broker
node.id=0
listeners=PLAINTEXT://:9092
advertised.listeners=PLAINTEXT://localhost:9092
listener.security.protocol.map=PLAINTEXT:PLAINTEXT,CONTROLLER:PLAINTEXT

# Don't set the IBP, KRaft uses "metadata.version" feature flag
# inter.broker.protocol.version=3.8

# Remove the migration enabled flag
# zookeeper.metadata.migration.enable=true

# Remove ZooKeeper client configuration
# zookeeper.connect=localhost:2181

# Keep the KRaft controller quorum configuration
controller.quorum.voters=3000@localhost:9093
controller.listener.names=CONTROLLER

Each broker is restarted with a KRaft configuration until the entire cluster is running in KRaft mode.

Finalizing the migration

Once all brokers have been restarted in KRaft mode, the last step to finalize the migration is to take the KRaft controllers out of migration mode. This is done by removing the “zookeeper.metadata.migration.enable” property from each of their configs and restarting them one at a time.

Once the migration has been finalized, you can safely deprovision your ZooKeeper cluster, assuming you are not using it for anything else. After this point, it is no longer possible to revert to ZooKeeper mode.

# Sample KRaft cluster controller.properties listening on 9093
process.roles=controller
node.id=3000
controller.quorum.voters=3000@localhost:9093
controller.listener.names=CONTROLLER
listeners=CONTROLLER://:9093

# Disable the migration
# zookeeper.metadata.migration.enable=true

# Remove ZooKeeper client configuration
# zookeeper.connect=localhost:2181

# Other configs ...

Reverting to ZooKeeper mode During the Migration

While the cluster is still in migration mode, it is possible to revert to ZooKeeper mode. The process to follow depends on how far the migration has progressed. In order to find out how to revert, select the final migration step that you have completed in this table.

Note that the directions given here assume that each step was fully completed, and they were done in order. So, for example, we assume that if “Enter Migration Mode on the Brokers” was completed, “Provisioning the KRaft controller quorum” was also fully completed previously.

If you did not fully complete any step, back out whatever you have done and then follow revert directions for the last fully completed step.

• Deprovision the KRaft controller quorum.
• Then you are done.

|
Enter Migration Mode on the brokers |

• Deprovision the KRaft controller quorum.
• Using zookeeper-shell.sh, run rmr /controller so that one of the brokers can become the new old-style controller.
• On each broker, remove the zookeeper.metadata.migration.enable, controller.listener.names, and controller.quorum.voters configurations, and replace node.id with broker.id. Then perform a rolling restart of all brokers.
• Then you are done.

| It is important to perform the zookeeper-shell.sh step quickly , to minimize the amount of time that the cluster lacks a controller. Until the /controller znode is deleted, you can also ignore any errors in the broker log about failing to connect to the Kraft controller. Those error logs should disappear after second roll to pure zookeeper mode.
Migrating brokers to KRaft |

• On each broker, remove the process.roles configuration, replace the node.id with broker.id and restore the zookeeper.connect configuration to its previous value. If your cluster requires other ZooKeeper configurations for brokers, such as zookeeper.ssl.protocol, re-add those configurations as well. Then perform a rolling restart of all brokers.
• Deprovision the KRaft controller quorum.
• Using zookeeper-shell.sh, run rmr /controller so that one of the brokers can become the new old-style controller.
• On each broker, remove the zookeeper.metadata.migration.enable, controller.listener.names, and controller.quorum.voters configurations. Then perform a second rolling restart of all brokers.
• Then you are done.

|

• It is important to perform the zookeeper-shell.sh step quickly , to minimize the amount of time that the cluster lacks a controller. Until the /controller znode is deleted, you can also ignore any errors in the broker log about failing to connect to the Kraft controller. Those error logs should disappear after second roll to pure zookeeper mode.
• Make sure that on the first cluster roll, zookeeper.metadata.migration.enable remains set to true. Do not set it to false until the second cluster roll.

Finalizing the migration | If you have finalized the ZK migration, then you cannot revert. | Some users prefer to wait for a week or two before finalizing the migration. While this requires you to keep the ZooKeeper cluster running for a while longer, it may be helpful in validating KRaft mode in your cluster.