Status

Current state: Under discussion

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

JIRA: here [Change the link from KAFKA-1 to your own ticket]

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

Motivation

The behavior of groups in Apache Kafka is more complicated and subtle than it first appears. To most users of Kafka, groups are synonymous with consumer groups. However, the "classic" consumer group protocol was extensible and there are several well-known extensions in use. For example, distributed workers in Kafka Connect also use groups as a coordination mechanism, and some applications such as schema registries have also built upon the consumer group protocol in interesting ways. These are all groups. Then, KIP-848 introduced the new consumer group protocol and modern consumer groups use this new protocol. KIP-932 introduces share groups, and there may well be additional types of group in the future.

All of these types of groups share a namespace for group IDs, but the manner in which you administer a group depends upon its type.

Here’s an example of the complexity. If you start up a distributed Kafka Connect worker using the default configuration, it creates a group called "connect-cluster" . This is a group, but it’s not a consumer group. You can’t see this group in the list of consumer groups with the kafka-consumer-groups.sh  tool, but if you try to describe a consumer group called "connect-cluster"  or even use this group ID with a consumer, you get an error.

This KIP tries to resolve some of these situations and make it easier to work out what’s going on with the groups on a cluster.

Public Interfaces

Client API changes

AdminClient

Add the following methods on the org.apache.kafka.client.admin.AdminClient  interface.

Here are the method signatures:

   /**
    * List the groups available in the cluster with the default options.
    *
    * <p>This is a convenience method for {@link #listGroups(ListGroupsOptions)} with default options.
    * See the overload for more details.
    *
    * @return The ListGroupsResult.
    */
   default ListGroupsResult listGroups() {
       return listGroups(new ListGroupsOptions());
   }
 
   /**
    * List the groups available in the cluster.
    *
    * @param options The options to use when listing the groups.
    * @return The ListGroupsResult.
    */
   ListGroupsResult listGroups(ListGroupsOptions options);

ListGroupOptions

package org.apache.kafka.client.admin;
 
import org.apache.kafka.common.GroupType;
 
/**
 * Options for {@link Admin#listGroups(ListGroupsOptions)}.
 *
 * The API of this class is evolving, see {@link Admin} for details.
 */
@InterfaceStability.Evolving
public class ListGroupsOptions extends AbstractOptions<ListGroupsOptions> {
}

ListGroupsResult

package org.apache.kafka.clients.admin;
   
/**
 * The result of the {@link Admin#listGroups(ListGroupsOptions)} call.
 * <p>
 * The API of this class is evolving, see {@link Admin} for details.
 */
@InterfaceStability.Evolving
public class ListGroupsResult {
    /**
     * Returns a future that yields either an exception, or the full set of group listings.
     */
    public KafkaFuture<Collection<GroupListing>> all() {
    }
  
    /**
     * Returns a future which yields just the valid listings.
     */
    public KafkaFuture<Collection<GroupListing>> valid() {
    }
   
    /**
     * Returns a future which yields just the errors which occurred.
     */
    public KafkaFuture<Collection<Throwable>> errors() {
    }
}

GroupListing

package org.apache.kafka.client.admin;
  
import org.apache.kafka.common.ShareGroupState;
  
/**
 * A listing of a group in the cluster.
 * <p>
 * The API of this class is evolving, see {@link Admin} for details.
 */
@InterfaceStability.Evolving
public class GroupListing {
  public GroupListing(String groupId, GroupType type, String protocolType);
  
  /**
   * The id of the group.
   */
  public String groupId();
  
  /**
   * The group type.
   */
  public GroupType type();
  
  /**
   * The protocol type.
   */
  public String protocolType();
}

Command-line tools

kafka-groups.sh

A new tool called kafka-groups.sh  is introduced for listing and describing groups of any kind. It has the following options:

Note that --group-type consumer  actually matches all groups whose protocol is "consumer" , and --group-type share  matches all groups whose type is Share. The filtering is done in the kafka-groups.sh  tool.

Here are some examples.

To list all of the groups:

$ bin/kafka-groups.sh --bootstrap-server localhost:9092 --list
old-consumer-group
new-consumer-group
connect-cluster
share-group

To describe all of the groups and their types:

$ bin/kafka-groups.sh --bootstrap-server localhost:9092 --describe
GROUP                   TYPE          PROTOCOL
old-consumer-group      Classic       consumer
new-consumer-group      Consumer      consumer
connect-cluster         Classic       connect
share-group             Share         share

To list all of the consumer groups:

$ bin/kafka-groups.sh --bootstrap-server localhost:9092 --list --group-type consumer
old-consumer-group
new-consumer-group

To describe all of the consumer groups:

$ bin/kafka-groups.sh --bootstrap-server localhost:9092 --describe --group-type consumer
GROUP                   TYPE          PROTOCOL
old-consumer-group      Classic       consumer
new-consumer-group      Consumer      consumer

kafka-consumer-groups.sh

For all operations which act on a single group, if that group exists but is not a consumer group, the command fails with a message indicating that the group type is incorrect, rather than the existing message that the group does not exist.

For example, if you try to describe a share group, the output will look like this:

$ bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --describe --group SG1
Error: Group 'SG1' is not a consumer group.

kafka-share-groups.sh

A new option --create  is added to this tool to create a share group. If the share group exists, the command succeeds. If the group does not exist, the share group is created. If the group exists but it's not a share group, the command fails.

$ bin/kafka-share-groups.sh --bootstrap-server localhost:9092 --create --group NewShareGroup
Share group 'NewShareGroup' exists.

$ bin/kafka-share-groups.sh --bootstrap-server localhost:9092 --create --group ExistingShareGroup
Share group 'ExistingShareGroup' exists.

$ bin/kafka-share-groups.sh --bootstrap-server localhost:9092 --create --group ConsumerGroup
Error: Group 'ConsumerGroup' is not a share group.

Under the covers, it uses the AlterShareGroupOffsets RPC with an empty Topics  array.

Also, for all operations which act on a single group, if that group exists but is not a share group, the command fails with a message indicating that the group type is incorrect, rather than the existing messages that the group does not exist.

For example, if you try to describe a consumer group, the output will look like this:

$ bin/kafka-share-groups.sh --bootstrap-server localhost:9092 --describe --group CG1
Error: Group 'CG1' is not a share group.

Proposed Changes

This KIP introduces a command-line tool for displaying all of the groups and their types.

Next, it introduces a way to create a share group administratively. If you create Kafka resources such as topics as part of deploying an application, you can now create share groups in the same way.

Finally, in situations where command-line tools are used to administer a group of the wrong type, you’ll now be told the group type is wrong, rather than the group does not exist.

Listing groups

The KIP introduces a new tool called kafka-groups.sh to show all of the groups in a cluster, their types and the protocols they use. This lets you see consumer groups, share groups, Kafka Connect cluster groups, and any other custom groups all together. It doesn't replace the specific tools for the different types of group, but it does shed light on what's actually going on for administrators. Note that this does not require any changes to the Kafka protocol. The information is already available, but not directly accessible by the administrator.

The ListGroups RPC response returns three pieces of information for each group: group ID, type and protocol. For the common types of group, here is what they mean:

The new kafka-groups.sh tool makes all of this information available.

Creating groups

Groups are created dynamically on first use. For example, when you connect the first consumer in a consumer group, the group coordinator creates the group as a consumer group. This is convenient, but it does mean that you need to ensure that different types of group use distinct group IDs or the results will be unpredictable. This is because the group type depends upon how the group was created, whether it was a consumer group member, a distributed Kafka Connect cluster, or whatever.

Today, you can create a consumer group administratively before the first member joins by resetting the offsets, such as like this:

$ bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --reset-offsets --to-latest --group CG1 --topic T1 --execute

That’s a slightly contorted way to create a consumer group, but it does already exist and it is known. As a result, this KIP does not introduce a new way to create consumer groups.

For share groups, a new --create  option is added to the kafka-share-groups.sh  tool to create a share group. You can use --reset-offsets  in a similar way as consumer groups, but if you want to create a share group without specifying any topics, use --create  instead. Note that the starting offset of a share group comes from the group's group.share.auto.offset.reset  configuration property as introduced in KIP-932. This defaults to "latest" .

$ bin/kafka-share-groups.sh --bootstrap-server localhost:9092 --create --group SG1 

Compatibility, Deprecation, and Migration Plan

The only change to existing behavior is the error messages issued by the kafka-consumer-groups.sh  and kafka-share-groups.sh  tools when working with groups of the wrong type.

Test Plan

The feature will be thoroughly tested with unit and integration tests.

Rejected Alternatives

None.