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Status

Current state: Under Discussion

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

JIRA: https://issues.apache.org/jira/browse/KAFKA-5693

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

Motivation

Kafka already has user configurable policies which can be used by a cluster administrator to limit how the cluster can be modified by non-administrator users (for example by using the AdminClient API):

  • CreateTopicPolicy can prevent a topic being created based on topic creation parameters (name, number of partitions & replication factor or replica assignments, topic configs)
  • AlterConfigPolicy can prevent a change to topic config (or, in theory, broker config, but it's current not possible to change broker configs via the AdminClient API)

As existing tools are migrated to using AdminClient APIs rather than interacting directly with ZooKeeper we need to apply policies to them, but the existing policy interfaces make it difficult to do this in a consistent way.

Problem 1 - Topic config is governed by both CreateTopicPolicy and AlterConfigPolicy

Currently the topic config is passed to the CreateTopicPolicy, but if a topic config is later modified the AlterConfigPolicy is applied. If an administrator wants to use the topic config in their policy decisions they have to implement this logic in two places. If the policy decision depends on both the topic config and another aspect of the topic the AlterConfigPolicy interface doesn't provide the necessary information.

Problem 2 - Creating more partitions is not currently covered by a policy

Changing the number of partitions in a topic was the subject of KIP-195 and is just one kind of topic modification. Consider two example use cases:

  1. It shouldn't be possible to create a topic, but then modify it so that it no longer conforms to the CreateTopicPolicy.
  2. An administrator who wants to prevent increasing the number of partitions entirely for topics with keys, because of the effect on partitioning.

To solve 1, we could simply apply the existing TopicCreationPolicy to modifications, but

  • this would obscure whether a particular invocation of the policy was for a topic creation or modification (the second bullet)
  • we would be left with a misleadingly named policy

So there needs to be a policy for specifically for modifying a topic. But it is confusing and error-prone if there are different policy classes for creation and modification (the CreateTopicPolicy and a new ModifyTopicPolicy, say): It would be easy for the code implementing a user's policies to get out of sync if it needs to be maintained in two places. It would also be easy to configure one policy but not the other. So it would be better if there were a single policy interface which is applied to both topic creation and modification.

Problem 3 - CreateTopicPolicy can govern partition assignment, but there is no policy for reassignment

Reassigning replicas is another kind of topic modification and the subject of KIP-179. By similar reasoning to example 2 it, too, should be covered by the same policy.

Problem 4 - There is no policy for topic deletion or message deletion

KIP-170 proposes a policy for topic deletion (see that KIP for the motivation behind this) and KIP-204 proposes to add an AdminClient API for the existing network protocol for deleting messages from the partitions of a topic.

It's pointless to add a policy for topic deletion if there is no policy for message deletion (deleting all the messages from the topic is practically equivalent to deleting the topic itself in most cases). If there were a separate TopicDeletePolicy and MessageDeletePolicy we have the similar problem as described above for separate topic creation and modification policies: It's unnecessarily difficult and tedious to keep the policies consistent.

Public Interfaces

Two new policy interfaces will be added:

  • TopicActionsPolicy will apply to topic creation and alteration and be configured by the new topic.actions.policy.class.name config
  • TopicDeletionPolicy will apply to message and topic deletion and be configured by the new topic.deletion.policy.class.name config.

The existing policy interfaces CreateTopicPolicy and AlterConfigPolicy will be deprecated, but will continue to be applied where they are currently applied until they are removed.

Proposed Changes

Add TopicActionsPolicy, TopicDeletionPolicy and supporting interfaces

The following policy interfaces and supporting classes will be added

/**
 * Represents the state of a topic either before, or as a result of,
 * an administrative request affecting a topic.
 */
interface TopicState {
    /**
     * The number of partitions of the topic.
     */
    int numPartitions();

    /**
     * The replication factor of the topic.
     */
    Short replicationFactor();

    /**
     * The replica assignments of the topic.
     */
    Map<Integer, List<Integer>> replicasAssignments();

    /**
     * The topic config.
     */
    Map<String,String> configs();

    /**
     * Returns whether the topic is marked for deletion.
     */
    boolean markedForDeletion();

}


/** The current state of the topics in the cluster, before the request takes effect. */
interface ClusterState {
    /**
     * Returns the current state of the given topic, or null if the topic does not exist.
     */
    TopicState topicState(String topicName);

    /**
     * Returns all the topics in the cluster, including internal topics if
     * {@code includeInternal} is true, and including those marked for deletion
     * if {@code includeMarkedForDeletion} is true.
     */
    Set<String> topics(boolean includeInternal, boolean includeMarkedForDeletion);
}

/**
 * A policy that is enforced on topic creation and alteration.
 * An implementation of this policy can be configured on a broker via the
 * {@code topic.actions.policy.class.name} broker config.
 * When this is configured the named class will be instantiated reflectively
 * using its nullary constructor and will then pass the broker configs to
 * its <code>configure()</code> method. During broker shutdown, the
 * <code>close()</code> method will be invoked so that resources can be
 * released (if necessary).
 *
 * @see TopicDeletionPolicy for the policy for deleting messages and topics.
 */
interface TopicActionsPolicy extends Configurable, AutoCloseable {

    /**
     * Parameters for a request to {@linkplain #isCreate() create} or
     * {@linkplain #isAlter() alter} the given {@linkplain #topic}.
     *
     * @see #validate(RequestMetadata, ClusterState)
     */
    static interface RequestMetadata {

        /**
         * Returns true if the request is for the creation of the given {@link #topic()}.
         */
        public boolean isCreate();

        /**
         * Returns true if the request is for the alteration of the given {@link #topic()}.
         */
        public boolean isAlter();

        /**
         * The topic the action is being performed upon.
         */
        public String topic();

        /**
         * The authenticated principal making the request, or null if the session is not authenticated.
         */
        public KafkaPrincipal principal();

        /**
         * The state the topic will have after the request.
         * <ul>
         * <li>When {@link #isCreate()} is true, this will be the requested state of the topic to be created.</li>
         * <li>When {@link #isAlter()} is true, this will be the state the topic will have after the alteration.</li>
         * </ul>
         */
        public TopicState requestedState();

    }

    /**
     * Validate the request parameters and throw a <code>PolicyViolationException</code> with a suitable error
     * message if the request parameters for the provided topic do not satisfy this policy.
     *
     * Clients will receive the POLICY_VIOLATION error code along with the exception's message.
     * Note that validation failure only affects the relevant topic,
     * other topics in the request will still be processed.
     *
     * @param requestMetadata the request parameters for the provided topic.
     * @param clusterState the current state of the cluster
     * @throws PolicyViolationException if the request parameters do not satisfy this policy.
     */
    void validate(RequestMetadata requestMetadata, ClusterState clusterState) throws PolicyViolationException;
}

/**
 * A policy that is enforced on message or topic deletion.
 * An implementation of this policy can be configured on a broker via the
 * {@code topic.deletion.policy.class.name} broker config.
 * When this is configured the named class will be instantiated reflectively
 * using its nullary constructor and will then pass the broker configs to
 * its {@link #configure(Map)}} method. During broker shutdown, the
 * {@link #close()} method will be invoked so that resources can be
 * released (if necessary).
 *
 * @see TopicActionsPolicy for the policy for creating and altering topics.
 */
interface TopicDeletionPolicy extends Configurable, AutoCloseable {
    /**
     * Parameters for a request to delete {@linkplain #isMessageDeletion()} messages} from the topic
     * or {@linkplain #isTopicDeletion() delete the entire topic}
     *
     * @see #validate(RequestMetadata, ClusterState)
     */
    static interface RequestMetadata {

        /**
         * The topic that is the subject of the deletion.
         */
        public String topic();

        /**
         * The authenticated principal making the request, or null if the session is not authenticated.
         */
        public KafkaPrincipal principal();

        /**
         * Returns true if the topic itself is being deleted, or false if
         * the topic is not being deleted but zero or more records from
         * one or more of the topic's partitions are being deleted.
         * This is mutually exclusive with {@link #isMessageDeletion()}.
         */
        boolean isTopicDeletion();

        /**
         * Returns true zero or more records from
         * one or more of the topic's partitions are being deleted, but the topic itself is
         * not being deleted.
         * This is mutually exclusive with {@link #isTopicDeletion()}.
         */
        boolean isMessageDeletion();

        /**
         * Returns a map of topic partitions and the corresponding offset of the last message
         * to be retained. Messages before this offset will be deleted.
         * Partitions which won't have messages deleted won't be present in the map.
         * When {@link #isTopicDeletion()} is true then all of the topic's partitions will be
         * present in the map and all the offsets will be {@link Long#MAX_VALUE}.
         */
        Map<Integer, Long> deletedMessageOffsets();
    }
    /**
     * Validate the request parameters and throw a <code>PolicyViolationException</code> with a suitable error
     * message if the request parameters for the provided topic do not satisfy this policy.
     *
     * Clients will receive the POLICY_VIOLATION error code along with the exception's message.
     * Note that validation failure only affects the relevant topic,
     * other topics in the request will still be processed.
     *
     * @param requestMetadata the request parameters for the provided topic.
     * @param clusterState the current state of the cluster
     * @throws PolicyViolationException if the request parameters do not satisfy this policy.
     */
    void validate(RequestMetadata requestMetadata, ClusterState clusterState) throws PolicyViolationException;
}

The TopicActionsPolicy will be applied:

  • On topic creation
  • On topic modification
    • Change in topic config, via AdminClient.alterConfigs() (this change done as part of this KIP).
    • Adding partitions to topics, via AdminClient.createPartitions() (see KIP-195, but this change done as part of this KIP)
    • Reassigning partitions to brokers, and/or changing the replication factor via AdminClient.reassignPartitions() (see KIP-179)

On topic deletion will be applied on topic and message deletion.

Note: Unlike previous policy interfaces the inner RequestMetadata is an interface rather than a class. This should simplify testing and better permit use sites to, for example, lazily fetch metadata when it's actually required by the policy implementation, rather than eagerly fetch information which the policy didn't actually require.

Deprecate existing policies

The existing CreateTopicPolicy and AlterConfigPolicy will be deprecated, but will continue to be applied when they are configured.

Using create.topic.policy.class.name or  alter.config.policy.class.name will result in an deprecation warning in the broker logs.

It will be a configuration time error if both create.topic.policy.class.name and topic.actions.policy.class.name are used at the same time, or both alter.config.policy.class.name and topic.actions.policy.class.name are used at the same time.

Internally, an adapter implementation of TopicActionsPolicy will be used when CreateTopicPolicy and AlterConfigPolicy are configured, so policy use sites won't be unnecessarily complicated.

If, in the future, AdminClient.alterConfigs()/AlterConfigsRequest is changed to support changing broker configs a separate policy interface can be applied to such changes.

Compatibility, Deprecation, and Migration Plan

Existing users will have to reimplement their policies in terms of the new TopicActionsPolicy interface, and reconfigure their brokers accordingly. Since the TopicActionsPolicy contains a superset of the existing information used by the deprecated policies such reimplementation should be trivial.

The deprecated policy interfaces and configuration keys will be removed in a future Kafka version. If this KIP is accepted for Kafka 1.1.0 this removal could happen in Kafka 2.0.0 or a later major release.

Rejected Alternatives

The objectives of this KIP could be achieved without deprecating the existing policy classes, but that:

  • incurs ongoing maintenance and testing costs on the project for not overall benefit
  • If two policies were in force it would be more confusing to users when a request was rejected (which policy rejected it?) possibly exacerbated if users didn't know two policies were in force.
  • If it were possible to have two policies in force administrators have not been relieved of the burden of maintaining two policies in sync.

The proposed TopicActionsPolicy doesn't have to cover the topic deletion case: That could still be handled by a separate policy, but it is desirable to have a single policy to cover the whole lifecycle of a topic, and for the same information to be made available about a topic being deleted as about a topic being modified.

The proposed TopicActionsPolicy doesn't cover the use case of records being deleted from a topic. This is not the same as the modification of a topic, and would require a different policy interface. It might be appropriate to use the same topic state in such a policy interface, however.

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