Status

Current state: Under Discussion

Discussion thread:

JIRA: KAFKA-10357 - Getting issue details... STATUS

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Motivation

Kafka Streams uses repartition topics to repartition the data when a key-based operation, e.g., aggregation or join, follows a key-changing operation, e.g., map. Additionally, Kafka Streams uses changelog topics to replicate the data in its state store for fault-tolerance. If any required repartition topic or changelog topic does not exist, it is created during a rebalance. More precisely these internal topics are created during the computation of the assignment. Consequently, if one of the internal topics is deleted between rebalances, it will be silently recreated as an empty topic and data might be lost. Kafka Streams users would either not notice the loss at all or notice it too late to limit the damage by stopping processing. Deletion of internal topics may happen by mistake. This silent recreation of internal topics could be avoided by creating the internal topics only once during the first-ever rebalance of the application (or after an application reset). However, determining the first-ever rebalance of an application is not always straightforward. For example, if all Kafka Streams clients of a Kafka Streams application are stopped, an internal topic is deleted, and then the Kafka Streams application is restarted, the deleted topic would be silently recreated as an empty topic because Kafka Streams does not have any information that it could leverage to recognize that the first rebalance after restarting is not the first-ever rebalance of the application.

We propose to move the creation of the internal topics to an initialization method that users can call before they start their application for the first time. That way, users have full control over when the internal topics are created and Kafka Streams can reliably notify the users about possible data loss by throwing an exception when internal topics do unexpectedly not exist. Additionally, we propose to add a configuration that allows to turn automatic creation of internal topics on or off. The configuration is needed for backward compatibility but also to keep the first steps with Kafka Streams simple for new users.

Public Interfaces

We propose to add a new exception that helps to discriminate errors originating from missing internal topics from other errors in the uncaught exception handler. For example, reacting on a missing source topic (i.e., MissingSourceTopicException) might be different from reacting on missing internal topics, because the process for re-creating source topics might differ from the process for re-creating internal topics. Furthermore, source topics might be owned by a different team than the internal topics, consequently different people need to be paged.

package org.apache.kafka.streams.errors;

public class MissingInternalTopicException extends StreamsException {
}

We propose to add an initialization method to the KafkaStreams class.

public class KafkaStreams {

	/**
	 * Initializes broker-side state.
     * 
     * If some but not all of the internal topics are absent it throws an exception.
     */
	public void init();

    /**
     * Initializes broker-side state.
     *
     * This methods takes parameters that specify which internal topics to setup if some
     * but not all of them are absent.
     */
	public void init(final InitParameters initParameters);

	public class InitParameters {
        
        public static InitParameters initParameters();                        // specifies to disable all setup of internal topics 

 		public InitParameters enableSetupRepartitionTopicsIfIncomplete();     // specifies to setup repartition topics if some are missing
 		public InitParameters disableSetupRepartitionTopicsIfIncomplete();    // specifies to throw if some but not all repartition topics are missing
 		public boolean setupRepartitionTopicsIfIncompleteEnabled();           // getter
 		public InitParameters enableSetupChangelogTopicsIfIncomplete();       // specifies to setup changelog topics if some are missing
 		public InitParameters disableSetupChangelogTopicsIfIncomplete();      // specifies to throw if some but not all changelog topics are missing
 		public boolean setupChangelogTopicsIfIncompleteEnabled();             // getter
 		public InitParameters enableSetupInternalTopicsIfIncomplete();        // specifies to setup repartition and changelog topics if some are missing
 		public InitParameters disableSetupInternalTopicsIfIncomplete();       // specifies to throw if some but not all repartition or changelog topics are missing
 		public boolean setupInternalTopicsIfIncompleteEnabled();              // getter
	}
}

We propose to add a new configuration to Kafka Streams to determine whether the internal topics should be setup during a rebalance or by users calling KafkaStreams#init().

public class StreamsConfig {

	// possible values
	public static final String AUTOMATIC_SETUP = "automatic";
	public static final String MANUAL_SETUP = "manual";

	// configuration
    public static final String INTERNAL_TOPIC_SETUP = "internal.topics.setup";  // default is AUTOMATIC_SETUP 
 
}

Proposed Changes

If configuration INTERNAL_TOPIC_SETUP is set to AUTOMATIC_SETUP, the internal topics will be set up during a rebalance. If the internal topics do not exist, they will be created. That corresponds to the current behavior of Kafka Streams. Users can also call KafkaStreams#init() to setup the internal topics when INTERNAL_TOPIC_SETUP is set to AUTOMATIC_SETUP, but the call is not necessary since the internal topics would be created anyways during the next rebalance.

If configuration INTERNAL_TOPIC_SETUP is set to MANUAL_SETUP, the internal topics will not be set up during a rebalance but users need to call KafkaStreams#init() to setup the internal topics. If the internal topics do not exist during a rebalance because KafkaStreams#init() has not been called or one or more internal topics have been deleted, a MissingInternalTopicException is thrown in each Kafka Streams client.

Method KafkaStreams#init() sets up the broker-side state for a Kafka Streams application. If the method:

• does not find all internal topics for the Kafka Streams client on the brokers, it will setup all internal topics
• finds all internal topics for the Kafka Streams client on the brokers, it will not setup any internal topics
• finds some of the internal topics it will
  • throw MissingInternalTopicsException if KafkaStreams#init() without any parameters is called or the parameters specify to throw for the missing internal topics
  • setup the missing internal topics if the parameters passed to KafkaStreams#init() specify so

For example, if a changelog topic is missing and init(initParameters.enableSetupChangelogTopicsIfIncomplete()) is called, the missing changelog topic will be setup. However, on a missing repartition topic the same call will throw. If KafkaStreams#init() is called, the call will throw if an internal topic is missing and other internal topics not.

In addition to setup internal topics, KafkaStreams#init() will make all checks that are currently done during a rebalance including checks for source and sink topics.

Compatibility, Deprecation, and Migration Plan

Since we introduce configuration INTERNAL_TOPIC_SETUP with default value AUTOMATIC_SETUP that ensures the current Kafka Streams behavior, this KIP should not affect backward-compatibility. 

We do not need to deprecate any public interfaces since we propose to add a new method to the public API that does not replace any other method.

Migrating from the current behavior to INTERNAL_TOPIC_SETUP set to MANUAL_SETUP can be done without any specific migration plan. Users need to set INTERNAL_TOPIC_SETUP to MANUAL_SETUP.  

Rejected Alternatives

• Persist a flag for the first-ever rebalance broker side: This approach was rejected because that would imply changes on the brokers which we thought we can avoid and still get a good solution with the approach proposed in this KIP.
• Use committed offsets for a repartition topic to verify if a repartition topic existed: This approach would not work since committed offsets are removed when a topic is deleted.