Status

Current state: "Under Discussion"

Discussion thread: [DISCUSS] KIP-591: Add Kafka Streams config to set default store type

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Released:

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Motivation

Kafka Streams supports persistent RocksDB stores as well as in-memory stores out of the box. By default all DSL operators use persistent RocksDB stores. Currently, it is only possible to switch out RocksDB stores with in-memory store on a per operator basis what is tedious if all stores should be switched (for example in a Kubernetes deployment without local disks). Furthermore, the current store interface via Materialized is very generic as it allows to pass in any custom state store implementation with the side effect that a user needs to name the store and thus also makes it queryable via "Interactive Queries".

We propose to simplify switching the store type via a new config to Kafka Streams that allows to set the default built-in or custom store implementation for the whole program as well as an improved API for Materialized.

Public Interfaces

We propose to add a new configuration parameter default.store.impl.class that defines the default store implementation class used by the DSL. Default to RocksDBPersistentStoreImplementation.class

public class StreamsConfig {     
    public static final String DEFAULT_STORE_IMPLEMENTATION_CLASS_CONFIG = "default.store.impl.class";
    private static final String DEFAULT_STORE_IMPLEMENTATION_CLASS_DOC = 
          "Store supplier implementation class to use. Default is set as RocksDBPersistentStoreImplementation. It can be overwritten dynamically during streaming operation.";                


	.define(DEFAULT_STORE_IMPLEMENTATION_CLASS_CONFIG,
            Type.CLASS,
            RocksDBPersistentStoreImplementation.class.getName(),
            Importance.MEDIUM,
            DEFAULT_STORE_IMPLEMENTATION_CLASS_DOC) 
}

The StoreImplementation interface:

/**
 * A state store supplier Implementation interface for all types of {@link StateStore}.
 *
 */
public interface StoreImplementation {
    /**
     * Create a {@link KeyValueBytesStoreSupplier}.
     *
     * @param name  name of the store (cannot be {@code null})
     * @return an instance of a {@link KeyValueBytesStoreSupplier} that can be used
     * to build a persistent key-value store
     */
    KeyValueBytesStoreSupplier keyValueSupplier(String name);

    /**
     * Create a {@link WindowBytesStoreSupplier}.
     *
     * @param name                  name of the store (cannot be {@code null})
     * @param retentionPeriod       length of time to retain data in the store (cannot be negative)
     *                              (note that the retention period must be at least long enough to contain the
     *                              windowed data's entire life cycle, from window-start through window-end,
     *                              and for the entire grace period)
     * @param windowSize            size of the windows (cannot be negative)
     * @param retainDuplicates      whether or not to retain duplicates. Turning this on will automatically disable
     *                              caching and means that null values will be ignored.
     * @return an instance of {@link WindowBytesStoreSupplier}
     * @throws IllegalArgumentException if {@code retentionPeriod} or {@code windowSize} can't be represented as {@code long milliseconds}
     * @throws IllegalArgumentException if {@code retentionPeriod} is smaller than {@code windowSize}
	 */
    WindowBytesStoreSupplier windowBytesStoreSupplier(String name,
                                                      Duration retentionPeriod,
                                                      Duration windowSize,
                                                      boolean retainDuplicates);


    /**
     * Create a {@link SessionBytesStoreSupplier}.
     * 
     * @param name              name of the store (cannot be {@code null})
     * @param retentionPeriod   length of time to retain data in the store (cannot be negative)
     *                          (note that the retention period must be at least as long enough to
     *                          contain the inactivity gap of the session and the entire grace period.)
     * @return an instance of a {@link  SessionBytesStoreSupplier}
     */
    SessionBytesStoreSupplier sessionBytesStoreSupplier(String name,
                                                        Duration retentionPeriod);
}

For the StoreImplementation, Kakfa provides 3 built-in kinds of implementation:

RocksDBPersistentStoreImplementation.java (default)

/**
 * A Rocks DB persistent state store supplier Implementation.
 *
 */
public class RocksDBPersistentStoreImplementation implements StoreImplementation {
    @Override
    public KeyValueBytesStoreSupplier keyValueSupplier(final String name) {
        return Stores.persistentTimestampedKeyValueStore(name);
    }

    @Override
    public WindowBytesStoreSupplier windowBytesStoreSupplier(final String name,
                                                             final Duration retentionPeriod,
                                                             final Duration windowSize,
                                                             final boolean retainDuplicates) {
        return Stores.persistentTimestampedWindowStore(name, retentionPeriod, windowSize, retainDuplicates);
    }

    @Override
    public SessionBytesStoreSupplier sessionBytesStoreSupplier(final String name, final Duration retentionPeriod) {
        return Stores.persistentSessionStore(name, retentionPeriod);
    }
}

RocksDBStoreImplementation.java

/**
 * A Rocks DB state store supplier Implementation.
 *
 */
public class RocksDBStoreImplementation implements StoreImplementation {
    @Override
    public KeyValueBytesStoreSupplier keyValueSupplier(final String name) {
        return Stores.persistentKeyValueStore(name);
    }

    @Override
    public WindowBytesStoreSupplier windowBytesStoreSupplier(final String name,
                                                             final Duration retentionPeriod,
                                                             final Duration windowSize,
                                                             final boolean retainDuplicates) {
        return Stores.persistentWindowStore(name, retentionPeriod, windowSize, retainDuplicates);
    }

    @Override
    public SessionBytesStoreSupplier sessionBytesStoreSupplier(final String name, final Duration retentionPeriod) {
        return Stores.persistentSessionStore(name, retentionPeriod);
    }
}

InMemoryStoreImplementation.java

/**
 * A In-memory state store supplier Implementation.
 *
 */
public class InMemoryStoreImplementation implements StoreImplementation {
    @Override
    public KeyValueBytesStoreSupplier keyValueSupplier(final String name) {
        return Stores.inMemoryKeyValueStore(name);
    }

    @Override
    public WindowBytesStoreSupplier windowBytesStoreSupplier(final String name,
                                                             final Duration retentionPeriod,
                                                             final Duration windowSize,
                                                             final boolean retainDuplicates) {
        return Stores.inMemoryWindowStore(name, retentionPeriod, windowSize, retainDuplicates);
    }

    @Override
    public SessionBytesStoreSupplier sessionBytesStoreSupplier(final String name, final Duration retentionPeriod) {
        return Stores.inMemorySessionStore(name, retentionPeriod);
    }
}

In Materialized class, we provided one more API to allow users to provide StoreImplementation dynamically.

	/**
     * Materialize a {@link StateStore} with the store implementation.
     *
     * @param storeImplementation  store implementation used to materialize the store
     * @param <K>       key type of the store
     * @param <V>       value type of the store
     * @param <S>       type of the {@link StateStore}

     * @return a new {@link Materialized} instance with the given storeImplementation
     */
    public static <K, V, S extends StateStore> Materialized<K, V, S> as(final StoreImplementation storeImplementation)

Proposed Changes

In order to leverage the new configuration, users will need to call the new proposed API in KAFKA-13281, to provide topology-config while construct the StreamBuilder .

Properties props = new Properties();
...
props.put(StreamsConfig.DEFAULT_STORE_IMPLEMENTATION_CLASS_CONFIG, InMemoryStoreImplementation.class.getName());
StreamsConfig streamsConfig = new StreamsConfig(props);

builder = new StreamsBuilder(new TopologyConfig(streamsConfig));
topology = builder.build();

If not set, it'll default to use RocksDBPersistentStoreImplementation , which is the same as current behavior. So this is a backward-compatible design.

All stateful operators (like aggregate(), reduce(), count(), or join(), will use the state store in the "default.store.impl.class" configuration. Of course, if the store is overwritten for an individual operator via Materialized the operator overwrite will be used. The improved Materialized interface allows to switch the store type more easily without the need to specify a store name.

KStream stream = ...

// current code to change from RocksDB to in-memory
stream.groupByKey().count(Materialized.as(Stores.inMemoryKeyValueStore("some-name"));

// new code
stream.groupByKey().count(Materialized.as(new InMemoryStoreImplementation()));

Compatibility, Deprecation, and Migration Plan

Because the default value of the config is "RocksDBPersistentStoreImplementation" there is no behavior change and thus there are not backward compatibility concerns.

Test Plan

Regular unit and integration testing is sufficient.

Rejected Alternatives

1. Use enum toset the default store type

We propose to add a new configuration parameter default.dsl.store.type that defines the default store type used by the DSL accepting two values "rocksDB" (default) and "in_memory" .

public class StreamsConfig {
    public static final String DEFAULT_DSL_STORE_TYPE_CONFIG = "default.dsl.store.type";
    private static final String DEFAULT_DSL_STORE_TYPE_DOC = "The default store type used by DSL operators.";

    public static final ROCKS_DB = "rocksDB";
    public static final IN_MEMORY = "in_memory";
    
    .define(DEFAULT_DSL_STORE_TYPE_CONFIG,
            Type.STRING,
            ROCKS_DB,
            in(ROCKS_DB, IN_MEMORY),
            Importance.LOW,
            DEFAULT_DSL_STORE_TYPE_DOC)
}

In addition, we propose to extend Materialized config object with a corresponding option to specify the store type:

public class Materialized<K, V, S extends StateStore> {

  public enum StoreType {
    ROCKS_DB,
    IN_MEMORY
  }

  public static <K, V, S extends StateStore> Materialized<K, V, S> as(StoryType storeType);

  public Materialized<K, V, S> withStoreType(StoryType storeType);
}

Finally, we propose to deprecate the StreamsBuilder#build() method.

public class StreamsBuilder {
  @Deprecate
  public synchronized Topology build();
}

In order to leverage the new configuration, users will need to call StreamsBuilder#build(Properties) instead of StreamsBuilder#build(). This is an already established pattern that users need to follow to enable topology optimization. As we predict that we might need to access the config in the DSL more often in the future, we also propose to deprecate StreamsBuilder#build() method to lift the mental burden for users to know which of both methods that should use.

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