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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: KAFKA-5651 

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

Motivation

As the Kafka Streams DSL has evolved, some of the APIs have become very overload heavy. For example, we have 8 different overloads for KStream#print. As we add more overloads it becomes harder for a developer using a modern IDE to discover the interfaces hence interrupting the flow and becoming an API usability issue.

Further, we'd like to provide users with a way to override certain StateStore features on a per operator basis, for example, enable caching or logging, for some but not all StateStores. Without a change in approach to the DSL this would add yet more overloads for every operation. Additionally, it should be simple to use the KafkaStreams Caching and Logging wrappers with custom StateStores. 

Before we go and add many more overloaded methods it is worth while exploring other options to see if we can provide a more concise and intuitive API.

Public Interfaces

New methods added to existing interfaces:

KStream
void print(final Printed<K, V> printed);

KStream<K, V> through(final String topic, final Partitioned<K, V> partitioned);

void to(final String topic, final Partitioned<V, V> partitioned);

KGroupedStream<K, V> groupByKey(final Serialized<K, V> serialized);

<KR> KGroupedStream<KR, V> groupBy(final KeyValueMapper<? super K, ? super V, KR> selector, Serialized<KR, V> serialized);

<VO, VR> KStream<K, VR> join(final KStream<K, VO> other, final ValueJoiner<? super V, ? super VO, ? extends VR> joiner, final JoinWindows windows, final Joined<K, V, VO> options);

<VT, VR> KStream<K, VR> join(final KTable<K, VT> other, final ValueJoiner<? super V, ? super VT, ? extends VR> joiner, final Joined<K, V, VT> options);

<VO, VR> KStream<K, VR> leftJoin(final KStream<K, VO> other, final ValueJoiner<? super V, ? super VO, ? extends VR> joiner, final JoinWindows windows, final Joined<K, V, VO> options);

<VT, VR> KStream<K, VR> leftJoin(final KTable<K, VT> other, final ValueJoiner<? super V, ? super VT, ? extends VR> joiner, final JoinWindows windows, final Joined<K, V, VT> options);

<VO, VR> KStream<K, VR> outerJoin(final KStream<K, VO> other, final ValueJoiner<? super V, ? super VO, ? extends VR> joiner, final JoinWindows windows, final Joined<K, V, VO> options);

<VT, VR> KStream<K, VR> outerJoin(final KTable<K, VT> other, final ValueJoiner<? super V, ? super VT, ? extends VR> joiner, final JoinWindows windows, final Joined<K, V, VT> options);
KTable
<KR, VR> KGroupedTable<KR, VR> groupBy(final KeyValueMapper<? super K, ? super V, KeyValue<KR, VR>> selector, Serialized<KR, VR> serialized);

KTable<K, V> filter(final Predicate<? super K, ? super V> predicate, final Materialized<K, V, KeyValueStore<K, V>> materialized);

KTable<K, V> filterNot(final Predicate<? super K, ? super V> predicate, final Materialized<K, V, KeyValueStore<K, V>> materialized);

<VR> KTable<K, VR> mapValues(final ValueMapper<? super V, ? extends VR> mapper, final Materialized<K, V, KeyValueStore<K, V>> materialized);

void to(final String topic, final Partitioned<V, V> options);

KTable<K, V> through(final String topic, final Materialized<K, V> options);

<VO, VR> KTable<K, VR> join(final KTable<K, VO> other,
                            final ValueJoiner<? super V, ? super VO, ? extends VR> joiner,
                            final Materialized<K, VR, KeyValueStore<K, VR>> materialized);

<VO, VR> KTable<K, VR> leftJoin(final KTable<K, VO> other,
                                final ValueJoiner<? super V, ? super VO, ? extends VR> joiner,
                                final Materialized<K, VR, KeyValueStore<K, VR>> materialized);

<VO, VR> KTable<K, VR> outerJoin(final KTable<K, VO> other,
                                 final ValueJoiner<? super V, ? super VO, ? extends VR> joiner,
                                 final Materialized<K, VR, KeyValueStore<K, VR>> materialized);

 

We add some new helper methods to Stores so people can conveniently and quickly create basic StateStoreSuppliers for use in the DSL or PAPI. We will also deprecate the existing Stores.create(...)

Stores
public static <K, V> StateStoreSupplier<KeyValueStore<K, V>> persistentKeyValueStore(final String name,
                                                                                     final Serde<K> keySerde,
                                                                                     final Serde<V> valueSerde) 

public static <K, V> StateStoreSupplier<KeyValueStore<K, V>> inMemoryKeyValueStore(final String name,
                                                                                final Serde<K> keySerde,
                                                                                final Serde<V> valueSerde)

public static <K, V> StateStoreSupplier<KeyValueStore<K, V>> lruMap(final String name,
                                                                final int capacity,
                                                                final Serde<K> keySerde,
                                                                final Serde<V> valueSerde) 

public static <K, V> StateStoreSupplier<WindowStore<K, V>> persistentWindowStore(final String name,
                                                                            final Windows windows,
                                                                            final Serde<K> keySerde,
                                                                            final Serde<V> valueSerde)

public static <K, V> StateStoreSupplier<SessionStore<K, V>> persistentSessionStore(final String name,
                                                                              final SessionWindows windows,
                                                                              final Serde<K> keySerde,
                                                                              final Serde<V> valueSerde) 

/**
 *  The following methods are for use with the PAPI. They allow building of StateStores that can be wrapped with
 *  caching, logging, and any other convenient wrappers provided by the KafkaStreams library
 */ 
public <K, V> StateStoreBuilder<WindowStore<K, V>> windowStoreBuilder(final StateStoreSupplier<WindowStore<K, V>> supplier)

public <K, V> StateStoreBuilder<KeyValueStore<K, V>> keyValueStoreBuilder(final StateStoreSupplier<KeyValueStore<K, V>> supplier)

public <K, V> StateStoreBuilder<SessionStore<K, V>> sessionStoreBuilder(final StateStoreSupplier<SessionStore<K, V>> supplier)
TopologyBuilder
public synchronized <K, V> TopologyBuilder addGlobalStore(final StateStoreBuilder storeSupplier,
                                                          final String sourceName,
                                                          final TimestampExtractor timestampExtractor,
                                                          final Deserializer keyDeserializer,
                                                          final Deserializer valueDeserializer,
                                                          final String topic,
                                                          final String processorName,
                                                          final ProcessorSupplier stateUpdateSupplier)
 
public synchronized final TopologyBuilder addStateStore(final StateStoreBuilder supplier, final String... processorNames)
 

 

 

KGroupedStream
<W extends Window> WindowedKStream<K, V> windowedBy(Windows<W> timeWindows);

SessionWindowedKStream<K, V> sessionWindowedBy(SessionWindows sessionWindows);

KTable<K, Long> count(final Materialized materialized);

KTable<K, V> reduce(final Reducer<V> reducer, final Materialized<K, V, KeyValueStore<K, V>> materialized);

<VR> KTable<K, VR> aggregate(final Initializer<VR> initializer,
                             final Aggregator<? super K, ? super V, VR> aggregator,
                             final Serde<VR> aggValueSerde,
                             final Materialized<K, VR, KeyValueStore<K, VR>> materialized);
KGroupedTable
KTable<K, Long> count(final Materialized<K, V, KeyValueStore<K, V>> materialized);

KTable<K, V> reduce(final Reducer<V> adder, final Reducer<V> subtractor, final Materialized<K, V, KeyValueStore<K, V>> materialized);

<VR> KTable<K, VR> aggregate(final Initializer<VR> initializer,
                             final Aggregator<? super K, ? super V, VR> aggregator,
                             final Aggregator<? super K, ? super V, VR> subtractor,
                             final Materialized<K, VR, KeyValueStore<K, VR>> materialized);

 

New classes and interfaces:

WindowedKStream
public interface WindowedKStream<K, V> {

    KTable<Windowed<K>, Long> count();

    KTable<Windowed<K>, Long> count(final Materialized<K, Long, WindowStore<K, Long>> materializedAs);

    <VR> KTable<Windowed<K>, VR> aggregate(final Initializer<VR> initializer,
                                           final Aggregator<? super K, ? super V, VR> aggregator);

    <VR> KTable<Windowed<K>, VR> aggregate(final Initializer<VR> initializer,
                                           final Aggregator<? super K, ? super V, VR> aggregator,
                                           final Materialized<K, VR, WindowStore<K, VR>> materializedAs);


    KTable<Windowed<K>, V> reduce(final Reducer<V> reducer);

    KTable<Windowed<K>, V> reduce(final Reducer<V> reducer,
                                  final Materialized<K, V, WindowStore<K, V>> materializedAs);


} 
SessionWindowedKStream
public interface SessionWindowedKStream<K, V> {

    KTable<Windowed<K>, Long> count();

    KTable<Windowed<K>, Long> count(final Materialized<K, Long, SessionStore<K, Long>> materializedAs);

    <VR, T> KTable<Windowed<K>, VR> aggregate(final Initializer<VR> initializer,
                                              final Aggregator<? super K, ? super V, VR> aggregator,
                                              final Merger<? super K, T> sessionMerger);

    <VR, T> KTable<Windowed<K>, VR> aggregate(final Initializer<VR> initializer,
                                              final Aggregator<? super K, ? super V, VR> aggregator,
                                              final Merger<? super K, T> sessionMerger,
                                              final Materialized<K, VR, SessionStore<K, VR>> materializedAs);


    KTable<Windowed<K>, V> reduce(final Reducer<V> reducer);

    KTable<Windowed<K>, V> reduce(final Reducer<V> reducer,
                                  final Materialized<K, V, SessionStore<K, V>> materializedAs);
}
Materialized
/**
 * Used when materializing a state store
 */
public class Materialized<K, V, S extends StateStore> {
    public static <K, V, S extends StateStore> Materialized<K, V, S> as(final String storeName)

    public static <K, V, S extends StateStore> Materialized<K, V, S> as(final StateStoreSupplier<S> supplier)

    public Materialized<K, V, S> withValueSerde(final Serde<V> valueSerde)

    public Materialized<K, V, S> withKeySerde(final Serde<K> valueSerde)

    public Materialized<K, V, S> withLoggingEnabled(final Map<String, String> topicConfig)

    public Materialized<K, V, S> withLoggingDisabled()

    public Materialized<K, V, S> withCachingEnabled()

    public Materialized<K, V, S> withCachingDisabled()

} 
Serialized
/**
 * Optional params that can be passed to groupBy and groupByKey operations
 */
public class Serialized<K, V> {

    public static <K, V> Serialized<K, V> with(final Serde<K> keySerde, final Serde<V> valueSerde)

    public static <K, V> Serialized<K, V> with(final Serde<K> keySerde, final Serde<V> valueSerde, final Serde<V> otherValueSerde)

    public Serialized<K, V> withKeySerde(final Serde<K> keySerde)

    public Serialized<K, V> withValueSerde(final Serde valueSerde)
}
Joined
/**
 * Optional params that can be passed to join, leftJoin, outerJoin operations
 */
public class Joined<K, V, VO> {

    public static <K, V, VO> Joined<K, V, VO> with(final Serde<K> keySerde, final Serde <V> valueSerde, final Serde<VO> otherValueSerde)

    public static <K, V, VO> Joined<K, V, VO> keySerde(final Serde<K> keySerde)

    public static <K, V, VO> Joined<K, V, VO> valueSerde(final Serde<V> valueSerde)

    public static <K, V, VO> Joined<K, V, VO> otherValueSerde(final Serde<V> valueSerde)

    public Joined<K, V, VO> withKeySerde(final Serde<K> keySerde)

    public Joined<K, V, VO> withValueSerde(final Serde<V> valueSerde)

    public Joined<K, V, VO> withOtherValueSerde(final Serde<VO> otherValueSerde)
}
Partitioned
/**
 * Optional arguments that can be specified when doing to and through operations
 */
public static <K, V> Partitioned<K, V> with(final Serde<K> keySerde, final Serde<V> valueSerde)

public static <K, V> Partitioned<K, V> with(final StreamPartitioner<K, V> partitioner, final Serde<K> keySerde, final Serde<V> valueSerde) 

public static <K, V> Partitioned<K, V> keySerde(final Serde<K> keySerde)

public static <K, V> Partitioned<K, V> valueSerde(final Serde<V> valueSerde) 

public static <K, V> Partitioned<K, V> streamPartitioner(final StreamPartitioner<K, V> partitioner) 

public Partitioned<K, V> withStreamPartitioner(final StreamPartitioner<K, V> partitioner) 
public Partitioned<K, V> withValueSerde(final Serde<V> valueSerde) 
public Partitioned<K, V> withKeySerde(final Serde<K> keySerde)
Printed
 
/**
 * options that can be used when printing to stdout our writing to a file
 */
public class Printed<K, V> {
    public static <K, V> Printed<K, V> toFile(final String filepath)

    public static <K, V> Printed<K, V> toSysOut()

    public Printed<K, V> withLabel(final String label)

    private Printed<K, V> withFile(final String filepath)

    public Printed<K, V> withKeySerde(final Serde<K> keySerde)

    public Printed<K, V> withValueSerde(final Serde<V> keySerde)

    public Printed<K, V> withKeyValueMapper(final KeyValueMapper<? super K, ? super V, String> mapper)

}

/**
 * Implementations of this will provide the ability to wrap a given StateStore
 * with or without caching/loggging etc.
 */
public interface StateStoreBuilder<T extends StateStore> {

    StateStoreBuilder<T> withCachingEnabled();
    StateStoreBuilder<T> withCachingDisabled();
    StateStoreBuilder<T> withLoggingEnabled(Map<String, String> config);
    StateStoreBuilder<T> withLoggingDisabled();
    T build();
}

 

Proposed Changes

Add the above methods, interfaces, classes to the DSL. Deprecate the existing overloads.

Compatibility, Deprecation, and Migration Plan

  • What impact (if any) will there be on existing users?
    • None - we will deprecate the existing methods so that existing users can continue until they decide to change

Rejected Alternatives

  • Using a more fluent api:  this approach always results in intermediate stages that require a final build or apply call to create the underlying KStream/KTable etc. We felt that this wasn't quite right.
  • Builder for all a params: Rather than specifying the required params and optional params separately we could make each method take a Builder that has all of the params. It was felt that this is a but onerous for users that just want to use the required params and don't care about the options
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