Status

Current state: Under Discussion

Discussion thread: [DISCUSS] KIP-120: Cleanup Kafka Streams builder API

Released: 0.11.0

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

Motivation

Currently, Kafka Streams public API leaks a bunch of internal methods that should not be public. Furthermore, DSL and PAPI abstraction are not completely separated at the moment and TopologyBuilder offers methods that belong to DSL only.

Public Interfaces

In order to get a clean refactoring, we will deprecate classes

org.apache.kafka.streams.processor.TopologyBuilder
org.apache.kafka.streams.kstream.KStreamBuilder

and add them again with new name and different package

org.apache.kafka.streams.Topology
org.apache.kafka.streams.StreamsTopologyBuilder

Furthermore, we add a new class to get a full description the a topology (to compensate for some internal methods that get removed)

org.apache.kafka.streams.TopologyDescription
- TopologyDescription will have public subclasses
- ```
Subtopology
```
- ```
GlobalStores
```
- Node (interface)
- Source
- Sink
- Processor

The following methods will not be available in the newly added classes:

TopologyBuilder -> Topology: setApplicationId, connectSourceStoreAndTopic, connectProcessors, addInternalTopic, copartitionSources, nodeGroups, build, buildGlobalStateTopology, globalStateStores, topicGroups, earliestResetTopicPattern, latestResetTopicPattern, stateStoreNamesToSourceTopics, copartitioneGroups, sourceTopicPattern, updateSubscriptions
KStreamBuilder -> StreamsTopologyBuilder: all methods from TopologyBuilder (as KStreamBuilder does not inherit from TopologyBuilder anymore) except addStateStore and addGlobalStore (which are both added explicitly to StreamsTopologyBuilder) plus newName
methods highlighted in TopologyBuilder list, are methods that actually belong to DSL abstraction

New org.apache.kafka.streams.Topology class:

public class Topology {

    // existing public API from current TopologyBuilder class that will not be changed

    public enum AutoOffsetReset {
        EARLIEST , LATEST
    }

    public Topology();


    public synchronized Topology addSource(String name, String... topics);
    public synchronized Topology addSource(String name, Pattern topicPattern);

    public synchronized Topology addSource(AutoOffsetReset offsetReset, String name,  String... topics);
    public synchronized Topology addSource(AutoOffsetReset offsetReset, String name,  Pattern topicPattern);

    public synchronized Topology addSource(String name, Deserializer keyDeserializer, Deserializer valDeserializer, String... topics);
    public synchronized Topology addSource(String name, Deserializer keyDeserializer, Deserializer valDeserializer, Pattern topicPattern);
   
    public synchronized Topology addSource(AutoOffsetReset offsetReset, String name, Deserializer keyDeserializer, Deserializer valDeserializer, String... topics);
    public synchronized Topology addSource(AutoOffsetReset offsetReset, String name,  Deserializer keyDeserializer, Deserializer valDeserializer, Pattern topicPattern;


    public synchronized Topology addSink(String name, String topic, String... parentNames);
    public synchronized Topology addSink(String name, String topic, Serializer keySerializer, Serializer valSerializer, String... parentNames);
 
    public synchronized Topology addSink(String name, String topic, StreamPartitioner partitioner, String... parentNames);
    public synchronized <K, V> Topology addSink(String name, String topic, Serializer<K> keySerializer, Serializer<V> valSerializer, StreamPartitioner<? super K, ? super V> partitioner, String... parentNames);


    public synchronized Topology addProcessor(String name, ProcessorSupplier supplier, String... parentNames);
 
    public synchronized Topology addStateStore(StateStoreSupplier supplier, String... processorNames);
 
    public synchronized Topology addGlobalStore(final StateStore store,
                                                final String sourceName,
                                                final Deserializer keyDeserializer,
                                                final Deserializer valueDeserializer,
                                                final String topic,
                                                final String processorName,
                                                final ProcessorSupplier stateUpdateSupplier);

    public synchronized Topology connectProcessorAndStateStores(String processorName, String... stateStoreNames);



    // newly added method to reveal topology structure
    
    public synchronized TopologyDescription describe();
}

New org.apache.kafka.streams.StreamTopologyBuilder class:

public class StreamsTopologyBuilder {

    // existing public API from current KStreamBuilder class that will not be changed
    // small change: adding `synchronized` to all methods to align with TopologyBuilder

    public StreamsTopologyBuilder();


    public synchronized <K, V> KStream<K, V> stream(final String... topics);
    public synchronized <K, V> KStream<K, V> stream(final Pattern topicPattern);

    public synchronized <K, V> KStream<K, V> stream(final AutoOffsetReset offsetReset, final String... topics);
    public synchronized <K, V> KStream<K, V> stream(final AutoOffsetReset offsetReset, final Pattern topicPattern);

    public synchronized <K, V> KStream<K, V> stream(final Serde<K> keySerde, final Serde<V> valSerde, final String... topics);
    public synchronized <K, V> KStream<K, V> stream(final Serde<K> keySerde, final Serde<V> valSerde, final Pattern topicPattern);

    public synchronized <K, V> KStream<K, V> stream(final AutoOffsetReset offsetReset, final Serde<K> keySerde, final Serde<V> valSerde, final String... topics);
    public synchronized <K, V> KStream<K, V> stream(final AutoOffsetReset offsetReset, final Serde<K> keySerde, final Serde<V> valSerde, final Pattern topicPattern);


    public synchronized <K, V> KTable<K, V> table(final String topic, final String storeName);
    public synchronized <K, V> KTable<K, V> table(final AutoOffsetReset offsetReset, final String topic, final String storeName);
    public synchronized <K, V> KTable<K, V> table(final Serde<K> keySerde, final Serde<V> valSerde, final String topic, final String storeName);
    public synchronized <K, V> KTable<K, V> table(final AutoOffsetReset offsetReset, final Serde<K> keySerde, final Serde<V> valSerde, final String topic, final String storeName);


    public synchronized <K, V> GlobalKTable<K, V> globalTable(final String topic, final String storeName);
    public synchronized <K, V> GlobalKTable<K, V> globalTable(final Serde<K> keySerde, final Serde<V> valSerde, final String topic, final String storeName);

    public synchronized <K, V> KStream<K, V> merge(final KStream<K, V>... streams);



    // newly added method

    public synchronized KStreamBuilder addStateStore(StateStoreSupplier supplier, String... processorNames);
    public synchronized KStreamBuilder addGlobalStore(final StateStore store,
                                                      final String sourceName,
                                                      final Deserializer keyDeserializer,
                                                      final Deserializer valueDeserializer,
                                                      final String topic,
                                                      final String processorName,
                                                      final ProcessorSupplier stateUpdateSupplier);

    public synchronized Topology build();
}

New org.apache.kafka.streams.TopologyDescription class:

public final class TopologyDescription {
    public final List<Subtopology> subtopologies;
    public final List<GlobalStore> globalStores;

    public final class Subtopology {
        public final List<Node> nodes;
    }

    public final class GlobalStore {
        public final String name;
        public final String topic;
    }

    public interface Node {
        List<Node> getPredecessors();
        List<Node> getSuccessors();
    }

    public final class Source implements Node {
        public final String name;
        public final String topic; // can be topic name or pattern (as String)
    }

    public final class Processor implements Node {
        public final String name;
        public final List<String> stores;
    }

    public final class Sink implements Node {
        public final String name;
        public final String topic;
    }

}

Proposed Changes

We will add two new internal classes

org.apache.kafka.streams.processor.internals.InternalTopologyBuilder
org.apache.kafka.streams.kstream.internals.InternalStreamsTopologyBuilder

that offer the methods remove from current API. Thus, both classes are the actual implementation. Old TopologyBuilder and KStreamBuilder are only proxy classes to both classes respectively, for backward compatibility.

The newly added Topology uses InternalTopologyBuilder as member.

The newly added StreamsTopologyBuilder uses the new Topology as a member (no class hierarchy anymore – using it as member gives a clear separation between PAPI and DSL).

Because the new StreamsTopologyBuilder does not inherit from new Topology we need to add StreamsTopologyBuilder#build() that returns the actual Topology to be passed into KafkaStreams client.

Note: because of backward compatibility, removed DSL specific classes offered by old TopologyBuilder must be offered by InternalTopologyBuilder for now. However, after both deprecated classes got removed, this cleanup can be done (and does not require a KIP anymore, because it's internal refactoring -- we just need to create a JIRA for this). Thus, this KIP falls short of separating PAPI and DSL completely. But it's a necessary first step to do the separation in a backward compatible way (backward compatibility requires a two step approach).

Compatibility, Deprecation, and Migration Plan

Because no classes/method will be removed but only deprecated, this change will be fully backward compatible
We intend to remove all deprecated classes/methods in 0.11.1, but we can keep them longer on user request

Test Plan

Tests need to be rewritten but no new tests are required. All tests for public API need to be updated to use new Topology and StreamsTopologyBuilder. All tests using internal API, need to be rewritten to use InternalTopologyBuilder and InternalStreamsTopologyBuilder.

Rejected Alternatives

None.

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