Status

Current state: Under Discussion

Discussion thread: 

JIRA: https://issues.apache.org/jira/browse/KAFKA-7523

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

Motivation

When writing low-level Processors and Transformers that are stateful using kafka streams, often the processors (or transformers, I'll use "processors" to refer to both for brevity) want to "own" one or more state stores, the details of which are not important to the business logic of the application.  However, when incorporating these into a topology defined by the high level DSL, using KStream:process, you're forced to specify the state store names so the topology is wired up correctly.  This creates a clumsy pattern where the "owned" state store's name must be passed alongside the TransformerSupplier, when the supplier itself could just as easily supply that information on their own.

An example of the clumsiness:

String stateStoreName = "my-store";
StoreBuilder<KeyValueStore> storeBuilder =
        Stores.keyValueStoreBuilder(Stores.inMemoryKeyValueStore(stateStoreName), keySerde, valSerde);
topology.addStateStore(storeBuilder);
ProcessorSupplier processorSupplier = new MyStatefulProcessorSupplier(stateStoreName, val -> businessLogic(val));
builder.stream("input.topic")
        .map(...)
        .filter(...)
        .process(processorSupplier, stateStoreName);

Both the main topology definition (the chained, high-level DSL calls on StreamBuilder, KStream, and KTable) and the internal implementation of MyStatefulProcessorSupplier need to know the state store name, when it should really only by MyStatefulProcessorSupplier that cares.  Additionally, topology.addStateStore(storeBuilder) and the creation of the StoreBuilder are required, all of which ought to be implicit when using MyStatefulProcessorSupplier.  Ultimately, because KStream:process requires store names as a separate argument, all of this "wiring" code is necessary alongside or nearby actual business logic.

Ideally, it would be reducible to something like:

builder.stream("input.topic")
        .map(...)
        .filter(...)
        .process(MyStatefulProcessorSupplier.make(val -> businessLogic(val)));

This allows for the same level of clarity as when using Processors (and Transformers) as when using the high-level DSL.

Public Interfaces

KStream

Deprecate methods that take String.. stateStoreNames and replace them with methods that don't:

@Deprecated
<K1, V1> NewKStream<K1, V1> transform(final TransformerSupplier<? super K, ? super V, KeyValue<K1, V1>> transformerSupplier,
                                      final String... stateStoreNames);

<K1, V1> NewKStream<K1, V1> transform(final TransformerSupplier<? super K, ? super V, KeyValue<K1, V1>> transformerSupplier);

@Deprecated
<VR> NewKStream<K, VR> transformValues(final ValueTransformerSupplier<? super V, ? extends VR> valueTransformerSupplier,
                                       final String... stateStoreNames);

<VR> NewKStream<K, VR> transformValues(final ValueTransformerSupplier<? super V, ? extends VR> valueTransformerSupplier);

@Deprecated
<VR> NewKStream<K, VR> transformValues(final ValueTransformerWithKeySupplier<? super K, ? super V, ? extends VR> valueTransformerSupplier,
                                       final String... stateStoreNames);

<VR> NewKStream<K, VR> transformValues(final ValueTransformerWithKeySupplier<? super K, ? super V, ? extends VR> valueTransformerSupplier);

@Deprecated
void process(final ProcessorSupplier<? super K, ? super V> processorSupplier,
             final String... stateStoreNames);

void process(final ProcessorSupplier<? super K, ? super V> processorSupplier);

Add method to supplier interfaces allowing them to specify state stores they own and how to build them. 

TransformerSupplier

public interface TransformerSupplier<K, V, R> {
    Transformer<K, V, R> get();
    default List<StoreBuilder> stateStores() {
        return Collections.emptyList();
    }
}

ValueTransformerSupplier

public interface ValueTransformerSupplier<V, VR> {
    ValueTransformer<V, VR> get();
    default List<StoreBuilder> stateStores() {
        return Collections.emptyList();
    }
}

ValueTransformerWithKeySupplier

public interface ValueTransformerWithKeySupplier<K, V, VR> {
    ValueTransformerWithKey<K, V, VR> get();
    default List<StoreBuilder> stateStores() {
        return Collections.emptyList();
    }
}

ProcessorSupplier

public interface ProcessorSupplier<K, V> {
    Processor<K, V> get();
    default List<StoreBuilder> stateStores() {
        return Collections.emptyList();
    }
}

Proposed Changes

The proposal is to change the ProcessorSupplier and TransformerSupplier interfaces by allowing them to provide information about what state stores they own, eliminating (and deprecating) the need to pass in state store names when constructing a topology using StreamsBuilder, KStream:process, KStream::transform, and KStream::transformValues.

The public interface changes above directly imply what needs to be changed in KStream:  The process etc methods would get state store names from the list of StoreBuilders that the supplier provides, rather than the var args stateStoreNames.  Additionally, the process method would add the StoreBuilders to the topology using builder.addStateStore() rather than requiring the user to do it themselves.

Compatibility, Deprecation, and Migration Plan

Deprecated methods will continue to behave as they previously have.  Because the added interface methods are default with a reasonable default, those additions are backwards compatible.  Ideally, the deprecated methods would throw an exception if state stores are provided via *Supplier::stateStores and the var args stateStoreNames, to reduce confusion.

No migration tools are required since it's a relatively minor library change.

Existing behavior (methods) could be removed at the next major release without a full year deprecation period, since KStream is InterfaceStability.Evolving.  While InterfaceStability.Evolving allows for a minor release breakage, it seems entirely unnecessary to rush the removal of those deprecated methods.

Alternatives

This section isn't called "rejected alternatives" yet like the template because I don't think they're necessarily rejected yet; they should be discussed.

Don't deprecate the existing KStream methods that take stateStoreNames

This may be desirable if users want to create StoreBuilders or specify store names far away from the Processor logic itself.  I can't think of a major reason why this would be desired - as is evidenced by this KIP, I have the opposite problem.  And I'm not sure that these changes were necessarily prevent that so much as just change how that would work.  In general I favor deprecation just to keep the API smaller and so there aren't two ways of doing things.

Have the added method on the Supplier interfaces only return store names, not builders

This solves the original issue only partially, but with perhaps less "API risk."  The String... stateStoreNames argument would no longer be needed on the KStream methods, but the user would still need to manually add the StoreBuilders to the Topology.  The downside is we don't achieve the full reduction of "wiring up" code required when building the topology (the user still needs to know to call topology.addStateStore()), but the upside is that the StoreBuilder is less coupled to the *Supplier.  I don't consider this upside significant, but perhaps there are other use cases I'm not considering.

Do nothing

This is a "quality of life" API improvement, and nothing more, so maybe it's unneeded churn.  I favor doing something (obviously) because I think that while small, this change can be a major usability improvement for the low-level API.