Status

Current state: Under Discussion Accepted

Discussion thread: here 

JIRA:

...

public interface/**
 * {@code DslStoreSuppliers {

    KeyValueBytesStoreSupplier keyValueStore(final DslKeyValueParams params);

    WindowBytesStoreSupplier windowStore(final DslWindowParams params);

    SessionBytesStoreSupplier sessionStore(final DslSessionParams params);
}

// the below are all "struct"-like classes with the following fields
class DslKeyValueParams(String name);
class DslWindowParams(String name, Duration retentionPeriod, Duration windowSize, boolean retainDuplicates, EmitStrategy emitStrategy);
class DslSessionParams(String name, Duration retentionPeriod, EmitStrategy emitStrategy);

OOTB Store Type Specs

We will provide a default implementations of the above interfaces:

• org.apache.kafka.streams.RocksDbDslStoreSuppliers
• org.apache.kafka.streams.InMemoryDslStoreSuppliers

Configuration

These interfaces will be specified by means of a new config, defined below. This takes precedence over the old default.dsl.store config, which will be deprecated to provide clear signal on the preferred method. Further discussion on this point is under "Rejected Alternatives":

public static final String DSL_STORE_SUPPLIERS_CLASS_CONFIG = "dsl.store.suppliers.class";
public static final String DSL_STORE_SUPPLIERS_CLASS_DOC = "Defines which store implementations to plug in to DSL operators. Must implement the <code>org.apache.kafka.streams.state.DslStoreSuppliers</code> interface.";
public static final Class DSL_STORE_SUPPLIERS_CLASS_DEFAULT = org.apache.kafka.streams.RocksDbDslStoreSuppliers.class;

Example usage of this configuration:

...

} defines a grouping of factories to construct
 * stores for each of the types of state store implementations in Kafka
 * Streams. This allows configuration of a default store supplier beyond
 * the builtin defaults of RocksDB and In-Memory.
 *
 * <p>There are various ways that this configuration can be supplied to
 * the application (in order of precedence):
 * <ol>
 *     <li>Passed in directly to a DSL operator via either
 *     {@link org.apache.kafka.streams.kstream.Materialized#as(DslStoreSuppliers)},
 *     {@link org.apache.kafka.streams.kstream.Materialized#withStoreType(DslStoreSuppliers)}, or
 *     {@link org.apache.kafka.streams.kstream.StreamJoined#withDslStoreSuppliers(DslStoreSuppliers)}</li>
 *
 *     <li>Passed in via a Topology configuration override (configured in a
 *     {@link org.apache.kafka.streams.TopologyConfig} and passed into the
 *     {@link org.apache.kafka.streams.StreamsBuilder#StreamsBuilder(TopologyConfig)} constructor</li>
 *
 *     <li>Configured as a global default in {@link org.apache.kafka.streams.StreamsConfig} using
 *     the {@link org.apache.kafka.streams.StreamsConfig#DSL_STORE_SUPPLIERS_CLASS_CONFIG}</li>
 *     configuration.
 * </ol>
 *
 * <p>Kafka Streams is packaged with some pre-existing {@code DslStoreSuppliers}
 * that exist in {@link BuiltInDslStoreSuppliers}
 */
public interface DslStoreSuppliers extends Configurable {

    KeyValueBytesStoreSupplier keyValueStore(final DslKeyValueParams params);

    WindowBytesStoreSupplier windowStore(final DslWindowParams params);

    SessionBytesStoreSupplier sessionStore(final DslSessionParams params);
}

// the below are all "struct"-like classes with the following fields
class DslKeyValueParams(String name);
class DslWindowParams(String name, Duration retentionPeriod, Duration windowSize, boolean retainDuplicates, EmitStrategy emitStrategy, boolean isSlidingWindow, boolean isTimestamped);
class DslSessionParams(String name, Duration retentionPeriod, EmitStrategy emitStrategy);

OOTB Store Type Specs

We will provide a default implementations of the above interfaces:

• org.apache.kafka.streams.RocksDbDslStoreSuppliers
• org.apache.kafka.streams.InMemoryDslStoreSuppliers

Configuration

These interfaces will be specified by means of a new config, defined below. This takes precedence over the old default.dsl.store config, which will be deprecated to provide clear signal on the preferred method. Further discussion on this point is under "Rejected Alternatives":

public static final String DSL_STORE_SUPPLIERS_CLASS_CONFIG = "dsl.store.suppliers.class";
public static final String DSL_STORE_SUPPLIERS_CLASS_DOC = "Defines which store implementations to plug in to DSL operators. Must implement the <code>org.apache.kafka.streams.state.DslStoreSuppliers</code> interface.";
public static final Class DSL_STORE_SUPPLIERS_CLASS_DEFAULT = org.apache.kafka.streams.RocksDbDslStoreSuppliers.class;

// we also add this constant for the old configuration so that it is easily referenceable
// across the code base
public static final String DEFAULT_DSL_STORE = ROCKS_DB;

Example usage of this configuration:

dsl.store.suppliers.class = com.my.company.MyCustomStoreSuppliers

TopologyConfig

We will expose the following method from TopologyConfig :

    /**
     * @return the DslStoreSuppliers if the value was explicitly configured (either by
     *         {@link StreamsConfig#DEFAULT_DSL_STORE} or {@link StreamsConfig#DSL_STORE_SUPPLIERS_CLASS_CONFIG})
     */
    public Optional<DslStoreSuppliers> resolveDslStoreSuppliers() {
        if (isTopologyOverride(DSL_STORE_SUPPLIERS_CLASS_CONFIG, topologyOverrides) || globalAppConfigs.originals().containsKey(DSL_STORE_SUPPLIERS_CLASS_CONFIG)) {
            return Optional.of(Utils.newInstance(dslStoreSuppliers, DslStoreSuppliers.class));
        } else if (isTopologyOverride(DEFAULT_DSL_STORE_CONFIG, topologyOverrides) || globalAppConfigs.originals().containsKey(DEFAULT_DSL_STORE_CONFIG)) {
            return Optional.of(MaterializedInternal.parse(storeType));
        } else {
            return Optional.empty();
        }

This allows us to determine whether or not a DslStoreSuppliers was configured or not, and also respects the hierarchy of resolution described in the javadocs to DslStoreSuppliers.

Materialized API

In the existing code, users are able to override the default store type by leveraging the existing Materialized.as and Materialized.withStoreType  methods. We will change the signature of these methods (see below on how we maintain compatibility) to take in a DslStoreSuppliers  instead of the StoreType  enum:

...

• What impact (if any) will there be on existing users?

Existing users will see deprecation warnings if they are using the old default.dsl.store configuration in their code. When 4.0 is released this configuration will no longer be respected.

• If we are changing behavior how will we phase out the older behavior?

N/A

• If we need special migration tools, describe them here.

N/A

• When will we remove the existing behavior?

4.0

Test Plan

Existing test coverage for the default.dsl.store configuration will be migrated to use the new configuration and ensure that the default behavior can be changed.

Rejected Alternatives

Support Both Configs

Don't deprecate the old default.dsl.store config and instead maintain it alongside the new config. The advantage here is obviously that we minimize disruption to anyone using this config already. However I believe the existing userbase is likely to be quite small, as in its current form this feature is only useful to those who (a) have many state stores to the point overriding them separately is cumbersome, and (b) also wishes to override all (or most) of their stores to be in-memory specifically. This seems to be a relatively uncommon pattern already, not least of which being that it's a recipe for OOM disaster unless their applications are extremely small and/or low traffic. In the absence of any evidence that this config is already in use despite such a niche application, it seems best to take this opportunity to deprecate it now, before it gains any more traction, and have everyone converge on the same, single configuration going forward. Whenever there are two configs affecting the same underlying feature, it's inevitable there will be confusion over their interaction/hierarchy which can often lead to misconfiguration. Deprecating the old config is a clear sign to users which of the two should be preferred and will take precedence.

Add CUSTOM Enum to Existing Config

Instead of positioning the new config as a full replacement for default.dsl.store, we could instead introduce it as a complementary config for custom stores specifically. For example, add a 3rd StoreType to the enum definition, such as CUSTOM. If (and only if) the CUSTOM StoreType is used, the value of the new default.dsl.store.type.spec config will be looked at and used to obtain the actual StoreSupplier instances of this custom type. This option has the same advantages as the above, though I would qualify it in the same way. In general, having two configs that do the same thing or are tied to the same feature will be confusing to new (or old!) users. However, between these two alternatives I would personally advocate for this one as the better option, as it at least solves the concern over potential misconfiguration due to unclear interaction between the two configs.

Deprecating StoreType Enum, Materialized.as(StoreType) and Materialized.withStoreType(StoreType) APIs

...