Status

Current state: Under Discussion Accepted

Discussion thread: TBD here

JIRA: TBD

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

...

• a new interface for versioned stores, and a helper class: VersionedKeyValueStore<K, V> extends StateStore  , with helper VersionedRecord
• a new interface for versioned store suppliers, and a helper interface: VersionedBytesStoreSupplier extends KeyValueBytesStoreSupplier , with helper VersionedBytesStore 
• three new methods in Stores.java:
  • two for creating a persistent, versioned store supplier: Stores#persistentVersionedKeyValueStore(...)  plus an overload
  • another for creating a StoreBuilder from a versioned supplier: Stores#versionedKeyValueStoreBuilder(...) 
• a new method in TopologyTestDriver.java for getting a versioned store from a topology
• a new static method in ValueAndTimestamp.java for creating ValueAndTimestamp instances where the value may be null: ValueAndTimestamp#makeAllowNullable(...) 

...

package org.apache.kafka.streams.state;  

/**
 * A key-value store that stores multiple record versions per key, and supports timestamp-based
 * retrieval operations to return the latest record (per key) as of a specified timestamp.
 * Only one record is stored per key and timestamp, i.e., a second call to
 * {@link #put(Object, Object, long)} with the same key and timestamp will replace the first.
 * <p>
 * @paramEach <K>store Theinstance keyhas type
an * @param <V> The value type
 */
public interface VersionedKeyValueStore<K, V> extends StateStore {

    /**
     * Add a new record version associated with this key.
     *
     * @param key       The key
     * @param value     The value, it can be {@code null};
     *                  if the serialized bytes are also {@code null} it is interpreted as a delete
     * @param timestamp The timestamp for this record version
     * @throws NullPointerException If {@code null} is used for key.
     */
    void put(K key, V value, long timestamp);associated, fixed-duration "history retention" which specifies
 * how long old record versions should be kept for. In particular, a versioned store guarantees
 * to return accurate results for calls to {@link #get(Object, long)} where the provided timestamp
 * bound is within history retention of the current observed stream time. (Queries with timestamp
 * bound older than the specified history retention are considered invalid.)
 * <p>
 * The store's "history retention" also doubles as its "grace period," which determines how far
 * back in time writes to the store will be accepted. A versioned store will not accept writes
 * (inserts, updates, or deletions) if the timestamp associated with the write is older than the
 * current observed stream time by more than the grace period.
 *
 * @param <K> The key type
 * @param <V> The value type
 */ 
public interface VersionedKeyValueStore<K, V> extends StateStore {

    /**
     * GetAdd thea latestnew (by timestamp) recordrecord version associated with this key.
     * <p>
     * @paramIf keythe Thetimestamp keyassociated towith fetch
the new record version is *older @returnthan The value and timestamp of the latest record associated with this key, or
     *   the store's
     * grace period (i.e., history retention) relative to the current observed stream time,
      {@code null} if either (1)* then the storerecord containswill nonot recordsbe foradded.
 this key or (2) the   *
     * @param key       latest record for this key is a tombstone.The key
     * @throws@param NullPointerExceptionvalue     The value, Ifit nullcan isbe used for key.{@code null};
     * @throws InvalidStateStoreException if the store is not initialized
     */
    ValueAndTimestamp<V> get(K key);

    /**
     * Get the latest record associated with this key with timestamp not exceeding the specified if the serialized bytes are also {@code null} it is interpreted as a delete
     * @param timestamp The timestamp for this record version
     * @throws NullPointerException timestamp bound.
     *If {@code null} is used for key.
     */
 @param key  void put(K key, V value, long timestamp);

 The key to fetch/**
     * @paramDelete timestampTothe Thevalue timestampassociated bound
with this key from the *store, @returnat Thethe value andspecified timestamp of the latest record associated with this key
     * (if there is such a value), and  satisfyingreturn the provideddeleted timestampvalue.
 bound, or {@code null} if any* of<p>
     * If the timestamp associated with this deletion is older (1)than the store contains no records for this key, (2) the latest record
     *    's grace period
     * (i.e., history retention) relative to the current observed stream time, then the deletion
     for* thiswill keynot satisfyingbe theperformed providedand timestamp{@code boundnull} iswill a tombstone, orbe returned.
     * <p>
     * This operation (3)is thesemantically storeequivalent noto longer contains data for the provided timestamp bound{@link #get(Object, long) #get(key, timestamp)}
     * followed by {@link #put(Object, Object, long)   #put(upkey, to store implementation discretion when this is the case).null, timestamp)}, with
     * @throwsa NullPointerExceptioncaveat that the return value is always If{@code null} isif usedthe fordeletion key.timestamp
     * @throwsis InvalidStateStoreExceptionolder ifthan the store's isgrace not initializedperiod (i.e., history retention), regardless of
     */
 what {@link  ValueAndTimestamp<V> get(K #get(Object, long) #get(key, long timestampTotimestamp);
}

Note that this proposal intentionally omits most methods from the existing KeyValueStore interface in order to keep the new interface simple. It could be nice to add additional methods in the future, such as rangeKey() methods to enable the foreign-key join subscription store use case, but this is deferred to a future KIP in order to align on these basic interfaces first.

Store Supplier/Builder Interfaces

The new Stores.java methods are as follows:

public final class Stores {

    // ... existing methods ...

    /**} would return.
     *
     * @param key       The key
     * @param timestamp The timestamp for this delete
     * Create@return aThe persistentvalue versionedand key-valuetimestamp storeof {@link VersionedBytesStoreSupplier}.
     * <p>the record associated with this key as of
     * This store supplier can be passed into a
 the deletion timestamp (inclusive), *or {@link #versionedKeyValueStoreBuilder(VersionedBytesStoreSupplier, Serde, Serde)}.
     *@code null} if no such record exists
     * @param name       (including if the deletion timestamp is nameolder ofthan thethis store (cannot be {@code null})
's history
     *     * @param historyRetention length ofretention time that old record versions are available for query, i.e., the store no longer contains data for the provided
     *         timestamp). Note that the record timestamp {@code r.timestamp()} of the
     *  (cannot be negative). If a timestamp bound providedreturned to
{@link VersionedRecord} may be smaller *than the provided deletion
     *         timestamp.
     VersionedRecord<V> delete(K key,   {@link VersionedKeyValueStore#get(Object, long)} is older than thislong timestamp);

    /**
     * Get the latest (by timestamp) record associated with this key.
     *
     * @param key The key specifiedto historyfetch
 retention, then the get operation* will@return notThe returnvalue data.
and timestamp of the latest *record @returnassociated anwith instancethis of {@link VersionedBytesStoreSupplier}key, or
     * @throws IllegalArgumentException if {@code historyRetention} or   {@code segmentIntervalnull} can'tif beeither represented(1) asthe {codestore longcontains milliseconds}
no records for this key */
  or (2) the
  public static VersionedBytesStoreSupplier persistentVersionedKeyValueStore(final* String name,
       latest record for this key is a tombstone.
     * @throws NullPointerException       If null is used for key.
     * @throws InvalidStateStoreException if the store is not initialized
     */
    VersionedRecord<V> get(K key);

    /**
     * Get the latest record associated with this key with timestamp not exceeding the finalspecified
 Duration historyRetention) {
  * timestamp bound.
    // ...*
     }

* @param key  /**
     * Create a persistent versionedThe key-value store {@link VersionedBytesStoreSupplier}.to fetch
     * <p>
@param asOfTimestamp The timestamp  *bound. This storebound supplieris caninclusive; beif passeda intorecord a
     * {@link #versionedKeyValueStoreBuilder(VersionedBytesStoreSupplier, Serde, Serde)}.
         *
     * @param name     (for the specified key) exists with this timestamp, namethen of
 the store (cannot be {@code null})
*      * @param historyRetention length of time that old record versions are available for query
   this is *the record that will be returned.
     * @return The value and timestamp of the record associated with this key
   (cannot be negative).* If a timestamp bound provided to
   as of *the provided timestamp, or {@code null} if no such record exists
     *          {@link VersionedKeyValueStore#get(Object, long)}(including if the provided timestamp bound is older than this store's history
     *         retention time, i.e., the store no longer contains data for the provided
     specified* history retention, then the get operation will not return datatimestamp).
 Note that the record *timestamp @param segmentInterval  size{@code r.timestamp()} of segmentsthe
 for storing old record versions* (must be positive). Old record versions
   returned {@link *VersionedRecord} may be smaller than the provided timestamp
     *         bound.
     * for@throws theNullPointerException same key in a single segment areIf storednull (updatedis andused accessed)for togetherkey.
     * @throws InvalidStateStoreException if the store is not initialized
     */
    VersionedRecord<V> get(K key,      Performance degrades as more record versions for the same key are collected
     *                         in a single segment. On the other hand, out-of-order writes and reads
     *                         which access older segments may slow down if there are too many segments.
     * @return an instance of {@link VersionedBytesStoreSupplier}long asOfTimestamp);
}

Note that this proposal intentionally omits most methods from the existing KeyValueStore interface in order to keep the new interface simple. It could be nice to add additional methods in the future, such as rangeKey() methods to enable the foreign-key join subscription store use case, but this is deferred to a future KIP in order to align on these basic interfaces first.

The VersionedRecord return type from the get() methods is essentially the same as the existing ValueAndTimestamp class today, but is its own separate class so that we can evolve it in the future. For example, we may want to add an additional timestamp to the VersionedRecord class to represent the expiry time of the record version (i.e., the timestamp of the next record version for this key) in addition to the existing timestamp.

package org.apache.kafka.streams.state;

/**
 * Combines a value from a {@link KeyValue} with a timestamp, for use as the return type
 * from {@link VersionedKeyValueStore#get(Object, long)} and related methods.
 *
 * @param <V> The value type
 */
public final class VersionedRecord<V> {
    private final V value;
    private final long timestamp;

    /**
     * @throwsCreate IllegalArgumentExceptiona ifnew {@code@link historyRetentionVersionedRecord} orinstance. {@code segmentIntervalvalue} can'tcannot be represented as {code@code long millisecondsnull}.
     */
    public static* VersionedBytesStoreSupplier@param persistentVersionedKeyValueStore(finalvalue String name,
    the value
     * @param timestamp  the timestamp
     * @return a new {@link VersionedRecord} instance
     */
    public VersionedRecord(final V value, final long timestamp) {
        this.value = Objects.requireNonNull(value);
        this.timestamp = timestamp;
    }

    public V value() {
        final Duration historyRetention,return value;
    }

    public long timestamp() {
        return timestamp;
    }

    @Override
    public String toString() {
        return "<" + value + "," + timestamp + ">";
    }

    @Override
    public boolean equals(final Object o) {
        if (this final Duration segmentInterval== o) {
		// ...
	}

 	/**
        * Creates a {@link StoreBuilder} that can be used to build a {@link VersionedKeyValueStore}.return true;
        }
     * 
  if (o == *null @param supplier   a {@link VersionedBytesStoreSupplier} (cannot be {@code null})|| getClass() != o.getClass()) {
     * @param keySerde   the key serde toreturn usefalse;
     * @param valueSerde the}
 value serde to use; if the serialized bytesfinal is {@code null} for put operations,VersionedRecord<?> that = (VersionedRecord<?>) o;
     *   return timestamp == that.timestamp &&
            it is treated as a deletionObjects.equals(value, that.value);
    }

    @Override
 * @param <K> public int      key typehashCode() {
     * @param <V>        value typereturn Objects.hash(value, timestamp);
    }
}

Store Supplier/Builder Interfaces

The new Stores.java methods are as follows:

public final class Stores {

    // ... existing methods ...
 * @return an instance of a {@link StoreBuilder} that can build a {@link VersionedKeyValueStore}
     /**/
    public static <K, V> StoreBuilder<VersionedKeyValueStore<K, V>> versionedKeyValueStoreBuilder(final VersionedBytesStoreSupplier supplier, * Create a persistent versioned key-value store {@link VersionedBytesStoreSupplier}.
     * <p>
     * This store supplier can be passed into a
     * {@link #versionedKeyValueStoreBuilder(VersionedBytesStoreSupplier, Serde, Serde)}.
     *
     * @param name             name of the store (cannot be {@code null})
     * @param historyRetention length of time that old record versions are available for query
     *               final Serde<K> keySerde,
        (cannot be negative). If a timestamp bound provided to
     *                         {@link VersionedKeyValueStore#get(Object, long)} is older than this
     *                         specified history retention, then the get operation will not return data.
     * @return finalan Serde<V>instance valueSerde)of {@link VersionedBytesStoreSupplier}
     * @throws IllegalArgumentException  // ...
if {@code historyRetention} or {@code segmentInterval} can't be represented as {code long milliseconds}
     }
}

To understand the history retention and segment interval parameters for the persistentVersionedKeyValueStore() methods requires brief discussion of the planned RocksDB implementation for versioned stores.

RocksDB Implementation Overview

Here's a high-level overview of the RocksDB versioned store implementation (details are outside the scope of this KIP).

Each store has an associated, fixed-duration history retention which specifies how long old record versions should be kept for. In particular, a versioned store guarantees to return accurate results for calls to get(key, timestampTo) where the provided timestamp bound is within history retention of the current observed stream time. (If the timestamp bound is outside the specified history retention, a warning is logged and null is returned.)

To achieve this, the store will consist of a "latest value store" and "segment stores." The latest record version for each key will be stored in the latest value store, while all older versions will be stored in the segment stores. 

Each record version has two associated timestamps:

• a validFrom timestamp. This timestamp is explicitly associated with the record as part of the put() call to the store; i.e., this is the record's timestamp.
• a validTo timestamp. This is the timestamp of the next record (or deletion) associated with the same key, and is implicitly associated with the record. This timestamp can change as new records are inserted into the store.

Old record versions are stored in segment stores according to their validTo timestamps. The use of segments here is analogous to that in the existing RocksDB implementation for windowed stores. Because records are stored in segments based on their validTo timestamps, this means that entire segments can be expired at a time once the records contained in the segment are no longer relevant based on the store's history retention. (A difference between the versioned store segments implementation and that of windowed stores today is that for versioned stores all segments will share the same physical RocksDB instance, in contrast to windowed stores where each segment is its own RocksDB, to allow for many more segments than windowed stores use today.)

The segment interval parameter for controlling segment size is (optionally) exposed to users in the static constructor methods above because benchmarking a prototype implementation showed that this parameter has significant effect on store performance based on workload characteristics.

VersionedBytesStoreSupplier Interface

Here's the VersionedBytesStoreSupplier interface used by the Stores.java methods above:

*/
    public static VersionedBytesStoreSupplier persistentVersionedKeyValueStore(final String name,
                                                                               final Duration historyRetention) {
        // ...
    }

    /**
     * Create a persistent versioned key-value store {@link VersionedBytesStoreSupplier}.
     * <p>
     * This store supplier can be passed into a
     * {@link #versionedKeyValueStoreBuilder(VersionedBytesStoreSupplier, Serde, Serde)}.
     *
     * @param name             name of the store (cannot be {@code null})
     * @param historyRetention length of time that old record versions are available for query
     *                         (cannot be negative). If a timestamp bound provided to
     *                         {@link VersionedKeyValueStore#get(Object, long)} is older than this
     *                         specified history retention, then the get operation will not return data.
     * @param segmentInterval  size of segments for storing old record versions (must be positive). Old record versions
     *                         for the same key in a single segment are stored (updated and accessed) together.
     *                         The only impact of this parameter is performance. If segments are large
     *                         and a workload results in many record versions for the same key being collected
     *                         in a single segment, performance may degrade as a result. On the other hand, 
     *                         reads and out-of-order writes which access older segments may slow down if 
     *                         there are too many segments.
     * @return an instance of {@link VersionedBytesStoreSupplier}
     * @throws IllegalArgumentException if {@code historyRetention} or {@code segmentInterval} can't be represented as {code long milliseconds}
     */
    public static VersionedBytesStoreSupplier persistentVersionedKeyValueStore(final String name,
                                                                               final Duration historyRetention,
                                                                               final Duration segmentInterval) {
		// ...
	}

 	/**
     * Creates a {@link StoreBuilder} that can be used to build a {@link VersionedKeyValueStore}.
     * 
     * @param supplier   a {@link VersionedBytesStoreSupplier} (cannot be {@code null})
     * @param keySerde   the key serde to use
     * @param valueSerde the value serde to use; if the serialized bytes is {@code null} for put operations,
     *                   it is treated as a deletion
     * @param <K>        key type
     * @param <V>        value type
     * @return an instance of a {@link StoreBuilder} that can build a {@link VersionedKeyValueStore}
     */
    public static <K, V> StoreBuilder<VersionedKeyValueStore<K, V>> versionedKeyValueStoreBuilder(final VersionedBytesStoreSupplier supplier,
                                                                                                  final Serde<K> keySerde,
                                                                                                  final Serde<V> valueSerde) {
        // ...
    }
}

To understand the history retention and segment interval parameters for the persistentVersionedKeyValueStore() methods requires brief discussion of the planned RocksDB implementation for versioned stores.

RocksDB Implementation Overview

Here's a high-level overview of the RocksDB versioned store implementation (details are outside the scope of this KIP).

Each store has an associated, fixed-duration history retention which specifies how long old record versions should be kept for. In particular, a versioned store guarantees to return accurate results for calls to get(key, asOfTimestamp) where the provided timestamp bound is within history retention of the current observed stream time. (If the timestamp bound is outside the specified history retention, then a record is still returned if the latest record version for the key satisfies the timestamp bound. Otherwise, a warning is logged and null is returned.)

To achieve this, the store will consist of a "latest value store" and "segment stores." The latest record version for each key will be stored in the latest value store, while all older versions will be stored in the segment stores. 

Each record version has two associated timestamps:

• a validFrom timestamp. This timestamp is explicitly associated with the record as part of the put() call to the store; i.e., this is the record's timestamp.
• a validTo timestamp. This is the timestamp of the next record (or deletion) associated with the same key, and is implicitly associated with the record. This timestamp can change as new records are inserted into the store.

The validity interval of a record is from validFrom (inclusive) to validTo (exclusive), and can change as new record versions are inserted into the store (and validTo changes as a result).

Old record versions are stored in segment stores according to their validTo timestamps. The use of segments here is analogous to that in the existing RocksDB implementation for windowed stores. Because records are stored in segments based on their validTo timestamps, this means that entire segments can be expired at a time once the records contained in the segment are no longer relevant based on the store's history retention. (A difference between the versioned store segments implementation and that of windowed stores today is that for versioned stores all segments will share the same physical RocksDB instance, in contrast to windowed stores where each segment is its own RocksDB, to allow for many more segments than windowed stores use today.)

The segment interval parameter for controlling segment size is (optionally) exposed to users in the static constructor methods above because benchmarking a prototype implementation showed that this parameter has significant effect on store performance based on workload characteristics.

VersionedBytesStoreSupplier Interface

Here's the VersionedBytesStoreSupplier interface used by the Stores.java methods above:

package org.apache.kafka.streams.state;

/**
 * A store supplier that can be used to create one or more versioned key-value stores,
 * specifically, {@link VersionedBytesStore} instances.
 * <p>
 * Rather than representing the returned store as a {@link VersionedKeyValueStore} of
 * type <Bytes, byte[]>, this supplier interface represents the returned store as a
 * {@link KeyValueStore} of type <Bytes, byte[]> (via {@link VersionedBytesStore}) in order to be compatible with
 * existing DSL methods for passing key-value stores such as {@link StreamsBuilder#table(String, Materialized)}
 * and {@link KTable#filter(Predicate, Materialized)}. A {@code VersionedKeyValueStore<Bytes, byte[]>}
 * is represented as a {@code KeyValueStore KeyValueStore<Bytes, byte[]>} by interpreting the
 * value bytes as containing record timestamp information in addition to raw record values.
 */
public interface VersionedBytesStoreSupplier extends KeyValueBytesStoreSupplier {

    /**
     * Returns the history retention (in milliseconds) that stores created from this supplier will have.
     * This value is used to set compaction configs on store changelog topics (if relevant).
	 *
     * @return history retention, i.e., length of time that old record versions are available for
     *         query from a versioned store
     */
    long historyRetentionMs();
}

As mentioned in the Javadoc, the reason that this supplier extends KeyValueBytesStoreSupplier and therefore returns a store of type KeyValueStore<Bytes, byte[]>  rather than a VersionedKeyValueStore<Bytes, byte[]>  (as the name suggests) is in order to be compatible with existing DSL methods for passing key-value stores, e.g., StreamsBuilder#table() and KTable methods, which are explicitly typed to accept Materialized<K, V, KeyValueStore<Bytes, byte[]> . The alternative to fitting VersionedKeyValueStore  into KeyValueStore  in this way is to introduce new versions of all relevant StreamsBuilder and KTable methods to relax the Materialized  type accepted by these methods. While this is possible, and we could even deprecate the existing methods in favor of the new ones introduced, this is a large surface area for public interface changes that it's best to avoid if possible.

The cost of fitting VersionedKeyValueStore  into KeyValueStore  as proposed is two additional layers of translation, for both DSL and PAPI users, whenever put() or get() is called on a versioned store: the record being written or read must be converted between (keyBytes, valueBytes, timestamp) and (keyBytes, valueBytes + serializedTimestamp) and back. It also means that users who wish to create their own VersionedKeyValueStore implementation (specifically, PAPI users who want to use the provided Stores#versionedKeyValueStoreBuilder method, and DSL users) also need to mimic this translation layer from VersionedKeyValueStore to KeyValueStore and back. 

To alleviate this pain, we could expose an additional helper method for the conversion and/or add an additional method to VersionedBytesStoreSupplier which directly returns a VersionedKeyValueStore<Bytes, byte[]>  if implemented. The latter allows us to save on the two additional layers of translation, at the expense of complicating one of the interfaces. Unless reviewers feel strongly about this (avoiding the extra translation and/or making it easier for users to create their own VersionedKeyValueStore implementations), I propose to leave these options out for now and we can always revisit them later.

For completeness, here's the new VersionedBytesStore interface which VersionedBytesStoreSupplier instances will return. Unless a user chooses to implement their own VersionedBytesStoreSupplier (i.e., in order to implement a custom versioned store to pass to the DSL or to the new Stores#versionedKeyValueStoreBuilder() method), then users will not need to interact with this interface.

package org.apache.kafka.streams.state;

/**
 * A representation of a versioned key-value store as a {@link KeyValueStore} of type <Bytes, byte[]>.
 * See {@link VersionedBytesStoreSupplier} for more.
 */
public interface VersionedBytesStore extends KeyValueStore<Bytes, byte[]>, TimestampedBytesStore {

    /**
     * The analog of {@link VersionedKeyValueStore#get(Object, long)}.
     */
    byte[] get(Bytes key, long asOfTimestamp);

	/**
     * The analog of {@link VersionedKeyValueStore#delete(Object, long)}.
     */
    byte[] delete(Bytes key, long timestamp);
}

Internally, this interface will be used to assist in the representation of VersionedKeyValueStore<Bytes, byte[]>  as KeyValueStore<Bytes, byte[]> .

Additional Interface Changes

TopologyTestDriver

TopologyTestDriver users should be able to get (and interact with) versioned stores from their topology, similar to the existing methods for other store types:

public class TopologyTestDriver implements Closeable {

	// ... existing methods ...

 	/**
     * Get the {@link VersionedKeyValueStore} with the given name.
     * The store can be a "regular" or global store.
     * <p>
     * This is often useful in test cases to pre-populate the store before the test case instructs the topology to
     * {@link TestInputTopic#pipeInput(TestRecord) process an input message}, and/or to check the store afterward.
     *
     * @param name the name of the store
     * @return the key value store, or {@code null} if no {@link VersionedKeyValueStore} has been registered with the given name
     * @see #getAllStateStores()
     * @see #getStateStore(String)
     * @see #getKeyValueStore(String)
     * @see #getTimestampedKeyValueStore(String)
     * @see #getWindowStore(String)
     * @see #getTimestampedWindowStore(String)
     * @see #getSessionStore(String)
     */
    public <K, V> VersionedKeyValueStore<K, V> getVersionedKeyValueStore(final String name) {
        // ...
    }
}

ValueAndTimestamp

Another additional interface change needed as part of this proposal is to add the following static constructor to ValueAndTimestamp :

public final class ValueAndTimestamp<V> {

	// ... existing methods ...

package org.apache.kafka.streams.state;

/**
 * A store supplier that can be used to create one or more versioned key-value stores.
 * <p>
 * Rather than representing the returned store as a {@link VersionedKeyValueStore<Bytes, byte[]>},
 * this supplier interface represents the returned store as a
 * {@link KeyValueStore KeyValueStore<Bytes, byte[]>} in order to be compatible with
 * existing DSL methods for passing key-value stores such as {@link StreamsBuilder#table(String, Materialized)}
 * and {@link KTable#filter(Predicate, Materialized)}. A {@link VersionedKeyValueStore<Bytes, byte[]>}
 * is represented as a {@link KeyValueStore KeyValueStore<Bytes, byte[]>} by interpreting the
 * value bytes as containing record timestamp information in addition to raw record values.
 */
public interface VersionedBytesStoreSupplier extends KeyValueBytesStoreSupplier {

    /**
     * ReturnsCreate thea historynew retention (in milliseconds) that stores created from this supplier will have{@link ValueAndTimestamp} instance. The provided {@code value} may be {@code null}.
     * This
 value is used to set* compaction@param configsvalue on store changelog topics (if relevant).
	 * the value
     * @return history retention, i.e., length of time that old record versions are available for@param timestamp  the timestamp
     * @param <V> the type of the value
     query* from@return a versioned store new {@link ValueAndTimestamp} instance
     */
    public static long historyRetentionMs();
}

As mentioned in the Javadoc, the reason that this supplier extends KeyValueBytesStoreSupplier and therefore returns a store of type KeyValueStore<Bytes, byte[]>  rather than a VersionedKeyValueStore<Bytes, byte[]>  (as the name suggests) is in order to be compatible with existing DSL methods for passing key-value stores, e.g., StreamsBuilder#table() and KTable methods, which are explicitly typed to accept Materialized<K, V, KeyValueStore<Bytes, byte[]> . The alternative to fitting VersionedKeyValueStore  into KeyValueStore  in this way is to introduce new versions of all relevant StreamsBuilder and KTable methods to relax the Materialized  type accepted by these methods. While this is possible, and we could even deprecate the existing methods in favor of the new ones introduced, this is a large surface area for public interface changes that it's best to avoid if possible.

The cost of fitting VersionedKeyValueStore  into KeyValueStore  as proposed is two additional layers of translation, for both DSL and PAPI users, whenever put() or get() is called on a versioned store: the record being written or read must be converted between (keyBytes, valueBytes, timestamp) and (keyBytes, valueBytes + serializedTimestamp) and back. It also means that users who wish to create their own VersionedKeyValueStore implementation (specifically, PAPI users who want to use the provided Stores#versionedKeyValueStoreBuilder method, and DSL users) also need to mimic this translation layer from VersionedKeyValueStore to KeyValueStore and back. 

To alleviate this pain, we could expose an additional helper method for the conversion and/or add an additional method to VersionedBytesStoreSupplier which directly returns a VersionedKeyValueStore<Bytes, byte[]>  if implemented. The latter allows us to save on the two additional layers of translation, at the expense of complicating one of the interfaces. Unless reviewers feel strongly about this (avoiding the extra translation and/or making it easier for users to create their own VersionedKeyValueStore implementations), I propose to leave these options out for now and we can always revisit them later.

Additional Interface Changes

One additional interface change needed as part of this proposal is to add the following static constructor to ValueAndTimestamp :

public final class ValueAndTimestamp<V> {

	// ... existing methods ...

    /**
     * Create a new {@link ValueAndTimestamp} instance. The provided {@code value} may be {@code null}.
     * 
     * @param value      the value
     * @param timestamp  the timestamp
     * @param <V> the type of the value
     * @return a new {@link ValueAndTimestamp} instance
     */
    public static <V> ValueAndTimestamp<V> makeAllowNullable(
        final V value, final long timestamp) {
        // ...
    }
}

The reason this addition is needed is an implementation detail. The existing DSL processor implementation represents all source table state stores as timestamped key-value stores (link) which means that, unless we want to lift this restriction and change a significant amount of code internally, then versioned key-value stores will have to fit the TimestampedKeyValueStore interface internally. TimestampedKeyValueStore  represents inserting a new record to the store as calling put(K key, ValueAndTimestamp<V> v). In order to allow inserting tombstones into versioned stores, ValueAndTimestamp therefore needs to allow null values.

Even though this is a public interface change (by virtue of ValueAndTimestamp being public), the usage of ValueAndTimestamp instances with null values will be purely internal. In other words, this change is not strictly needed as a public interface change and could also be achieved through refactoring. That said, the cost of introducing this new method seems low and I'd like to propose it.

Compatibility, Deprecation, and Migration Plan

This KIP introduces a new type of store without deprecating any existing interfaces. Unless a user explicitly updates their application code to use the new store, this KIP will have no effect on their applications (versioned stores are not used anywhere by default).

The RocksDB format used for versioned stores is not compatible with the existing format for non-versioned stores.

However, RocksDB-based versioned and non-versioned stores will use the same changelog topic format, though their changelog topic configurations will differ. Specifically, the changelog bytes format for RocksDB-based versioned and non-versioned stores is the same, but changelog topics for versioned stores need min.compaction.lag.ms set to a value suitable for the desired history retention of the versioned store. The RocksDB versioned store implementation will set min.compaction.lag.ms equal to history retention plus 24 hours, where the purpose of this additional buffer is to account for the broker's usage of wall clock time in topic compactions (analogous to the extra 24 hours changelog retention for windowed stores today). Changelog topic configs will be set only on changelog topic creation, and will not be verified if the changelog topic already exists.

In light of the above, users can use the following manual procedure to update an existing application with a non-versioned store to use a versioned store instead:

1. Stop the application
2. Delete all local state (for the store being updated) from all instances
3. Update the changelog topic configurations to set min.compaction.lag.ms to a value suitable for the desired history retention (e.g., history retention plus some buffer to account for broker wall clock time usage in topic cleanup)
4. Update the application code to use a versioned store
5. Restart the app.

There are no plans to support a non-manual upgrade procedure or a live migration procedure at this time. In the future, it could be nice to make versioned stores the default since a non-versioned store is simply a special case of a versioned store (with history retention 0) but that's far out of scope for this KIP.

Test Plan

The RocksDB-based versioned store implementation will be tested with the Processor API: put, get, and timestamp-based get methods will have their results validated. 

The manual procedure described above for updating an application using a non-versioned store to use a versioned store will be tested as well. 

Rejected Alternatives

Versioned Store Interface

History retention and get(key, timestampTo)

In the event that get(key, timestampTo) is called with a timestamp bound older than the specified history retention, instead of returning null (and logging a warning) as proposed above, other design options include (1) throwing an exception or (2) updating the return type from ValueAndTimestamp<V> to Optional<ValueAndTimestamp<V>> and returning an empty optional to indicate that the timestamp bound was invalid. The first option is not very user-friendly. The second option complicates the interface and diverges the return types of get(key) and get(key, timestampTo) .

Additional return timestamps from get(key, timestampTo)

The proposed return type from get(key, timestampTo) of ValueAndTimestamp<V> represents returning the record value and timestamp (i.e., validFrom timestamp) found for the given key (and timestamp bound). In some situations, it may be useful for users to additionally have the validTo timestamp associated with the record. In order to provide this additional timestamp to users, the return type of get(key, timestampTo) could instead be a new type, e.g., VersionedRecord , which has a value and two associated timestamps. This again complicates the interface and diverges the return types of get(key)  and get(key, timestampTo) . (It's not meaningful to return a validTo timestamp associated with records returned from get(key) since the validTo timestamp will always be a sentinel value that signals that the record is the latest record associated with the given key.)

Return null with timestamp from get()

<V> ValueAndTimestamp<V> makeAllowNullable(
        final V value, final long timestamp) {
        // ...
    }
}

The reason this addition is needed is an implementation detail. The existing DSL processor implementation represents all source table state stores as timestamped key-value stores (link) which means that, unless we want to lift this restriction and change a significant amount of code internally, then versioned key-value stores will have to fit the TimestampedKeyValueStore interface internally. TimestampedKeyValueStore  represents inserting a new record to the store as calling put(K key, ValueAndTimestamp<V> v). In order to allow inserting tombstones into versioned stores, ValueAndTimestamp therefore needs to allow null values.

Even though this is a public interface change (by virtue of ValueAndTimestamp being public), the usage of ValueAndTimestamp instances with null values will be purely internal. In other words, this change is not strictly needed as a public interface change and could also be achieved through refactoring. That said, the cost of introducing this new method seems low and I'd like to propose it.

Compatibility, Deprecation, and Migration Plan

This KIP introduces a new type of store without deprecating any existing interfaces. Unless a user explicitly updates their application code to use the new store, this KIP will have no effect on their applications (versioned stores are not used anywhere by default).

The RocksDB format used for versioned stores is not compatible with the existing format for non-versioned stores.

However, RocksDB-based versioned and non-versioned stores will use the same changelog topic format, though their changelog topic configurations will differ. Specifically, the changelog bytes format for RocksDB-based versioned and non-versioned stores is the same, but changelog topics for versioned stores need min.compaction.lag.ms set to a value suitable for the desired history retention of the versioned store. The RocksDB versioned store implementation will set min.compaction.lag.ms equal to history retention plus 24 hours, where the purpose of this additional buffer is to account for the broker's usage of wall clock time in topic compactions (analogous to the extra 24 hours changelog retention for windowed stores today). Changelog topic configs will be set only on changelog topic creation, and will not be verified if the changelog topic already exists.

In light of the above, users can use the following manual procedure to update an existing application with a non-versioned store to use a versioned store instead:

1. Stop the application
2. Delete all local state (for the store being updated) from all instances
3. Update the changelog topic configurations to set min.compaction.lag.ms to a value suitable for the desired history retention (e.g., history retention plus some buffer to account for broker wall clock time usage in topic cleanup)
4. Update the application code to use a versioned store
5. Restart the app.

There are no plans to support a non-manual upgrade procedure or a live migration procedure at this time. In the future, it could be nice to make versioned stores the default since a non-versioned store is simply a special case of a versioned store (with history retention 0) but that's far out of scope for this KIP.

Test Plan

The RocksDB-based versioned store implementation will be tested with the Processor API: put, get, and timestamp-based get methods will have their results validated. 

The manual procedure described above for updating an application using a non-versioned store to use a versioned store will be tested as well. 

Rejected Alternatives

Versioned Store Interface

History retention and get(key, asOfTimestamp)

In the event that get(key, asOfTimestamp) is called with a timestamp bound older than the specified history retention, instead of returning null (and logging a warning) as proposed above, other design options include (1) throwing an exception or (2) updating the return type from VersionedRecord<V> to Optional<VersionedRecord<V>> and returning an empty optional to indicate that the timestamp bound was invalid. The first option is not very user-friendly. The second option complicates the interface and diverges the return types of get(key) and get(key, asOfTimestamp) .

Regarding the edge case where get(key, asOfTimestamp) is called with a timestamp bound older than the specified history retention but the latest record version for the key satisfies the timestamp bound, the proposal above says that the latest record version should be returned in this case, rather than rejecting the timestamped query and returning null. Returning the record is preferable because its existence (as the latest value for the key) is guaranteed in the store, and is accessible from get(key) anyway. The alternative of returning null, i.e., strict enforcement of the store's history retention, is not very user-friendly as users would then have to determine whether to call get(key) or get(key, timestamp) to account for this edge case.

ValueAndTimestamp as return type of get(key, asOfTimestamp) / Additional return timestamps from get(key, asOfTimestamp)

The proposed return type from get(key, asOfTimestamp) of VersionedRecord<V> returns the record value and timestamp (i.e., validFrom timestamp) found for the given key (and timestamp bound). In some situations, it may be useful for users to additionally have the validTo timestamp associated with the record. In order to allow for this possibility in the future, the return type of get(key, asOfTimestamp) is a new type, VersionedRecord , rather than the existing ValueAndTimestamp<V>  type, even though the two are largely the same today. We considered keeping the interface simple by not introducing a new type, but felt that the flexibility of evolving this type in the future was worth the addition of a new class. However, we will not add additional return timestamps at this time. They can be added once we have more confidence that they will be useful for users.

Return null with timestamp from get()

In the event that get(key) or get(key, asOfTimestamp) finds that the latest record version associated with a particular key (and possible timestamp bound) is a tombstone, rather than returning null the versioned store could instead return a non-null VersionedRecord with null value (and relevant timestamp). This would allow users to distinguish between the key not being found in the store at all (null VersionedRecord ) versus the key being found with a tombstone for the latest record (non-null VersionedRecord with null value). This proposal was rejected since the use cases for making such a distinction are limited.

Grace period separately configurable from history retention

"History retention" and "grace period" control how far back in time (relative to the current observed stream time) old reads and writes, respectively, will be accepted by the store. In the proposal above, users specify a single value which is used for both parameters, though in the future we could add an additional option for users to specify the two separately. (Today, users specify an explicit value for history retention, and grace period is automatically set to the same value. There are no compatibility concerns with introducing a new option for grace period in the future.)In the event that get(key) or get(key, timestampTo) finds that the latest record version associated with a particular key (and possible timestamp bound) is a tombstone, rather than returning null the versioned store could instead return a non-null ValueAndTimestamp with null value (and relevant timestamp). This would allow users to distinguish between the key not being found in the store at all (null ValueAndTimestamp ) versus the key being found with a tombstone for the latest record (non-null ValueAndTimestamp with null value). This proposal was rejected since the use cases for making such a distinction are limited.

Support for Upgrades

Additional support for upgrading a non-versioned store to a versioned store beyond the manual steps above were rejected on the basis of complexity. Automatic upgrades are too complex, and it's not clear that additional tooling for manual upgrades would be valuable to users at this time. It's better to get the new versioned interfaces out sooner in order to let them bake/iterate, rather than block on additional complexity for introducing the first version at this time.

...