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

Motivation

We propose to add a new type of state that can be used to create simpler and more efficient implementations of the operators that either maintain a MapState<?, List<?>>, or that require range operations. This includes temporal joins, operators that sort their input streams by timestamp, and a few other cases:

TemporalRowTimeJoinOperator
RowTimeSortOperator
CepOperator
IntervalJoinOperator
AbstractStreamArrowPythonOverWindowAggregateFunctionOperator
The various over/rank window operators
Min/max with retractions
Custom window implementations using keyed process functions

Several of these operators are using MapState<Long, List<T>> to store lists of records associated with timestamps. Updating those lists is expensive, especially when they are stored in RocksDB, as that requires deserializing and reserializing the lists when appending new list entries.

With this proposal, MapState<Long, List<T>> becomes BinarySortedState<Long, T>. We can use the timestamps that were the MapState’s keys as namespaces in the state backends, and the MapState’s values can be stored as lists using each state backend’s native ListState implementation.

Another opportunity for optimization is to take advantage of the fact that RocksDB stores its key/value pairs in order, sorted by key. We can use this to implement efficient range scans.

This FLIP also makes it possible for users to create custom time windows that are implemented as keyed process functions that achieve performance similar to that of the built-in window operators. This isn’t currently possible because the window operators take advantage of the state backends’ namespaces to optimize state access in a way that can’t be done with Flink’s public APIs. By exposing these namespaces in this limited, controlled way, users will be able to achieve comparable performance.

An implementation is underway. POC benchmarking has shown ~130% speedup for sorting, and ~60% for temporal joins.

Public Interfaces

The complete proposed interface for BinarySortedState<UK, UV> is shown below. In summary, it includes:

Point operations:

valuesAt(key)
add(key, value)
put(key, values)
clearEntryAt(key)

Lookups:

firstEntry(), firstEntry(fromKey)
lastEntry(), lastEntry(UK toKey)

Range-operations:

readRange(fromKey, toKey, inclusiveToKey)
readRangeUntil(toKey, inclusiveToKey)
readRangeFrom(fromKey)

Range-deletes:

clearEntryAt(key)
clearRange(fromKey, toKey, inclusiveToKey)
clearRangeUntil(toKey, inclusiveToKey)
clearRangeFrom(fromKey)

Cleanup:

clear()

@PublicEvolving
public interface BinarySortedState<UK, UV> extends State {

    /**
     * Returns the current values associated with the given key.
     *
     * @param key The key of the mapping
     * @return The values of the mapping with the given key (or <tt>null</tt> if there is none)
     * @throws Exception Thrown if the system cannot access the state.
     */
    @Nullable
    Iterable<UV> valuesAt(UK key) throws Exception;

    /**
     * Returns the current values associated with the first key in state, based on the state's
     * ordering function.
     *
     * @return The entry of the first key (or <tt>null</tt> if there is none)
     * @throws Exception Thrown if the system cannot access the state.
     */
    @Nullable
    Map.Entry<UK, Iterable<UV>> firstEntry() throws Exception;

    /**
     * Returns the current values associated with the first key in state at or after {@code
     * fromKey}, based on the state's ordering function.
     *
     * @param fromKey The query key (inclusive)
     * @return The entry of the first key after {@code fromKey} (or <tt>null</tt> if there is none)
     * @throws Exception Thrown if the system cannot access the state.
     */
    @Nullable
    Map.Entry<UK, Iterable<UV>> firstEntry(UK fromKey) throws Exception;

    /**
     * Returns the current values associated with the last key in state, based on the state's
     * ordering function.
     *
     * @return The entry of the last key (or <tt>null</tt> if there is none)
     * @throws Exception Thrown if the system cannot access the state.
     */
    @Nullable
    Map.Entry<UK, Iterable<UV>> lastEntry() throws Exception;

    /**
     * Returns the current values associated with the last key in state at or before {@code toKey},
     * based on the state's ordering function.
     *
     * @param toKey The query key (inclusive)
     * @return The entry of the last key at or before {@code toKey} (or <tt>null</tt> if there is
     *     none)
     * @throws Exception Thrown if the system cannot access the state.
     */
    @Nullable
    Map.Entry<UK, Iterable<UV>> lastEntry(UK toKey) throws Exception;

    /**
     * Associates a new set of values with the given key (overwrites the previous mapping).
     *
     * @param key The key of the mapping
     * @param values The new values of the mapping
     * @throws Exception Thrown if the system cannot access the state.
     */
    void put(UK key, Iterable<UV> values) throws Exception;

    /**
     * Adds a new value to the mapping with the given key.
     *
     * @param key The key of the mapping
     * @param value The new value to add to the mapping
     * @throws Exception Thrown if the system cannot access the state.
     */
    void add(UK key, UV value) throws Exception;

    /**
     * Returns all the mappings in the state for the given range of keys.
     *
     * @param fromKey The first key of the mappings to return (inclusive)
     * @param toKey The last key of the mappings to return
     * @param inclusiveToKey Whether {@code toKey} should be inclusive or not
     * @return An iterable view of all the state's key-values pairs in the given range.
     * @throws Exception Thrown if the system cannot access the state.
     */
    Iterable<Map.Entry<UK, Iterable<UV>>> readRange(UK fromKey, UK toKey, boolean inclusiveToKey)
            throws Exception;

    /**
     * Returns all the mappings in the state for the range of the smallest key until the given key.
     *
     * @param toKey The last key of the mappings to return
     * @param inclusiveToKey Whether {@code toKey} should be inclusive or not
     * @return An iterable view of all the state's key-values pairs in the given range.
     * @throws Exception Thrown if the system cannot access the state.
     */
    Iterable<Map.Entry<UK, Iterable<UV>>> readRangeUntil(UK toKey, boolean inclusiveToKey)
            throws Exception;

    /**
     * Returns all the mappings in the state for the range of the given key (inclusive) until the
     * largest key.
     *
     * @param fromKey The first key of the mappings to return
     * @return An iterable view of all the state's key-values pairs in the given range.
     * @throws Exception Thrown if the system cannot access the state.
     */
    Iterable<Map.Entry<UK, Iterable<UV>>> readRangeFrom(UK fromKey) throws Exception;

    /**
     * Deletes all values for the mapping with the given key.
     *
     * @param key The key of the mapping
     * @throws Exception Thrown if the system cannot access the state.
     */
    void clearEntryAt(UK key) throws Exception;

    /**
     * Deletes all values in the given range of keys.
     *
     * @param fromKey The first key of the mappings to remove (inclusive)
     * @param toKey The last key of the mappings to remove
     * @param inclusiveToKey Whether {@code toKey} should be inclusive or not
     * @throws Exception Thrown if the system cannot access the state.
     */
    void clearRange(UK fromKey, UK toKey, boolean inclusiveToKey) throws Exception;

    /**
     * Deletes all values in the range of the smallest key until the given key.
     *
     * @param toKey The last key of the mappings to remove
     * @param inclusiveToKey Whether {@code toKey} should be inclusive or not
     * @throws Exception Thrown if the system cannot access the state.
     */
    void clearRangeUntil(UK toKey, boolean inclusiveToKey) throws Exception;

    /**
     * Deletes all values in the range of the given key (inclusive) until the largest key.
     *
     * @param fromKey The first key of the mappings to remove
     * @throws Exception Thrown if the system cannot access the state.
     */
    void clearRangeFrom(UK fromKey) throws Exception;

    /** Deletes all values from the state. */
    void clear();
}

Under this proposal, a new state descriptor type is added:

@PublicEvolving
public class BinarySortedStateDescriptor<UK, UV> extends ListStateDescriptor<UV> {

    /**
     * Create a new {@code BinarySortedStateDescriptor} with the given name and the given type
     * serializers.
     *
     * @param name The name of the {@code MapStateDescriptor}.
     * @param keySerializer The type serializer for the keys in the state.
     * @param valueSerializer The type serializer for the values in the state.
     */
    public BinarySortedStateDescriptor(
            String name,
            LexicographicalTypeSerializer<UK> keySerializer,
            TypeSerializer<UV> valueSerializer);

    /**
     * Create a new {@code BinarySortedStateDescriptor} with the given name and the given type
     * information.
     *
     * @param name The name of the {@code MapStateDescriptor}.
     * @param keySerializer The type serializer for the keys in the state.
     * @param valueTypeInfo The type information for the values in the state.
     */
    public BinarySortedStateDescriptor(
            String name,
            LexicographicalTypeSerializer<UK> keySerializer,
            TypeInformation<UV> valueTypeInfo);

    /**
     * Create a new {@code BinarySortedStateDescriptor} with the given name and the given type
     * information.
     *
     * <p>If this constructor fails (because it is not possible to describe the type via a class),
     * consider using the {@link #BinarySortedStateDescriptor(String, LexicographicalTypeSerializer,
     * TypeInformation)} constructor.
     *
     * @param name The name of the {@code MapStateDescriptor}.
     * @param keySerializer The type serializer for the keys in the state.
     * @param valueType The class of the type of values in the state.
     */
    public BinarySortedStateDescriptor(
            String name, LexicographicalTypeSerializer<UK> keySerializer, Class<UV> valueType);

    /**
     * Gets the serializer for the keys in the state.
     *
     * @return The serializer for the keys in the state.
     */
    public LexicographicalTypeSerializer<UK> getKeySerializer();
}

Here the LexicographicalTypeSerializer is a special TypeSerializer which defines a binary sort order to allow retrieving and iterating values in order. The proposal is to initially provide only an internal implementation of a LexicographicLongSerializer that supports BinarySortedState<Long, T> out of the box:

private transient BinarySortedState<Long, Long> events;

BinarySortedStateDescriptor<Long, Long> desc =
    new BinarySortedStateDescriptor<>(
        "Events",
        LexicographicLongSerializer.INSTANCE,
        Types.LONG);

events = getRuntimeContext()
    .getBinarySortedState(desc);

As illustrated above, RuntimeContext (and KeyedStateStore) need to be extended with a method for registering BinarySortedState:

    @PublicEvolving
    <UK, UV> BinarySortedState<UK, UV> getBinarySortedState(
        BinarySortedStateDescriptor<UK, UV> stateProperties);

Proposed Changes

EmbeddedRocksDBStateBackend

The RocksDB state backend is extended to use the state descriptor's LexicographicalTypeSerializer as the namespace serializer.

A RocksDBBinarySortedNavigator is added to support the various lookup and range operations. It takes advantage of RocksDB's iterators.

HashMapStateBackend

A HeapBinarySortedNavigator is needed to provide the same lookup and range operations. The proposed implementation uses a

HashMap<FK, TreeSet<UKI>> to keep track of the namespaces that need to be iterated over.

Operators that can potentially be improved by using BinarySortedState

As outlined in the Motivation section, many of Flink's operators can benefit by being updated to leverage this new state type.

However, in a first release, we plan to limit the scope, and only include the new state type.

New Documentation

In addition to javadocs, state#using-keyed-state should be updated to include BinarySortedState.

Compatibility, Deprecation, and Migration Plan

Deprecation

While this FLIP does open the door for eventually deprecating some of the existing (and often abused) mechanisms for customizing DataStream windows, we are not proposing to deprecate anything as part of this FLIP.

Compatibility/Migration

Any operators that are reworked to use binary sorted state will thereafter use state in an incompatible way, compared to their current implementations.

To support upgrades to any SQL operator that is reworked to use binary sorted state, the mechanisms of FLIP-190 for handling operator migrations need to be followed. This involves annotating the ExecNode of the operator with an updated version number, and then modifying the operator so that it works regardless of which version of the state it finds available.

These operators can then either migrate the state, or directly support both the old and new versions of the state. In the context of this FLIP, migrating from any MapState<Long, List<T>> that is found to BinarySortedState<Long, T> will be simpler than continuing to maintain the old state as MapState, so that is the preferred approach.

Test Plan

We will include unit tests for the new state types that are tested against both state backends.

We should verify that any operators that are updated to use temporal state have sufficient test coverage. Tests will need to be added for state migration.

Rejected Alternatives

Directly expose the state backend namespaces

By itself, this wouldn’t accomplish much, and the interfaces proposed here satisfy the objectives without exposing this internal API.

Limit the scope to only offering TemporalState

An earlier draft of this FLIP proposed adding TemporalValueState and TemporalListState types, roughly equivalent to BinarySortedState<Long, ?>. After some discussion on the mailing list, this was determined to be too limiting.

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FLIP-220: Temporal State