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Status

Current state: "Under Discussion"Adopted

Discussion thread: here 

JIRA: KAFKA-13494

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

Motivation

Provide an implementation of the Query interface, introduced in KIP-796: Interactive Query v2 , to support session and window queries.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

A public interface is any change to the following:

• Binary log format

• The network protocol and api behavior

• Any class in the public packages under clientsConfiguration, especially client configuration

  • org/apache/kafka/common/serialization

  • org/apache/kafka/common

  • org/apache/kafka/common/errors

  • org/apache/kafka/clients/producer

  • org/apache/kafka/clients/consumer (eventually, once stable)

• Monitoring

• Command line tools and arguments

• Anything else that will likely break existing users in some way when they upgrade

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Rejected Alternatives

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In this KIP we propose two new public classes:

WindowKeyQuery

This type used to search occurrences of keys in window stores in combination with IQv2. In particular the following mappings are established:

• WindowStore.fetch(K key, Instant timeFrom, Instant timeTo) ← WindowKeyQuery.withKeyAndWindowStartRange(key, from, to)

WindowRangeQuery

The query type is used to search per window aggregates of keys in window and session stores in combination with IQv2. In particular the following mappings are established:

• WindowStore.fetchAll(Instant timeFrom, Instant timeTo) ← WindowRangeQuery.withWindowStartRange(from, to)
• SessionStore.fetch(Bytes key) ← WindowRangeQuery.withKey(key)

There are multiple discussion points here:

• One reason we cannot use WindowKeyQuery to support SessionStore.fetch(key) is because SessionStore.fetch(key) returns a windowed key value iterator, KeyValueIterator<Windowed<Bytes>, byte[]>, whereas WindowKeyQuery is bound to Query<WindowStoreIterator<V>>. 
• WindowRangeQuery does not currently support key ranges. We could add more cases here to get better coverage of WindowStore and SessionStore API. In this KIP, however, we focus on a subset of queries and defer support for additional cases to future KIPs.

Proposed Changes

The WindowKeyQuery class:

@Evolving
public class WindowKeyQuery<K, V> implements Query<WindowStoreIterator<V>> {

    public static <K, V> WindowKeyQuery<K, V> withKeyAndWindowStartRange(final K key, final Instant timeFrom, final Instant timeTo);

    public K getKey();

    public Optional<Instant> getTimeFrom();

    public Optional<Instant> getTimeTo();
}

The WindowRangeQuery class:

@Evolving
public class WindowRangeQuery<K, V> implements Query<KeyValueIterator<Windowed<K>, V>> {

    public static <K, V> WindowRangeQuery<K, V> withKey(final K key);          

    public static <K, V> WindowRangeQuery<K, V> withWindowStartRange(final Instant timeFrom, final Instant timeTo);

    public K getKey();

    public Optional<Instant> getTimeFrom();

    public Optional<Instant> getTimeTo();
}

Examples

The following example illustrates the use of the WindowKeyQuey class.

final Instant timeTo = Instant.now();
final Instant timeFrom = timeTo.minusSeconds(60);
final WindowKeyQuery<GenericKey, ValueAndTimestamp<GenericRow>> query = WindowKeyQuery.withKeyAndWindowStartRange(key, timeFrom, timeTo);

final StateQueryRequest<WindowStoreIterator<ValueAndTimestamp<GenericRow>>> request = inStore("rocksdb-window-store").withQuery(query);
final StateQueryResult<WindowStoreIterator<ValueAndTimestamp<GenericRow>>> result = kafkaStream.query(request);
final WindowStoreIterator<ValueAndTimestamp<GenericRow>> iterator = result.getGlobalResult().getResult();

The following example illustrates the use of the WindowQuery class to query a window store.

final Instant timeTo = Instant.now();
final Instant timeFrom = timeTo.minusSeconds(60);

final WindowRangeQuery<GenericKey, ValueAndTimestamp<GenericRow>> query = WindowRangeQuery.withWindowStartRange(timeFrom, timeTo);

final StateQueryRequest<KeyValueIterator<Windowed<GenericKey>, ValueAndTimestamp<GenericRow>>> request = inStore("inmemory-window-store").withQuery(query);
final StateQueryResult<KeyValueIterator<Windowed<GenericKey>, ValueAndTimestamp<GenericRow>>> result = kafkaStreams.query(request);
final KeyValueIterator<Windowed<GenericKey>, ValueAndTimestamp<GenericRow>> iterator = result.getGlobalResult().getResult();

Compatibility, Deprecation, and Migration Plan

• Since this is a completely new set of APIs, no backward compatibility concerns are anticipated. 
• Since nothing is deprecated in this KIP, users have no need to migrate unless they want to.