Current state: "Under Discussion"

Discussion thread: here


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Fetching range of records from Kafka Streams state stores comes with an iterator to traverse elements from oldest to newest, e.g ReadOnlyWindowStore#fetch(K key, long fromTime, long toTime) mentions:

For each key, the iterator guarantees ordering of windows, starting from the oldest/earliest”

Similar guarantees are provided on other fetch and range operations.

This APIs constraint the usage of local state store for some use-cases:

When storing records on time windows, or records by key; and an operation wants to return the last N values inserted withing a time range containing M records: Currently there is no alternative other than iterating records from oldest to newest–traversing M records, where M » N.

If a backward read direction option becomes available, then we could start from the latest record within a time range and go backwards, returning the first N value more efficiently.

At Zipkin Kafka-based storage, we are planning to use this feature to replace two KeyValueStores (one for traces indexed by id, and another with trace_ids indexed by timestamp) for one WindowStore. A backward read direction will allow to support queries like: “within this time range, find the last traces that match this criteria”, and return latest values quickly.

Reference issues

Proposed Changes

Introduce a new StreamsConfig configuration to flag support for backwards iteration:

public class StreamsConfig extends AbstractConfig {
    public static final String ENABLE_BACKWARD_ITERATION_CONFIG = "enable.backward.iteration";
    private static final String ENABLE_BACKWARD_ITERATION_DOC = "If true any range operations will accept (from, to) arguments to be from > to, returning recent records first";

If true, ReadOnlyKeyValueStore will support (from, to) argument pairs to be: from > to, returning iteration in reverse order.

To complete support for backwards iteration, `all` operations will be companioned by a `reverseAll`:

public interface ReadOnlyKeyValueStore<K, V> { KeyValueIterator<K, V> reverseAll(); } 
public interface ReadOnlyWindowStore<K, V> { KeyValueIterator<Windowed<K>, V> reverseAll(); } 

StreamConfig flag will be passed to Stores via `ProcessorContext`.

Internally, both implementations: persistent (RocksDB), and in-memory (TreeMap) support reverse/descending iteration:

final RocksIterator iter = db.newIterator(); 
final RocksIterator reverse = db.newIterator(); 
final TreeMap<String, String> map = new TreeMap<>(); 
final NavigableSet<String> nav = map.navigableKeySet(); 
final NavigableSet<String> rev = map.descendingKeySet();

Compatibility, Deprecation, and Migration Plan

StreamsConfig will mitigate affecting users that are relying on current behaviour: if from>to, then return empty iterator. Only when users are enabling flag from>to will return a reversed iterator.

Therefore this change is backwards compatible.

Rejected Alternatives