Package org.apache.kafka.streams.kstream

Interface GlobalKTable<K,V>

Type Parameters:
K - the key type of this table
V - the value type of this table
public interface GlobalKTable<K,V>
GlobalKTable is an abstraction of a changelog stream from a primary-keyed table. Each record in this changelog stream is an update on the primary-keyed table with the record key as the primary key. Primary-keys in a table cannot be null, and thus, null-key key-value pairs are not supported, and corresponding records will be dropped. KTables follow Kafka "tombstone" semantics, and null-value key-value pairs are interpreted and processed as deletes for the corresponding key.

A GlobalKTable is defined from a single Kafka topic that is consumed message by message.

A GlobalKTable can only be used as right-hand side input for a stream-globalTable join.

In contrast to a KTable that is partitioned over all KafkaStreams instances, a GlobalKTable is fully replicated per KafkaStreams instance. Every partition of the underlying topic is consumed by each GlobalKTable, such that the full set of data is available in every KafkaStreams instance. This provides the ability to perform joins with KStream without having to repartition the input stream. Furthermore, GlobalKTable are "bootstrapped" on startup, and are maintained by a separate thread. Thus, updates to a GlobalKTable are not "stream-time synchronized" what may lead to non-deterministic results.

Furthermore, all GlobalKTable have an internal state store which can be accessed from "outside" using the Interactive Queries (IQ) API (see KafkaStreams#store(...) and KafkaStreams#query(...) [new API; evolving] for details). For example: 


 builder.globalTable("topic-name", "queryable-store-name");
 ...
 KafkaStreams streams = ...;
 streams.start()
 ...
 StoreQueryParameters<ReadOnlyKeyValueStore<K, ValueAndTimestamp<V>>> storeQueryParams =
   StoreQueryParameters.fromNameAndType("queryable-store-name", QueryableStoreTypes.timestampedKeyValueStore());
 ReadOnlyKeyValueStore view = streams.store(storeQueryParams);

 // query the value for a key
 ValueAndTimestamp value = view.get(key);
Note that in contrast to KTable a GlobalKTable's state holds a full copy of the underlying topic, thus all keys can be queried locally.
See Also:

  • Method Summary

    Modifier and Type
    Method
    Description
    String
    queryableStoreName()
    Get the name of the local state store that can be used to query this GlobalKTable.

  • Method Details

    • queryableStoreName

      String queryableStoreName()
      Get the name of the local state store that can be used to query this GlobalKTable.
      Returns:
      the underlying state store name, or null if this GlobalKTable cannot be queried.