Kafka Streams is a library for developing stream processing applications based on Apache Kafka. Quoting its docs, "a Kafka Streams application processes record streams through a topology in real-time, processing data continuously, concurrently, and in a record-by-record manner". The Kafka Streams DSL provides a range of stream processing operations such as a map, filter, join, and aggregate.
Non-Key Joins in Kafka Streams
Debezium’s CDC source connectors make it easy to capture data changes in databases and push them towards sink systems such as Elasticsearch in near real-time. By default, this results in a 1:1 relationship between tables in the source database, the corresponding Kafka topics, and a representation of the data at the sink side, such as a search index in Elasticsearch.
In case of 1:n relationships, say between a table of customers and a table of addresses, consumers often are interested in a view of the data that is a single, nested data structure, e.g. a single Elasticsearch document representing a customer and all their addresses.
This is where KIP-213 ("Kafka Improvement Proposal") and its foreign key joining capabilities come in: it was introduced in Apache Kafka 2.4 "to close the gap between the semantics of KTables in streams and tables in relational databases". Before KIP-213, in order to join messages from two Debezium change event topics, you’d typically have to manually re-key at least one of the topics, so to make sure the same key is used on both sides of the join.
Thanks to KIP-213, this isn’t needed any longer, as it allows to join two Kafka topics on fields extracted from the Kafka message value, taking care of the required re-keying automatically, in a fully transparent way. Comparing to previous approaches, this drastically reduces the effort for creating aggregated events from Debezium’s CDC events.