ksqlDB

ksqlDB is an open-source streaming database engine built on Apache Kafka that provides a SQL-based interface for building and running real-time data processing pipelines. It enables users to query and manipulate streams of events stored in Kafka topics using continuous, event-driven queries, reducing the need to write standalone applications.

Core capabilities include creating streams and tables from Kafka topics using CREATE STREAM and CREATE TABLE, defining persistent queries that run indefinitely, and routing results to new topics or tables with INSERT INTO. It supports windowed aggregations (such as tumbling and hopping windows), joins between streams and tables, and the creation of materialized views that persist state as Kafka topics. Queries are continuously updated as new events arrive. ksqlDB can leverage the Confluent Schema Registry for schema management and can publish results to sinks through Kafka topics.

The ksqlDB engine offers a SQL-like dialect and exposes a HTTP REST API, along with a web UI and a command-line interface. Users interact by issuing statements to define streams, execute persistent queries, and manage functions. The engine operates on top of Kafka’s log-based storage, benefiting from Kafka’s durability and strongly-coupled processing semantics where available.

Typical use cases include real-time analytics, event-driven architectures, ETL/ELT pipelines, data enrichment, alerting, and live dashboards. ksqlDB integrates with the broader Kafka ecosystem, including Kafka Connect and the Schema Registry, to ingest data from and export data to various systems. It represents a practical option for users seeking real-time, SQL-powered stream processing without building custom streaming applications.