Kafka SQL or KSQL is a streaming SQL engine that provides SQL interface to the streams in Apache Kafka. It is developed by Confluent Inc. and is built on the Kafka Streams API, which supports joins, aggregations, windowing and sessionization on streaming data. The data is stored in a Kafka cluster, which is a collection of Kafka brokers, segregated into topics. Each topic consists of a defined number of partitions, where each partition is an immutable sequence of messages. KSQL can be used as a library by applications to run SQL queries on top of the stream data stored in the Kafka cluster.

As opposed to streaming systems like Spark Streaming which require using Java/Scala for development, KSQL provides a completely interactive, SQL only interface improving the ease of access. KSQL processes one message at a time, making it a true stream processing system instead of a micro-batching system.

Query Interface

Custom API SQL Command-line / Shell

KSQL is a SQL-like query language with certain extensions for streams. As defined in the data model, tables and streams are the two major abstractions in KSQL.

KSQL provides the following execution modes/interfaces:

  • Interactive mode: Using command line interface or REST API
  • Application mode: A list of queries can be provided as an input to the KSQL jar
  • Embedded mode: KSQL queries can be embedded within the statements of Kafka Streams API, similar to Spark SQL.

Storage Organization


The basic unit of data in Apache Kafka is a message. Messages are organized into topics, with each topic split into a configurable number of partitions. A partition can be considered as a log that is immutable and append-only, with a guarantee that a message will be part of only one partition in a topic. The data is stored in a Kafka cluster, which is a distributed collection of Kafka brokers. Each partition has one leader broker and other replication/follower brokers for fault tolerance.

Streams in KSQL are stateless entities, hence not requiring any additional state data. Tables, on the other hand, are stateful entities, and KSQL uses RocksDB for storing the state of the table. Any incoming message that updates the table will update this state store.

Analogous to a catalog in an RDBMS, KSQL maintains a metastore that contains information about all the tables and streams in the Kafka cluster. This metastore itself can be configured to be stored as a Kafka topic for fault tolerance.

Data Model

Relational Key/Value

The basic unit of storage in Kafka is a message, which consists of a key, value, timestamp, partition number and its offset in the partition. Key and value are just arrays of bytes, hence there is no restriction on the type of values they can hold. A schema can be associated with each topic, which is imposed upon the value part of the message. Hence, inherently it follows a key-value based model, with relational schema optionally enforced upon. ToDo: Verify who validates the schema i.e. can schema be applied on top of an existing Kafka topic.

KSQL provides two different concepts of organizing a topic's data, streams, and table. Messages within a stream are independent of each other and unbounded. A table, on the other hand, is a stateful entity where a new message is considered either as a new entry in the table or an update to the entry in the existing table with the same key. Hence, in the case of a KSQL table, the messages in the topic can be considered as a changelog/redo-log.

Storage Architecture


Every message in the Kafka cluster is stored at the brokers on disks. In spite of disk-oriented storage, it performs comparably to in-memory message queues because Kafka only allows for sequential processing of messages, avoiding random disk access. Since Kafka is implemented in Java, instead of caching data on JVM heap, it relies on OS page cache and batching techniques for improved performance.


Not Supported

KSQL doesn't implement any additional type of logging since it just provides a SQL layer on top of the streams in Kafka which themselves are distributed and fault-tolerant. The additional metadata that KSQL maintains is the state stores and meta store which are stored on RocksDB and Kafka topic respectively.

ToDo: Research more about this.

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Open Source

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Compatible With

Spark SQL