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KSQL Architecture

You can use KSQL to build event streaming applications from Apache Kafka® topics by using only SQL statements and queries. KSQL is built on Kafka Streams, so a KSQL application communicates with a Kafka cluster like any other Kafka Streams application.

KSQL Components

KSQL has these main components:

  • KSQL engine – processes KSQL statements and queries
  • REST interface – enables client access to the engine
  • KSQL CLI – console that provides a command-line interface (CLI) to the engine
  • KSQL UI – enables developing KSQL applications in Confluent Control Center

KSQL Server comprises the KSQL engine and the REST API. KSQL Server instances communicate with the Kafka cluster, and you can add more of them as necessary without restarting your applications.

Diagram showing architecture of KSQL
KSQL Engine

The KSQL engine executes KSQL statements and queries. You define your application logic by writing KSQL statements, and the engine builds and runs the application on available KSQL servers. Each KSQL server instance runs a KSQL engine. Under the hood, the engine parses your KSQL statements and builds corresponding Kafka Streams topologies.

The KSQL engine is implemented in the class.


The KSQL CLI provides a console with a command-line interface for the KSQL engine. Use the KSQL CLI to interact with KSQL Server instances and develop your streaming applications. The KSQL CLI is designed to be familiar to users of MySQL, Postgres, and similar applications.

The KSQL CLI is implemented in the io.confluent.ksql.cli package.

REST Interface

The REST server interface enables communicating with the KSQL engine from the CLI, Confluent Control Center, or from any other REST client. For more information, see KSQL REST API Reference.

The KSQL REST server is implemented in the class.

When you deploy your KSQL application, it runs on KSQL Server instances that are independent of one another, are fault-tolerant, and can scale elastically with load. For more information, see KSQL Deployment Modes.

Diagram showing architecture of KSQL

KSQL and Kafka Streams

KSQL is built on Kafka Streams, a robust stream processing framework that is part of Apache Kafka. You can use KSQL and Kafka Streams together in your event streaming applications. For more information on their relationship, see KSQL and Kafka Streams. For more information on Kafka Streams, see Streams Architecture.

Also, you can implement custom logic and aggregations in your KSQL applications by implementing User Defined Functions in Java. For more information, see KSQL Custom Function Reference (UDF and UDAF).

KSQL Language Elements

Like traditional relational databases, KSQL supports two categories of statements: Data Definition Language (DDL) and Data Manipulation Language (DML).

These categories are similar in syntax, data types, and expressions, but they have different functions in KSQL Server.

Data Definition Language (DDL) Statements

Imperative verbs that define metadata on the KSQL server by adding, changing, or deleting streams and tables. Data Definition Language statements modify metadata only and don’t operate on data. You can use these statements with declarative DML statements.

The DDL statements include:

Data Manipulation Language (DML) Statements

Declarative verbs that read and modify data in KSQL streams and tables. Data Manipulation Language statements modify data only and don’t change metadata. The KSQL engine compiles DML statements into Kafka Streams applications, which run on a Kafka cluster like any other Kafka Streams application.

The DML statements include:


The CSAS and CTAS statements occupy both categories, because they perform both a metadata change, like adding a stream, and they manipulate data, by creating a derivative of existing records.

For more information, see KSQL Syntax Reference.

KSQL Deployment Modes

You can use these modes to deploy your KSQL streaming applications:

  • Interactive – data exploration and pipeline development
  • Headless – long-running production environments

In both deployment modes, KSQL enables distributing the processing load for your KSQL applications across all KSQL Server instances, and you can add more KSQL Server instances without restarting your applications.

Interactive Deployment

Use the interactive mode to develop your KSQL applications. When you deploy a KSQL server in interactive mode, the REST interface is available for the KSQL CLI and Confluent Control Center to connect to.

Diagram showing interactive KSQL deployment

In interactive mode, you can:

  • Write statements and queries on the fly
  • Start any number of server nodes: <path-to-confluent>/bin/ksql-server-start
  • Start one or more CLIs or REST Clients and point them to a server: <path-to-confluent>/bin/ksql https://<ksql-server-ip-address>:8090

Headless Deployment

Use headless mode to deploy your KSQL application to a production environment. When you deploy a KSQL server in headless mode, the REST interface isn’t available, so you assign workloads to KSQL servers by using a SQL file. The SQL file contains the KSQL statements and queries that define your application. Headless mode is ideal for streaming ETL application deployments.

Diagram showing headless KSQL deployment

In headless mode you can:

  • Start any number of server nodes
  • Pass a SQL file with KSQL statements to execute: <path-to-confluent>bin/ksql-node query-file=path/to/myquery.sql
  • Version-control your queries and transformations as code
  • Ensure resource isolation
  • Leave resource management to dedicated systems, like Kubernetes

Supported Operations in Headless and Interactive Modes

The following table shows which KSQL operations are supported in headless and interactive deployments.

KSQL Operation Interactive KSQL Headless KSQL
Describe a stream or table, including runtime stats (DESCRIBE, DESCRIBE EXTENDED) Supported Not Supported
Explain a query, including runtime stats (EXPLAIN) Supported Not Supported
CREATE a stream or table Supported Supported
DROP a stream or table Supported Not Supported
List existing streams and tables (SHOW STREAMS, SHOW TABLES) Supported Not Supported
List running queries (SHOW QUERIES) Supported Not Supported
Run a script (RUN SCRIPT) Supported Not Supported
Set query properties (SET) Supported Supported
Show contents of a Kafka topic (PRINT) Supported Not Supported
Show contents of a stream or table (SELECT) Supported Not Supported
Show properties of a query (SHOW PROPERTIES) Supported Not Supported
Show results of a query (SELECT) Supported Not Supported
TERMINATE a query Supported Not Supported
Start and stop a KSQL Server instance Not with KSQL API Not with KSQL API
Cleanup and delete internal data (internal topics) of a KSQL cluster or application Supported (KSQL REST API) Not with KSQL API


You can perform operations listed as “Not with KSQL API” manually. Also, you can use deployment tools, like Kubernetes or Ansible, and you can use the Kafka tools, like kafka-delete-records.

Dedicating Resources

Join KSQL engines to the same service pool by using the property. The following diagram shows a Kafka cluster with separate workloads for a finance pool and a sales pool. For more information, see

Diagram showing how to join KSQL engines to the same service pool

To scale out, just add more KSQL server instances. There’s no master node or coordination among them required. For more information, see KSQL Capacity Planning.

KSQL Query Lifecycle

To create a streaming application with KSQL, you write KSQL statements and queries. Each statement and query has a lifecycle with the following steps:

  1. You register a KSQL stream or table from an existing Kafka topic with a DDL statement, like CREATE STREAM <my-stream> WITH <topic-name>.
  2. You express your app by using a KSQL statement, like CREATE TABLE AS SELECT FROM <my-stream>.
  3. KSQL parses your statement into an abstract syntax tree (AST).
  4. KSQL uses the AST and creates the logical plan for your statement.
  5. KSQL uses the logical plan and creates the physical plan for your statement.
  6. KSQL generates and runs the Kafka Streams application.
  7. You manage the application as a STREAM or TABLE with its corresponding persistent query.
Diagram showing how the KSQL query lifecycle for a KSQL statement

Register the Stream

Register a stream or table by using the DDL statements, CREATE STREAM and CREATE TABLE. For example, the following KSQL statement creates a stream named authorization_attempts that’s backed by a topic named authorizations.

CREATE STREAM authorization_attempts                        \
  (card_number VARCHAR, attemptTime BIGINT, ...)            \
  WITH (kafka_topic='authorizations', value_format=‘JSON’);

KSQL writes DDL and DML statements to the command topic. Each KSQL Server reads the statement from the command topic, parsing and analyzing it.

Diagram showing deployment of a KSQL file to a command topic

The CREATE STREAM statement is a DDL statement, so the action is to update the KSQL metadata.

Each KSQL server has an internal, in-memory metadata store, or metastore, that it builds as it receives DDL statements. The metastore is an in-memory map. For each new DDL statement, the KSQL engine adds an entry to the metastore.

For example, the metastore entry for the previous CREATE STREAM statement might resemble:

Source Name Structured Data Source
AUTHORIZATION_ATTEMPTS [DataSourceType: STREAM], [Schema:(card_number VARCHAR, attemptTime BIGINT, attemptRegion VARCHAR, …)], [Key: null], [KsqlTopic: AUTHORIZATIONS], …

The KSQL metastore is implemented in the io.confluent.ksql.metastore package.

Express Your Application as a KSQL Statement

Now that you have a stream, express your application’s business logic by using a KSQL statement. The following DML statement creates a possible_fraud table from the authorization_attempts stream:

CREATE TABLE possible_fraud AS     \
  SELECT card_number, count(*)     \
  FROM authorization_attempts      \
  WHERE region = ‘west’            \
  GROUP BY card_number             \
  HAVING count(*) > 3;

The KSQL engine translates the DML statement into a Kafka Streams application. The application reads the source topic continuously, and whenever the count(*) > 3 condition is met, it writes records to the possible_fraud table.

KSQL Parses Your Statement

To express your DML statement as a Kafka Streams application, the KSQL engine starts by parsing the statement. The parser creates an abstract syntax tree (AST). The KSQL engine uses the AST to plan the query.

The KSQL statement parser is based on ANTLR and is implemented in the io.confluent.ksql.parser package.

KSQL Creates the Logical Plan

The KSQL engine creates the logical plan for the query by using the AST. For the previous possible_fraud statement, the logical plan has the following steps:

  1. Define the source – FROM node
  2. Apply the filter – WHERE clause
  3. Apply aggregation – GROUP BY
  4. Project – WINDOW
  5. Apply post-aggregation filter – HAVING, applied to the result of GROUP BY
  6. Project – for the result
Diagram showing how the KSQL engine creates a logical plan for a KSQL statement

KSQL Creates the Physical Plan

From the logical plan, the KSQL engine creates the physical plan, which is a Kafka Streams DSL application with a schema.

The generated code is based on the KSQL classes, SchemaKStream and SchemaKTable:

The KSQL engine traverses the nodes of the logical plan and emits corresponding Kafka Streams API calls:

  1. Define the source – a SchemaKStream or SchemaKTable with info from the KSQL metastore
  2. Filter – produces another SchemaKStream
  3. Project – select() method
  4. Apply aggregation – Multiple steps: rekey(), groupby(), and aggregate() methods. KSQL may re-partition data if it’s not keyed with a GROUP BY phrase.
  5. Filter – filter() method
  6. Project – select() method for the result
Diagram showing how the KSQL engine creates a physical plan for a KSQL statement

If the DML statement is CREATE STREAM AS SELECT or CREATE TABLE AS SELECT, the result from the generated Kafka Streams application is a persistent query that writes continuously to its output topic until the query is terminated.