ksqlDB can read and write messages in Avro and Protobuf formats by integrating with Confluent Schema Registry. ksqlDB automatically retrieves (reads) and registers (writes) schemas as needed, which saves you from having to manually define columns and data types in SQL and from manual interaction with Schema Registry.

Supported functionality

ksqlDB supports Avro and Protobuf data in the Apache Kafka® message values.

Schemas with nested fields are supported. You can read nested data, in Avro, PROTOBUF, JSON, and JSON_SR formats, by using the STRUCT type. Also, you can create new nested STRUCT data as the result of a query. For more info, see STRUCT.

The following functionality is not supported:

  • Message keys in Avro or Protobuf formats are not supported. Message keys in ksqlDB are always interpreted as STRING format, which means ksqlDB ignores and Protobuf schemas that have been registered for message keys, and the key is read by using StringDeserializer.

Configure ksqlDB for Avro or Protobuf

You must configure the REST endpoint of Schema Registry by setting ksql.schema.registry.url (default: http://localhost:8081) in the ksqlDB Server configuration file (<path-to-confluent>/etc/ksqldb/ksql-server.properties). For more information, see Installation Instructions.


Don't use the SET statement in the ksqlDB CLI to configure the registry endpoint.

Use Avro or Protobuf in ksqlDB

Before using Avro or Protobuf in ksqlDB, make sure that Schema Registry is up and running and that ksql.schema.registry.url is set correctly in the ksqlDB properties file (defaults to http://localhost:8081). Schema Registry is included by default with Confluent Platform.


By default, ksqlDB-registered Avro schemas have the same name (KsqlDataSourceSchema) and the same namespace (io.confluent.ksql.avro_schemas). You can override this behavior by providing a VALUE_AVRO_SCHEMA_FULL_NAME property in the WITH clause, where you set the VALUE_FORMAT to 'AVRO'. As the name suggests, this property overrides the default name/namespace with the provided one. For example, com.mycompany.MySchema registers a schema with the MySchema name and the com.mycompany namespace.

Here's what you can do with Avro and Protobuf in ksqlDB:

  • Declare streams and tables on Kafka topics with Avro- or Protobuf- formatted data by using CREATE STREAM and CREATE TABLE statements.
  • Read from and write into Avro- or Protobuf-formatted data by using CREATE STREAM AS SELECT and CREATE TABLE AS SELECT statements.
  • Create derived streams and tables from existing streams and tables with CREATE STREAM AS SELECT and CREATE TABLE AS SELECT statements.
  • Convert data to different formats with CREATE STREAM AS SELECT and CREATE TABLE AS SELECT statements. For example, you can convert a stream from Avro to JSON.

Example SQL Statements with Avro

The following statements show how to create streams and tables that have Avro-formatted data. If you want to use Protobuf-formatted data, substitute PROTOBUF for AVRO in each statement.

Create a New Stream by Reading Avro-formatted Data

The following statement shows how to create a new pageviews stream by reading from a Kafka topic that has Avro-formatted messages.

  WITH (KAFKA_TOPIC='pageviews-avro-topic',

Create a New Table by Reading Avro-formatted Data

The following statement shows how to create a new users table by reading from a Kafka topic that has Avro-formatted message values.

  WITH (KAFKA_TOPIC='users-avro-topic',

In this example, you don't need to define any columns or data types in the CREATE statement. ksqlDB infers this information automatically from the latest registered Avro schema for the pageviews-avro-topic topic. ksqlDB uses the most recent schema at the time the statement is first executed.

Note: the Avro schema must be registered in the Schema Registry under the subject users-avro-topic-value.

By default, the key of the data will be assumed to be a single KAFKA serialized STRING called ROWKEY. If the key schema differs, then you can provide just the key column in the statement. For example, the following creates the users table with a 64-bit integer key and infers the value columns from the Avro schema.

  WITH (KAFKA_TOPIC='users-avro-topic',

Create a New Stream with Selected Fields of Avro-formatted Data

If you want to create a STREAM or TABLE with only a subset of all the available fields in the Avro schema, you must explicitly define the columns and data types.

The following statement shows how to create a new pageviews_reduced stream, which is similar to the previous example, but with only a few of the available fields in the Avro data. In this example, only the viewtime and pageid columns are picked.

CREATE STREAM pageviews_reduced (viewtime BIGINT, pageid VARCHAR)
  WITH (KAFKA_TOPIC='pageviews-avro-topic',

Convert a JSON Stream to an Avro Stream

ksqlDB enables you to work with streams and tables regardless of their underlying data format. This means that you can easily mix and match streams and tables with different data formats and also convert between data formats. For example, you can join a stream backed by Avro data with a table backed by JSON data.

In this example, only the VALUE_FORMAT is required for Avro to achieve the data conversion. ksqlDB automatically generates an appropriate Avro schema for the new pageviews_avro stream, and it registers the schema with Schema Registry.

CREATE STREAM pageviews_json (viewtime BIGINT, userid VARCHAR, pageid VARCHAR)
  WITH (KAFKA_TOPIC='pageviews_kafka_topic_json', VALUE_FORMAT='JSON');

CREATE STREAM pageviews_avro
  SELECT * FROM pageviews_json

For more information, see Changing Data Serialization Format from JSON to Avro in the Stream Processing Cookbook.

Last update: 2020-03-19