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How to change the serialization format of messages with ksqlDB

ksqlDBKafka Streams
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How to change the serialization format of messages with ksqlDB

If you have a stream of Avro-formatted events in a Kafka topic, it's trivial to convert the events to Protobuf format by using a CREATE STREAM AS SELECT (CSAS) statement to populate the Protobuf-formatted stream with values from the Avro-formatted stream.

For example, suppose that you have a stream with Avro-formatted values that represent movie releases:

CREATE STREAM movies_avro (movie_id BIGINT KEY, title VARCHAR, release_year INT)
    WITH (KAFKA_TOPIC='movies_avro',
          PARTITIONS=1,
          VALUE_FORMAT='AVRO');

Then the analogous stream with Protobuf-formatted values can be created and populated as follows:

CREATE STREAM movies_proto
    WITH (KAFKA_TOPIC='movies_proto', VALUE_FORMAT='PROTOBUF') AS
    SELECT * FROM movies_avro;

Running the example

You can run the example backing this tutorial in one of two ways: locally with the ksql CLI against Kafka and ksqlDB running in Docker, or with Confluent Cloud.

Local With Docker

Prerequisites

Run the commands

Clone the confluentinc/tutorials GitHub repository (if you haven't already) and navigate to the tutorials directory:

git clone git@github.com:confluentinc/tutorials.git
cd tutorials

Start ksqlDB and Kafka:

docker compose -f ./docker/docker-compose-ksqldb.yml up -d

Next, open the ksqlDB CLI:

docker exec -it ksqldb-cli ksql http://ksqldb-server:8088

Run the following SQL statements to create the movies_avro stream backed by Kafka running in Docker and populate it with test data.

CREATE STREAM movies_avro (movie_id BIGINT KEY, title VARCHAR, release_year INT)
    WITH (KAFKA_TOPIC='movies_avro',
          PARTITIONS=1,
          VALUE_FORMAT='AVRO');
INSERT INTO movies_avro (movie_id, title, release_year) VALUES (1, 'Lethal Weapon', 1992);
INSERT INTO movies_avro (movie_id, title, release_year) VALUES (2, 'The Batman', 2022);
INSERT INTO movies_avro (movie_id, title, release_year) VALUES (3, 'Beetlejuice Beetlejuice', 2024);

Finally, run the CREATE STREAM AS SELECT query to create an analogous stream with Protobuf-formatted values. Note that we first tell ksqlDB to consume from the beginning of the stream.

SET 'auto.offset.reset'='earliest';

CREATE STREAM movies_proto
    WITH (KAFKA_TOPIC='movies_proto', VALUE_FORMAT='PROTOBUF') AS
    SELECT * FROM movies_avro;

Query the new topic and observe the same events as in the source topic:

SELECT *
FROM movies_proto;

When you are finished, exit the ksqlDB CLI by entering CTRL-D and clean up the containers used for this tutorial by running:

docker compose -f ./docker/docker-compose-ksqldb.yml down
Confluent Cloud

Prerequisites

Create Confluent Cloud resources

Login to your Confluent Cloud account:

confluent login --prompt --save

Install a CLI plugin that will streamline the creation of resources in Confluent Cloud:

confluent plugin install confluent-cloud_kickstart

Run the following command to create a Confluent Cloud environment and Kafka cluster. This will create resources in AWS region us-west-2 by default, but you may override these choices by passing the --cloud argument with a value of aws, gcp, or azure, and the --region argument that is one of the cloud provider's supported regions, which you can list by running confluent kafka region list --cloud <CLOUD PROVIDER>

confluent cloud-kickstart --name ksqldb-tutorial \
  --environment-name ksqldb-tutorial \
  --output-format stdout

Now, create a ksqlDB cluster by first getting your user ID of the form u-123456 when you run this command:

confluent iam user list

And then create a ksqlDB cluster called ksqldb-tutorial with access linked to your user account:

confluent ksql cluster create ksqldb-tutorial \
  --credential-identity <USER ID>

Run the commands

Login to the Confluent Cloud Console. Select Environments in the lefthand navigation, and then click the ksqldb-tutorial environment tile. Click the ksqldb-tutorial Kafka cluster tile, and then select ksqlDB in the lefthand navigation.

The cluster may take a few minutes to be provisioned. Once its status is Up, click the cluster name and scroll down to the editor.

In the query properties section at the bottom, change the value for auto.offset.reset to Earliest so that ksqlDB will consume from the beginning of the stream we create.

Enter the following statements in the editor and click Run query. This creates the movies_avro stream and populates it with test data.

CREATE STREAM movies_avro (movie_id BIGINT KEY, title VARCHAR, release_year INT)
    WITH (KAFKA_TOPIC='movies_avro',
          PARTITIONS=1,
          VALUE_FORMAT='AVRO');

INSERT INTO movies_avro (movie_id, title, release_year) VALUES (1, 'Lethal Weapon', 1992);
INSERT INTO movies_avro (movie_id, title, release_year) VALUES (2, 'The Batman', 2022);
INSERT INTO movies_avro (movie_id, title, release_year) VALUES (3, 'Beetlejuice Beetlejuice', 2024);

Now paste the CREATE STREAM AS SELECT query in the editor and click Run query:

CREATE STREAM movies_proto
    WITH (KAFKA_TOPIC='movies_proto', VALUE_FORMAT='PROTOBUF') AS
    SELECT * FROM movies_avro;

Query the new topic and observe the same events as in the source topic:

SELECT *
FROM movies_proto;

Clean up

When you are finished, delete the ksqldb-tutorial environment by first getting the environment ID of the form env-123456 corresponding to it:

confluent environment list

Delete the environment, including all resources created for this tutorial:

confluent environment delete <ENVIRONMENT ID>