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How to transform events with ksqlDB scalar functions

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How to transform events with ksqlDB scalar functions

If you have a stream of events in a Kafka topic and wish to transform a field in each event, you an use ksqlDB's scalar functions or implement your own scalar UDF if your needs aren't met by the built-in functions.

Setup

As a concrete example, consider a stream containing events that represent movies.

CREATE STREAM movies (id INT KEY, title VARCHAR, genre VARCHAR)
    WITH (KAFKA_TOPIC='movies',
          PARTITIONS=1,
          VALUE_FORMAT='AVRO');

Each event has a title attribute that combines its title and its release year into a string, e.g., Inside Out 2::2024.

Transform events

Given the stream of movies, we can break the title field into separate attributes for the title and release year using the SPLIT function. CAST is also used to convert the resulting release year's data type from string to integer.

SELECT id,
       SPLIT(title, '::')[1] AS title,
       CAST(SPLIT(title, '::')[2] AS INT) AS year,
       genre
FROM movies
EMIT CHANGES;

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 stream backed by Kafka running in Docker and populate it with test data.

CREATE STREAM movies (id INT KEY, title VARCHAR, genre VARCHAR)
    WITH (KAFKA_TOPIC='movies',
          PARTITIONS=1,
          VALUE_FORMAT='AVRO');
INSERT INTO movies (id, title, genre) VALUES (1, 'Twisters::2024', 'drama');
INSERT INTO movies (id, title, genre) VALUES (2, 'Unfrosted::2024', 'comedy');
INSERT INTO movies (id, title, genre) VALUES (3, 'Family Switch::2023', 'comedy');

Next, run the event transformation query to split the title field into the actual movie title and release year. Note that we first tell ksqlDB to consume from the beginning of the stream.

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

SELECT id,
       SPLIT(title, '::')[1] AS title,
       CAST(SPLIT(title, '::')[2] AS INT) AS year,
       genre
FROM movies
EMIT CHANGES;

The query output should look like this:

+---------------------+---------------------+---------------------+---------------------+
|ID                   |TITLE                |YEAR                 |GENRE                |
+---------------------+---------------------+---------------------+---------------------+
|1                    |Twisters             |2024                 |drama                |
|2                    |Unfrosted            |2024                 |comedy               |
|3                    |Family Switch        |2023                 |comedy               |
+---------------------+---------------------+---------------------+---------------------+

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 stream and populates it with test data.

CREATE STREAM movies (id INT KEY, title VARCHAR, genre VARCHAR)
    WITH (KAFKA_TOPIC='movies',
          PARTITIONS=1,
          VALUE_FORMAT='AVRO');

INSERT INTO movies (id, title, genre) VALUES (1, 'Twisters::2024', 'drama');
INSERT INTO movies (id, title, genre) VALUES (2, 'Unfrosted::2024', 'comedy');
INSERT INTO movies (id, title, genre) VALUES (3, 'Family Switch::2023', 'comedy');

Now paste the the event transformation query to split the title field into the actual movie title and release year, and click Run query:

SELECT id,
       SPLIT(title, '::')[1] AS title,
       CAST(SPLIT(title, '::')[2] AS INT) AS year,
       genre
FROM movies
EMIT CHANGES;

The query output should look like this:

+---------------------+---------------------+---------------------+---------------------+
|ID                   |TITLE                |YEAR                 |GENRE                |
+---------------------+---------------------+---------------------+---------------------+
|1                    |Twisters             |2024                 |drama                |
|2                    |Unfrosted            |2024                 |comedy               |
|3                    |Family Switch        |2023                 |comedy               |
+---------------------+---------------------+---------------------+---------------------+

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>