Get Started Free
Tutorial

How to filter messages in a Kafka topic with ksqlDB

Flink SQLksqlDBKafka Streams
‹ Back to Tutorials

How to filter messages in a Kafka topic with ksqlDB

How do you filter messages in a Kafka topic to contain only those that you're interested in? In this tutorial, we will filter a stream of book publications down to those by a particular author.

Setup

First, let's create a base stream of events containing book publications:

CREATE STREAM all_publications (bookid BIGINT KEY, 
                                author VARCHAR, 
                                title VARCHAR)
    WITH (KAFKA_TOPIC='publication_events'
          PARTITIONS=1,
          VALUE_FORMAT='JSON');

To create a new topic containing only events for a particular author, we issue a CREATE STREAM AS SELECT query containing the appropriate WHERE clause:

CREATE STREAM george_martin WITH (KAFKA_TOPIC='george_martin_books') AS
    SELECT *
      FROM all_publications
      WHERE author = 'George R. R. Martin';

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

CREATE STREAM all_publications (book_id BIGINT KEY, 
                                author VARCHAR, 
                                title VARCHAR)
    WITH (KAFKA_TOPIC='publication_events',
          PARTITIONS=1,
          VALUE_FORMAT='JSON');
INSERT INTO all_publications (book_id, author, title) VALUES (1, 'C.S. Lewis', 'The Silver Chair');
INSERT INTO all_publications (book_id, author, title) VALUES (2, 'George R. R. Martin', 'A Song of Ice and Fire');
INSERT INTO all_publications (book_id, author, title) VALUES (3, 'C.S. Lewis', 'Perelandra');
INSERT INTO all_publications (book_id, author, title) VALUES (4, 'George R. R. Martin', 'Fire & Blood');
INSERT INTO all_publications (book_id, author, title) VALUES (5, 'J. R. R. Tolkien', 'The Hobbit');
INSERT INTO all_publications (book_id, author, title) VALUES (6, 'J. R. R. Tolkien', 'The Lord of the Rings');
INSERT INTO all_publications (book_id, author, title) VALUES (7, 'George R. R. Martin', 'A Dream of Spring');
INSERT INTO all_publications (book_id, author, title) VALUES (8, 'J. R. R. Tolkien', 'The Fellowship of the Ring');
INSERT INTO all_publications (book_id, author, title) VALUES (9, 'George R. R. Martin', 'The Ice Dragon');

Finally, run the filter query to find books by George R. R. Martin and write the events to a new topic. Note that we first tell ksqlDB to consume from the beginning of the stream.

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

CREATE STREAM george_martin WITH (KAFKA_TOPIC='george_martin_books') AS
  SELECT *
    FROM all_publications
    WHERE author = 'George R. R. Martin';

Query the new topic:

SELECT *
FROM george_martin
EMIT CHANGES;

The query output should look like this:

+-------------------------------+-------------------------------+-------------------------------+
|BOOK_ID                        |AUTHOR                         |TITLE                          |
+-------------------------------+-------------------------------+-------------------------------+
|2                              |George R. R. Martin            |A Song of Ice and Fire         |
|4                              |George R. R. Martin            |Fire & Blood                   |
|7                              |George R. R. Martin            |A Dream of Spring              |
|9                              |George R. R. Martin            |The Ice Dragon                 |
+-------------------------------+-------------------------------+-------------------------------+

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 all_publications stream and populates it with test data.

CREATE STREAM all_publications (book_id BIGINT KEY, 
                                author VARCHAR, 
                                title VARCHAR)
    WITH (KAFKA_TOPIC='publication_events',
          PARTITIONS=1,
          VALUE_FORMAT='JSON');

INSERT INTO all_publications (book_id, author, title) VALUES (1, 'C.S. Lewis', 'The Silver Chair');
INSERT INTO all_publications (book_id, author, title) VALUES (2, 'George R. R. Martin', 'A Song of Ice and Fire');
INSERT INTO all_publications (book_id, author, title) VALUES (3, 'C.S. Lewis', 'Perelandra');
INSERT INTO all_publications (book_id, author, title) VALUES (4, 'George R. R. Martin', 'Fire & Blood');
INSERT INTO all_publications (book_id, author, title) VALUES (5, 'J. R. R. Tolkien', 'The Hobbit');
INSERT INTO all_publications (book_id, author, title) VALUES (6, 'J. R. R. Tolkien', 'The Lord of the Rings');
INSERT INTO all_publications (book_id, author, title) VALUES (7, 'George R. R. Martin', 'A Dream of Spring');
INSERT INTO all_publications (book_id, author, title) VALUES (8, 'J. R. R. Tolkien', 'The Fellowship of the Ring');
INSERT INTO all_publications (book_id, author, title) VALUES (9, 'George R. R. Martin', 'The Ice Dragon');

Now paste the filter query to find books by George R. R. Martin and write the events to a new topic and click Run query:

CREATE STREAM george_martin WITH (KAFKA_TOPIC='george_martin_books') AS
  SELECT *
    FROM all_publications
    WHERE author = 'George R. R. Martin';

Query the new topic:

SELECT *
FROM george_martin
EMIT CHANGES;

The query output should look like this:

+-------------------------------+-------------------------------+-------------------------------+
|BOOK_ID                        |AUTHOR                         |TITLE                          |
+-------------------------------+-------------------------------+-------------------------------+
|2                              |George R. R. Martin            |A Song of Ice and Fire         |
|4                              |George R. R. Martin            |Fire & Blood                   |
|7                              |George R. R. Martin            |A Dream of Spring              |
|9                              |George R. R. Martin            |The Ice Dragon                 |
+-------------------------------+-------------------------------+-------------------------------+

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>