You have an Apache Kafka topic that you want to consume via REST APIs (think curl or just a web browser), how to do it?

The reply is Karapace an Open Source (Apache 2.0) tool providing Schema Registry and REST proxy functionality.

Let's check out how to quickly run it on Docker.

Step 1: the Apache Kafka® Cluster

You need an Apache Kafka cluster up & running, if you have one already you can skip to the next section.

The Karapace GitHub repo contains a Docker compose version of it. Alternatively you can spin a Kafka cluster up in minutes with Aiven by:

• Navigating to the Aiven Console
• Selecting a new Apache Kafka service
• Selecting the cloud provider and region of your preference
• Selecting the plan
• Providing the service name
• Clicking on Create service

After a couple minutes the Apache Kafka® cluster will be up&running.

Step 2: the connection parameters

In order to connect to Apache Kafka, you'll need:

• the bootstrap URI information
• the security information (it being PLAINTEXT, SSL or SASL) with the required certificates

If you are using Aiven, you can download the three certificates (ca.pem, service.crt, service.key) in the Aiven Console, in the Kafka service overview.

Create a folder named certs and place the certificates there.

Step 3: Run Karapace REST proxy in Docker

Once you collected the required information and certificates, it's time to run the REST proxy part of Karapace on Docker with:

docker run -it  \
    -e KARAPACE_BOOTSTRAP_URI='<KAFKA_HOST>:<KAFKA_PORT>' \
    -e KARAPACE_SSL_CAFILE="/certs/ca.pem" \
    -e KARAPACE_SSL_CERTFILE="/certs/service.crt" \
    -e KARAPACE_SSL_KEYFILE="/certs/service.key" \
    -e KARAPACE_SECURITY_PROTOCOL="SSL" \
    -e KARAPACE_HOST='0.0.0.0' \
    -e KARAPACE_PORT=8082 \
    -v $(pwd)/certs:/certs \
    -p 8082:8082 \
    ghcr.io/aiven/karapace:latest ./start.sh rest

The command above is:

• Running the latest Karapace image
• Starting the rest part of Karapace with the ./start.sh command
• Passing the parameters:
  • KARAPACE_BOOTSTRAP_URI: for the Kafka bootstrap URI, you will need to replace the placeholders
  • KARAPACE_SECURITY_PROTOCOL: the protocol to use to connect to Kafka, SSL in this example
  • KARAPACE_SSL_*: the list of certificates used for the SSL connection
  • KARAPACE_HOST and KARAPACE_PORT: the host and port used to start Karapace in the Docker container
  • -v $(pwd)/certs:/certs: mapping the certs folder created locally to /certs in the Docker container
  • -p 8082:8082 mapping the port 8082 to the Docker port 8082

If the above command is successful you should see the below message appearing

======== Running on http://0.0.0.0:8082 ========

And should be able to execute

curl http://0.0.0.0:8082 

Returning

[]

Step 3: Consume Apache Kafka® messages via REST APIs

The last step is to consume messages via REST APIs.

We can list the topics in Kafka with:

curl "http://localhost:8082/topics"

In my test example the response is only a test topic:

["test"]

Next we create a consumer my_consumer with:

Create JSON consumer
curl -X POST \
    -H "Content-Type: application/vnd.kafka.v2+json" \
    -H "Accept: application/vnd.kafka.v2+json" \
    --data '{"name": "my_consumer",  "format": "json", "auto.offset.reset": "earliest"}' \
    http://localhost:8082/consumers/json_consumers

Now we subscribe the my_consumer to test topic with:

curl -X POST \
    -H "Content-Type: application/vnd.kafka.v2+json" \
    --data '{"topics":["test"]}' \
    http://localhost:8082/consumers/json_consumers/instances/my_consumer/subscription

Finally we consume the data from the test topic with:

curl -X GET -H "Accept: application/vnd.kafka.json.v2+json" \
  http://localhost:8082/consumers/json_consumers/instances/my_consumer/records?timeout=1000

The output will showcase the existing data in the topic like

[
  {
    "key": {
      "id": 1
    },
    "offset": 0,
    "partition": 0,
    "timestamp": 1693386610137,
    "topic": "test",
    "value": {
      "id": 1,
      "name": "francesco"
    }
  },
  {
    "key": {
      "id": 1
    },
    "offset": 1,
    "partition": 0,
    "timestamp": 1693387327332,
    "topic": "test",
    "value": {
      "id": 2
    }
  }
]

Karapace will sync the consumer offset to a topic in Kafka named __consumer_offsets, therefore the next time we issue the consume command, we'll see only the messages appeared in the Kafka topic since the last poll (the output will be empty if no new messages are in the topic).