Apache Kafka Channel Example

You can install and configure the Apache Kafka CRD (KafkaChannel) as the default channel configuration in Knative Eventing.


You must ensure that you meet the prerequisites listed in the Apache Kafka overview.

You must also have the following tools installed:

  • curl
  • sed

Creating a KafkaChannel channel CRD

Install the KafkaChannel sub-component on your Knative Eventing cluster:

curl -L "https://storage.googleapis.com/knative-releases/eventing-contrib/latest/kafka-channel.yaml" \
 | sed 's/REPLACE_WITH_CLUSTER_URL/my-cluster-kafka-bootstrap.kafka:9092/' \
 | kubectl apply --filename -

Note: The above assumes that you have Apache Kafka installed in the kafka, as discussed here!

Once the KafkaChannel API is available, create a new object by configuring the YAML file as follows:

cat <<-EOF | kubectl apply -f -
apiVersion: messaging.knative.dev/v1alpha1
kind: KafkaChannel
  name: my-kafka-channel
  numPartitions: 3
  replicationFactor: 1

You can now set the KafkaChannel CRD as the default channel configuration.

Specifying the default channel configuration

To configure the usage of the KafkaChannel CRD as the default channel configuration, edit the default-ch-webhook ConfigMap as follows:

cat <<-EOF | kubectl apply -f -
apiVersion: v1
kind: ConfigMap
  name: default-ch-webhook
  namespace: knative-eventing
  # Configuration for defaulting channels that do not specify CRD implementations.
  default-ch-config: |
      apiVersion: messaging.knative.dev/v1alpha1
      kind: KafkaChannel

Creating an Apache Kafka channel using the default channel configuration

Now that KafkaChannel is set as the default channel configuration, you can use the channels.messaging.knative.dev CRD to create a new Apache Kafka channel, using the generic Channel:

cat <<-EOF | kubectl apply -f -
apiVersion: messaging.knative.dev/v1alpha1
kind: Channel
  name: testchannel-one

Check Kafka for a testchannel topic. With Strimzi this can be done by using the command:

kubectl -n kafka exec -it my-cluster-kafka-0 -- bin/kafka-topics.sh --zookeeper localhost:2181 --list

The result is:


The Apache Kafka topic that is created by the channel implementation is prefixed with knative-messaging-kafka. This indicates it is an Apache Kafka channel from Knative. It contains the name of the namespace, default in this example, followed by the actual name of the channel.

Configuring the Knative broker for Apache Kafka channels

To setup a broker that will use the new default Kafka channels, you must inject a new default broker, using the command:

kubectl label namespace default knative-eventing-injection=enabled

This will give you two pods, such as:

default-broker-filter-64658fc79f-nf596                            1/1     Running     0          15m
default-broker-ingress-ff79755b6-vj9jt                            1/1     Running     0          15m

Inside the Apache Kafka cluster you should see two new topics, such as:


Creating a service and trigger to use the Apache Kafka broker

To use the Apache Kafka based broker, let's take a look at a simple demo. Use theApiServerSource to publish events to the broker as well as the Trigger API, which then routes events to a Knative Service.

  1. Install ksvc, using the command:

    kubectl apply -f 000-ksvc.yaml
  2. Install a source that publishes to the default broker

    kubectl apply --filename 020-k8s-events.yaml
  3. Create a trigger that routes the events to the ksvc:

    kubectl apply -f 030-trigger.yaml

Verifying your Apache Kafka channel and broker

Now that your Eventing cluster is configured for Apache Kafka, you can verify your configuration with the following options.

Receive events via Knative

Now you can see the events in the log of the ksvc using the command:

kubectl logs --selector='serving.knative.dev/service=broker-kafka-display' -c user-container