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Apache Kafka Channels

Kafka channels are those backed by Apache Kafka topics.

Deployment steps

  1. Setup Knative Eventing

  2. Install an Apache Kafka cluster, if you have not done so already.

    For Kubernetes a simple installation is done using the Strimzi Kafka Operator. Its installation guides provide content for Kubernetes and Openshift.

    Note: This KafkaChannel is not limited to Apache Kafka installations on Kubernetes. It is also possible to use an off-cluster Apache Kafka installation.

  3. Now that Apache Kafka is installed, you need to configure the bootstrapServers value in the config-kafka ConfigMap, located inside the config/400-kafka-config.yaml file.

    apiVersion: v1
kind: ConfigMap
metadata:
  name: config-kafka
  namespace: knative-eventing
data:
  # Broker URL. Replace this with the URLs for your kafka cluster,
  # which is in the format of my-cluster-kafka-bootstrap.my-kafka-namespace:9092.
  bootstrapServers: REPLACE_WITH_CLUSTER_URL

  4. Apply the Kafka config:

    ko apply -f config

  5. Create the KafkaChannel custom objects:

    apiVersion: messaging.knative.dev/v1beta1
kind: KafkaChannel
metadata:
  name: my-kafka-channel
spec:
  numPartitions: 1
  replicationFactor: 1

    You can configure the number of partitions with numPartitions, as well as the replication factor with replicationFactor. If not set, both will default to 1.

Components

The major components are:

  • Kafka Channel Controller
  • Kafka Channel Dispatcher
  • Kafka Webhook
  • Kafka Config Map

The Kafka Channel Controller is located in one Pod:

kubectl get deployment -n knative-eventing kafka-ch-controller

The Kafka Channel Dispatcher receives and distributes all events to the appropriate consumers:

kubectl get deployment -n knative-eventing kafka-ch-dispatcher

The Kafka Webhook is used to validate and set defaults to KafkaChannel custom objects:

kubectl get deployment -n knative-eventing kafka-webhook

The Kafka Config Map is used to configure the bootstrapServers of your Apache Kafka installation:

kubectl get configmap -n knative-eventing config-kafka

Namespace Dispatchers

By default events are received and dispatched by a single cluster-scoped dispatcher components. You can also specify whether events should be received and dispatched by the dispatcher in the same namespace as the channel definition by adding the eventing.knative.dev/scope: namespace annotation.

First, you need to create the configMap config-kafka in the same namespace as the KafkaChannel.

apiVersion: v1
kind: ConfigMap
metadata:
  name: config-kafka
  namespace: <YOUR_NAMESPACE>
data:
  # Broker URL. Replace this with the URLs for your kafka cluster,
  # which is in the format of my-cluster-kafka-bootstrap.my-kafka-namespace:9092.
  bootstrapServers: REPLACE_WITH_CLUSTER_URL

Note: the bootstrapServers value does not have to be the same as the one specified in knative-eventing/config-kafka.

Then create a KafkaChannel:

apiVersion: messaging.knative.dev/v1beta1
kind: KafkaChannel
metadata:
  name: my-kafka-channel
  namespace: <YOUR_NAMESPACE>
  annotations:
    eventing.knative.dev/scope: namespace
spec:
  numPartitions: 1
  replicationFactor: 1

The dispatcher is created in <YOUR_NAMESPACE>:

kubectl get deployment -n <YOUR_NAMESPACE> kafka-ch-dispatcher

Both cluster-scoped and namespace-scoped dispatcher can coexist. However once the annotation is set (or not set), its value is immutable.