Default channels

A channel provides an event delivery mechanism that can fan-out received events to multiple destinations. The default channel configuration allows channels to be created without specifying an underlying implementation. This is useful for users that do not care about the properties a particular channel provides (e.g., ordering, persistence, etc.), but are rather fine with using the
implementation selected by the the operator. The operator controls the default settings via a ConfigMap. For example, when Brokers or Sequences are created, an operator can configure a default channel to use for their underlying channel-based implementations.

Even though this default channel mechanism aims to ease the usability of the system, users can still create their own channels directly by instantiating one of the supported channel objects (e.g., InMemoryChannel, KafkaChannel, etc.).

Setting the default channel configuration

The default channel configuration is specified in the ConfigMap named default-ch-webhook in the knative-eventing namespace. This ConfigMap may specify a cluster-wide default channel and namespace-specific channel implementations.

The namespace-specific defaults override the cluster default for channels created in the specified namespace.

Note that default channel spec fields can also be specified.

The default options are specified like this:

apiVersion: v1
kind: ConfigMap
metadata:
  name: default-ch-webhook
  namespace: knative-eventing
data:
  default-ch-config: |
    clusterDefault:
      apiVersion: messaging.knative.dev/v1alpha1
      kind: InMemoryChannel
    namespaceDefaults:
      some-namespace:
        apiVersion: messaging.knative.dev/v1alpha1
        kind: KafkaChannel
        spec:
          numPartitions: 2
          replicationFactor: 1

Namespace-specific defaults take precedence over cluster defaults when matched.

Creating a channel with cluster or namespace defaults

To create a channel using the cluster or namespace defaults set by the operator, create a generic Channel custom object. This is typically useful if you do not care what kind of channel it is, and if you are comfortable using the ones that the operator has selected for you.

For example, this is a valid Channel object:

apiVersion: messaging.knative.dev/v1alpha1
kind: Channel
metadata:
  name: my-channel
  namespace: default

When the above Channel is created, a mutating admission webhook sets spec.channelTemplate based on the default channel implementation chosen by the operator. The Channel controller will then create the backing channel instance based on that spec.channelTemplate. The spec.channelTemplate property cannot be changed after creation, and it will be normally set by the default channel mechanism instead of the user.

For example, this is the output when the default channel is set using the above ConfigMap configuration:

apiVersion: messaging.knative.dev/v1alpha1
kind: Channel
metadata:
  name: my-channel
  namespace: default
spec:
  channelTemplate:
    apiVersion: messaging.knative.dev/v1alpha1
    kind: InMemoryChannel

When this mechanism is used, two objects are created, a generic Channel and an InMemoryChannel channel. The former acts as a proxy of the latter: it copies its subscriptions to the InMemoryChannel one and sets its status with the backing InMemoryChannel status.

Further, note that as the Channel was created in the default namespace, it uses the cluster defaults, i.e., InMemoryChannel. If the Channel would have been created in the some-namespace namespace, it would have been backed by an underlying KafkaChannel instead (i.e., the default for that namespace).

Defaults only apply on object creation

Defaults are applied by the webhook only on Channel, Broker, or Sequence creation. If the default settings change, the new defaults will only apply to newly-created channels, brokers, or sequences. Existing ones will not change.