Creating a channel using cluster or namespace defaults

Developers can create channels of any supported implementation type by creating an instance of a Channel object.

1. Create a Channel object:

   apiVersion: messaging.knative.dev/v1
   kind: Channel
   metadata:
     name: my-channel
     namespace: default

   Since this object is created in the default namespace, according to the default ConfigMap example in the previous section, it will be an InMemoryChannel channel implementation.

2. After the Channel object is created, a mutating admission webhook sets the spec.channelTemplate based on the default channel implementation:

   apiVersion: messaging.knative.dev/v1
   kind: Channel
   metadata:
     name: my-channel
     namespace: default
   spec:
     channelTemplate:
       apiVersion: messaging.knative.dev/v1
       kind: InMemoryChannel

   NOTE: The spec.channelTemplate property cannot be changed after creation, since it is set by the default channel mechanism, not the user.

3. The channel controller creates a backing channel instance based on the spec.channelTemplate.

   When this mechanism is used, two objects are created, a generic Channel object, and an InMemoryChannel object. The generic object acts as a proxy for the InMemoryChannel object, by copying its subscriptions to and setting its status to that of the InMemoryChannel object.

NOTE: Defaults only apply on object creation. Defaults are applied by the webhook only on Channel 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.

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