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
created, an operator can configure a default channel to use for their underlying
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.,
Setting the default channel configuration
The default channel configuration is specified in the
default-ch-webhook in the
knative-eventing namespace. This
specify a cluster-wide default channel and namespace-specific channel
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
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
Channel controller will then create the backing channel instance
based on that
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
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
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
KafkaChannel instead (i.e., the default for that namespace).
Defaults only apply on object creation
Defaults are applied by the webhook only on
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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