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docs/eventing/channels/default-channels.md
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@ -5,23 +5,27 @@ weight: 2
type: "docs" type: "docs"
--- ---
A channel provides an event delivery mechanism that can fan-out received events to multiple destinations. A channel provides an event delivery mechanism that can fan-out received events
The default channel configuration allows channels to be created without to multiple destinations. The default channel configuration allows channels to
specifying an underlying implementation. This is useful for users that do not care about the properties a particular be created without specifying an underlying implementation. This is useful for
channel provides (e.g., ordering, persistence, etc.), but are rather fine with using the users that do not care about the properties a particular channel provides (e.g.,
implementation selected by the the operator. The operator controls the default settings via a `ConfigMap`. ordering, persistence, etc.), but are rather fine with using the
For example, when `Brokers` or `Sequences` are created, an operator can configure a default channel to use implementation selected by the the operator. The operator controls the default
for their underlying channel-based implementations. 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 Even though this default channel mechanism aims to ease the usability of the
channels directly by instantiating one of the supported channel objects (e.g., `InMemoryChannel`, `KafkaChannel`, etc.). 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 ## Setting the default channel configuration
The default channel configuration is specified in the `ConfigMap` named The default channel configuration is specified in the `ConfigMap` named
`default-ch-webhook` in the `knative-eventing` namespace. This `ConfigMap` `default-ch-webhook` in the `knative-eventing` namespace. This `ConfigMap` may
may specify a cluster-wide default channel and namespace-specific specify a cluster-wide default channel and namespace-specific channel
channel implementations. implementations.
_The namespace-specific defaults override the cluster default for channels _The namespace-specific defaults override the cluster default for channels
created in the specified namespace._ created in the specified namespace._
@ -54,8 +58,9 @@ Namespace-specific defaults take precedence over cluster defaults when matched.
## Creating a channel with cluster or namespace defaults ## 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. To create a channel using the cluster or namespace defaults set by the operator,
This is typically useful if you do not care what kind of channel it is, and if you are comfortable using the ones that 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. the operator has selected for you.
For example, this is a valid `Channel` object: For example, this is a valid `Channel` object:
@ -68,12 +73,15 @@ metadata:
namespace: default namespace: default
``` ```
When the above `Channel` is created, a mutating admission webhook sets `spec.channelTemplate` based on the default channel When the above `Channel` is created, a mutating admission webhook sets
implementation chosen by the operator. The `Channel` controller will then create the backing channel instance based on `spec.channelTemplate` based on the default channel implementation chosen by the
that `spec.channelTemplate`. The `spec.channelTemplate` property cannot be changed after creation, and it will be normally operator. The `Channel` controller will then create the backing channel instance
set by the default channel mechanism instead of the user. 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: For example, this is the output when the default channel is set using the above
`ConfigMap` configuration:
```yaml ```yaml
apiVersion: messaging.knative.dev/v1alpha1 apiVersion: messaging.knative.dev/v1alpha1
@ -87,16 +95,18 @@ spec:
 kind: InMemoryChannel  kind: InMemoryChannel
``` ```
When this mechanism is used, two objects are created, a generic `Channel` and an `InMemoryChannel` channel. When this mechanism is used, two objects are created, a generic `Channel` and an
The former acts as a proxy of the latter: it copies its subscriptions to the `InMemoryChannel` one and sets its `status` `InMemoryChannel` channel. The former acts as a proxy of the latter: it copies
with the backing `InMemoryChannel` `status`. 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`. Further, note that as the `Channel` was created in the `default` namespace, it
If the `Channel` would have been created in the `some-namespace` namespace, it would have been backed by an underlying uses the cluster defaults, i.e., `InMemoryChannel`. If the `Channel` would have
`KafkaChannel` instead (i.e., the default for that namespace). 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 only apply on object creation
Defaults are applied by the webhook only on `Channel`, `Broker`, or `Sequence` creation. If the default Defaults are applied by the webhook only on `Channel`, `Broker`, or `Sequence`
settings change, the new defaults will only apply to newly-created channels, brokers, or sequences. creation. If the default settings change, the new defaults will only apply to
Existing ones will not change. newly-created channels, brokers, or sequences. Existing ones will not change.