knative
/
docs
mirror of https://github.com/knative/docs.git
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

Format markdown (#1634) 

Produced via:
  `prettier --write --prose-wrap=always $(find -name '*.md' | grep -v vendor | grep -v .github)`
/assign @samodell
This commit is contained in:
mattmoor-sockpuppet 2019-07-31 20:02:44 -07:00 committed by Knative Prow Robot
parent 31b5c780c1
commit 353f6fe300
1 changed files with 38 additions and 28 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

66
docs/eventing/channels/default-channels.md
View File

@ -5,23 +5,27 @@ weight: 2
type: "docs"
---
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.
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.).
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.
`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._
@ -54,8 +58,9 @@ 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
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:
@ -68,12 +73,15 @@ metadata:
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.
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:
For example, this is the output when the default channel is set using the above
`ConfigMap` configuration:
```yaml
apiVersion: messaging.knative.dev/v1alpha1
@ -87,16 +95,18 @@ spec:
 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`.
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).
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.
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.