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Chapter 7. Subscriptions

7.1. Creating subscriptions

After you have created a channel and an event sink, you can create a subscription to enable event delivery. Subscriptions are created by configuring a Subscription object, which specifies the channel and the sink (also known as a subscriber) to deliver events to.

7.1.1. Creating a subscription by using the Administrator perspective

After you have created a channel and an event sink, also known as a subscriber, you can create a subscription to enable event delivery. Subscriptions are created by configuring a Subscription object, which specifies the channel and the subscriber to deliver events to. You can also specify some subscriber-specific options, such as how to handle failures.

Prerequisites

  • The OpenShift Serverless Operator and Knative Eventing are installed on your OpenShift Container Platform cluster.
  • You have logged in to the web console and are in the Administrator perspective.
  • You have cluster administrator permissions on OpenShift Container Platform, or you have cluster or dedicated administrator permissions on Red Hat OpenShift Service on AWS or OpenShift Dedicated.
  • You have created a Knative channel.
  • You have created a Knative service to use as a subscriber.

Procedure

  1. In the Administrator perspective of the OpenShift Container Platform web console, navigate to Serverless Eventing.
  2. In the Channel tab, select the Options menu kebab for the channel that you want to add a subscription to.
  3. Click Add Subscription in the list.
  4. In the Add Subscription dialogue box, select a Subscriber for the subscription. The subscriber is the Knative service that receives events from the channel.
  5. Click Add.

7.1.2. Creating a subscription by using the Developer perspective

After you have created a channel and an event sink, you can create a subscription to enable event delivery. Using the OpenShift Container Platform web console provides a streamlined and intuitive user interface to create a subscription.

Prerequisites

  • The OpenShift Serverless Operator, Knative Serving, and Knative Eventing are installed on your OpenShift Container Platform cluster.
  • You have logged in to the web console.
  • You have created an event sink, such as a Knative service, and a channel.
  • You have created a project or have access to a project with the appropriate roles and permissions to create applications and other workloads in OpenShift Container Platform.

Procedure

  1. In the Developer perspective, navigate to the Topology page.

  2. Create a subscription using one of the following methods:

    1. Hover over the channel that you want to create a subscription for, and drag the arrow. The Add Subscription option is displayed.

      Create a subscription for the channel
      1. Select your sink in the Subscriber list.
      2. Click Add.
    2. If the service is available in the Topology view under the same namespace or project as the channel, click on the channel that you want to create a subscription for, and drag the arrow directly to a service to immediately create a subscription from the channel to that service.

Verification

  • After the subscription has been created, you can see it represented as a line that connects the channel to the service in the Topology view:

    Subscription in the Topology view

7.1.3. Creating a subscription by using YAML

After you have created a channel and an event sink, you can create a subscription to enable event delivery. Creating Knative resources by using YAML files uses a declarative API, which enables you to describe subscriptions declaratively and in a reproducible manner. To create a subscription by using YAML, you must create a YAML file that defines a Subscription object, then apply it by using the oc apply command.

Prerequisites

  • The OpenShift Serverless Operator and Knative Eventing are installed on the cluster.
  • Install the OpenShift CLI (oc).
  • You have created a project or have access to a project with the appropriate roles and permissions to create applications and other workloads in OpenShift Container Platform.

Procedure

  • Create a Subscription object:

    • Create a YAML file and copy the following sample code into it: 

      apiVersion: messaging.knative.dev/v1
kind: Subscription
metadata:
  name: my-subscription 1
  namespace: default
spec:
  channel: 2
    apiVersion: messaging.knative.dev/v1
    kind: Channel
    name: example-channel
  delivery: 3
    deadLetterSink:
      ref:
        apiVersion: serving.knative.dev/v1
        kind: Service
        name: error-handler
  subscriber: 4
    ref:
      apiVersion: serving.knative.dev/v1
      kind: Service
      name: event-display
      1
      Name of the subscription.
      2
      Configuration settings for the channel that the subscription connects to.
      3
      Configuration settings for event delivery. This tells the subscription what happens to events that cannot be delivered to the subscriber. When this is configured, events that failed to be consumed are sent to the deadLetterSink. The event is dropped, no re-delivery of the event is attempted, and an error is logged in the system. The deadLetterSink value must be a Destination.
      4
      Configuration settings for the subscriber. This is the event sink that events are delivered to from the channel.

    • Apply the YAML file: 

      $ oc apply -f <filename>

7.1.4. Creating a subscription by using the Knative CLI

After you have created a channel and an event sink, you can create a subscription to enable event delivery. Using the Knative (kn) CLI to create subscriptions provides a more streamlined and intuitive user interface than modifying YAML files directly. You can use the kn subscription create command with the appropriate flags to create a subscription.

Prerequisites

  • The OpenShift Serverless Operator and Knative Eventing are installed on your OpenShift Container Platform cluster.
  • You have installed the Knative (kn) CLI.
  • You have created a project or have access to a project with the appropriate roles and permissions to create applications and other workloads in OpenShift Container Platform.

Procedure

  • Create a subscription to connect a sink to a channel: 

    $ kn subscription create <subscription_name> \
  --channel <group:version:kind>:<channel_name> \ 1
  --sink <sink_prefix>:<sink_name> \ 2
  --sink-dead-letter <sink_prefix>:<sink_name> 3
    1
    --channel specifies the source for cloud events that should be processed. You must provide the channel name. If you are not using the default InMemoryChannel channel that is backed by the Channel custom resource, you must prefix the channel name with the <group:version:kind> for the specified channel type. For example, this will be messaging.knative.dev:v1beta1:KafkaChannel for an Apache Kafka backed channel.
    2
    --sink specifies the target destination to which the event should be delivered. By default, the <sink_name> is interpreted as a Knative service of this name, in the same namespace as the subscription. You can specify the type of the sink by using one of the following prefixes:
    ksvc
    A Knative service.
    channel
    A channel that should be used as destination. Only default channel types can be referenced here.
    broker
    An Eventing broker.
    3
    Optional: --sink-dead-letter is an optional flag that can be used to specify a sink which events should be sent to in cases where events fail to be delivered. For more information, see the OpenShift Serverless Event delivery documentation.

    Example command

    $ kn subscription create mysubscription --channel mychannel --sink ksvc:event-display

    Example output

    Subscription 'mysubscription' created in namespace 'default'.

Verification

  • To confirm that the channel is connected to the event sink, or subscriber, by a subscription, list the existing subscriptions and inspect the output: 

    $ kn subscription list

    Example output

    NAME            CHANNEL             SUBSCRIBER           REPLY   DEAD LETTER SINK   READY   REASON
mysubscription   Channel:mychannel   ksvc:event-display                              True

Deleting a subscription

  • Delete a subscription: 

    $ kn subscription delete <subscription_name>

7.1.5. Next steps

7.2. Managing subscriptions

7.2.1. Describing subscriptions by using the Knative CLI

You can use the kn subscription describe command to print information about a subscription in the terminal by using the Knative (kn) CLI. Using the Knative CLI to describe subscriptions provides a more streamlined and intuitive user interface than viewing YAML files directly.

Prerequisites

  • You have installed the Knative (kn) CLI.
  • You have created a subscription in your cluster.

Procedure

  • Describe a subscription: 

    $ kn subscription describe <subscription_name>

    Example output

    Name:            my-subscription
Namespace:       default
Annotations:     messaging.knative.dev/creator=openshift-user, messaging.knative.dev/lastModifier=min ...
Age:             43s
Channel:         Channel:my-channel (messaging.knative.dev/v1)
Subscriber:
  URI:           http://edisplay.default.example.com
Reply:
  Name:          default
  Resource:      Broker (eventing.knative.dev/v1)
DeadLetterSink:
  Name:          my-sink
  Resource:      Service (serving.knative.dev/v1)

Conditions:
  OK TYPE                  AGE REASON
  ++ Ready                 43s
  ++ AddedToChannel        43s
  ++ ChannelReady          43s
  ++ ReferencesResolved    43s

7.2.2. Listing subscriptions by using the Knative CLI

You can use the kn subscription list command to list existing subscriptions on your cluster by using the Knative (kn) CLI. Using the Knative CLI to list subscriptions provides a streamlined and intuitive user interface.

Prerequisites

  • You have installed the Knative (kn) CLI.

Procedure

  • List subscriptions on your cluster: 

    $ kn subscription list

    Example output

    NAME             CHANNEL             SUBSCRIBER           REPLY   DEAD LETTER SINK   READY   REASON
mysubscription   Channel:mychannel   ksvc:event-display                              True

7.2.3. Updating subscriptions by using the Knative CLI

You can use the kn subscription update command as well as the appropriate flags to update a subscription from the terminal by using the Knative (kn) CLI. Using the Knative CLI to update subscriptions provides a more streamlined and intuitive user interface than updating YAML files directly.

Prerequisites

  • You have installed the Knative (kn) CLI.
  • You have created a subscription.

Procedure

  • Update a subscription: 

    $ kn subscription update <subscription_name> \
  --sink <sink_prefix>:<sink_name> \ 1
  --sink-dead-letter <sink_prefix>:<sink_name> 2
    1
    --sink specifies the updated target destination to which the event should be delivered. You can specify the type of the sink by using one of the following prefixes:
    ksvc
    A Knative service.
    channel
    A channel that should be used as destination. Only default channel types can be referenced here.
    broker
    An Eventing broker.
    2
    Optional: --sink-dead-letter is an optional flag that can be used to specify a sink which events should be sent to in cases where events fail to be delivered. For more information, see the OpenShift Serverless Event delivery documentation.

    Example command

    $ kn subscription update mysubscription --sink ksvc:event-display

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