Chapter 7. Subscriptions

Prerequisites

• You have installed the OpenShift Serverless Operator, Knative Serving, and Knative Eventing 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 privileges to create applications and other workloads in OpenShift Container Platform.

Verification

• After you create the subscription, the Topology view shows it as a line that connects the channel to the service.

  

Prerequisites

• You have installed the OpenShift Serverless Operator and Knative Eventing 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 

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      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. These settings define how the subscription handles events that the subscriber cannot receive. When you configure this option, the system sends failed events to the deadLetterSink. The system drops the event, does not try redelivery, and logs an error. The deadLetterSink value must be a Destination.

    4

    Configuration settings for the subscriber. The subscriber is the event sink that receives events from the channel.

  • Apply the YAML file:

    $ oc apply -f <filename>

Prerequisites

• You have installed the OpenShift Serverless Operator and Knative Eventing 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> \ 

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    --sink <sink_prefix>:<sink_name> \ 

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    --sink-dead-letter <sink_prefix>:<sink_name> 

  3

  1

  Use --channel to specify the source of CloudEvents to process. You can give the channel name. If you do not use the default InMemoryChannel defined by the Channel custom resource, prefix the channel name with <group:version:kind> for the channel type. For example, use messaging.knative.dev:v1beta1:KafkaChannel for an Apache Kafka channel.

  2

  Use --sink to specify the event destination. By default, the system treats <sink_name> as a Knative service with that name in the same namespace as the subscription. Use one of the following prefixes to specify the sink type:

  ksvc

  A Knative service.

  channel

  Specify the channel to use as the destination. Reference only default channel types.

  broker

  An Eventing broker.

  3

  Optional: Use the optional --sink-dead-letter flag to specify a sink that receives events when delivery fails.

  Example command

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

  Example output

  Subscription 'mysubscription' created in namespace 'default'.

Verification

• To confirm that a subscription connects the channel to the event sink, or subscriber, list the existing subscriptions and inspect the output by running the following command:

  $ 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>

• Configure event delivery parameters that are applied in cases where an event fails to be delivered to an event sink.

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

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> \ 

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    --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