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Chapter 10. Event workflows

10.1. Event delivery workflows using brokers and triggers

Brokers can be used in combination with triggers to deliver events from an event source to an event sink.

Broker event delivery overview

Events can be sent from an event source to a broker as an HTTP POST request.

After events have entered the broker, they can be filtered by CloudEvent attributes using triggers, and sent as an HTTP POST request to an event sink.

10.1.1. Creating a broker

OpenShift Serverless provides a default Knative broker that can be created by using the Knative CLI. You can also create the default broker by adding the eventing.knative.dev/injection=enabled label to a namespace if you are a cluster administrator, or by adding the eventing.knative.dev/injection: enabled annotation to a trigger if you are a developer.

Important

Although both developers and cluster administrators can add a broker by injection, only cluster administrators can permanently delete brokers that were created using this method.

10.1.1.1. Creating a broker using the Knative CLI

Prerequisites

  • The OpenShift Serverless Operator and Knative Eventing are installed on your OpenShift Container Platform cluster.
  • You have installed the kn CLI.

Procedure

  • Create the default broker:

    $ kn broker create default

Verification

  1. Use the kn command to list all existing brokers:

    $ kn broker list

    Example output

    NAME      URL                                                                     AGE   CONDITIONS   READY   REASON
    default   http://broker-ingress.knative-eventing.svc.cluster.local/test/default   45s   5 OK / 5     True

  2. Optional: If you are using the OpenShift Container Platform web console, you can navigate to the Topology view in the Developer perspective, and observe that the broker exists:

    View the broker in the web console Topology view

10.1.1.2. Creating a broker by annotating a trigger

You can create a broker by adding the eventing.knative.dev/injection: enabled annotation to a Trigger object.

Important

If you create a broker by using the eventing.knative.dev/injection: enabled annotation, you cannot delete this broker without cluster administrator permissions. If you delete the broker without having a cluster administrator remove this annotation first, the broker is created again after deletion.

Prerequisites

  • The OpenShift Serverless Operator and Knative Eventing are installed on your OpenShift Container Platform cluster.

Procedure

  1. Create a Trigger object as a .yaml file that has the eventing.knative.dev/injection: enabled annotation:

    apiVersion: eventing.knative.dev/v1
    kind: Trigger
    metadata:
      annotations:
        eventing.knative.dev/injection: enabled
      name: <trigger-name>
    spec:
      broker: default
      subscriber: 1
        ref:
          apiVersion: serving.knative.dev/v1
          kind: Service
          name: <service-name>
    1
    Specify details about the event sink, or subscriber, that the trigger sends events to.
  2. Apply the .yaml file:

    $ oc apply -f <filename>

Verification

You can verify that the broker has been created successfully by using the oc CLI, or by observing it in the Topology view in the web console.

  1. Use the oc command to get the broker:

    $ oc -n <namespace> get broker default

    Example output

    NAME      READY     REASON    URL                                                                     AGE
    default   True                http://broker-ingress.knative-eventing.svc.cluster.local/test/default   3m56s

  2. Navigate to the Topology view in the web console, and observe that the broker exists:

    View the broker in the web console Topology view

10.1.1.3. Creating a broker by labeling a namespace

If you have cluster administrator permissions, you can create the default broker automatically by labeling a namespace.

Note

Brokers created using this method will not be removed if you remove the label. You must manually delete them.

Prerequisites

  • The OpenShift Serverless Operator and Knative Eventing are installed on your OpenShift Container Platform cluster.
  • You have cluster administrator permissions for OpenShift Container Platform.

Procedure

  • Label a namespace with eventing.knative.dev/injection=enabled:

    $ oc label namespace <namespace> eventing.knative.dev/injection=enabled

Verification

You can verify that the broker has been created successfully by using the oc CLI, or by observing it in the Topology view in the web console.

  1. Use the oc command to get the broker:

    $ oc -n <namespace> get broker <broker_name>

    Example command

    $ oc -n default get broker default

    Example output

    NAME      READY     REASON    URL                                                                     AGE
    default   True                http://broker-ingress.knative-eventing.svc.cluster.local/test/default   3m56s

  2. Navigate to the Topology view in the web console, and observe that the broker exists:

    View the broker in the web console Topology view

10.1.2. Managing brokers

The kn CLI provides commands that can be used to list, describe, update, and delete brokers. Cluster administrators can also permanently delete a broker that was created using injection.

10.1.2.1. Listing existing brokers using the Knative CLI

Prerequisites

  • The OpenShift Serverless Operator, Knative Serving and Knative Eventing are installed on your OpenShift Container Platform cluster.
  • You have installed the kn CLI.

Procedure

  • List all existing brokers:

    $ kn broker list

    Example output

    NAME      URL                                                                     AGE   CONDITIONS   READY   REASON
    default   http://broker-ingress.knative-eventing.svc.cluster.local/test/default   45s   5 OK / 5     True

10.1.2.2. Describing an existing broker using the Knative CLI

Prerequisites

  • The OpenShift Serverless Operator, Knative Serving and Knative Eventing are installed on your OpenShift Container Platform cluster.
  • You have installed the kn CLI.

Procedure

  • Describe an existing broker:

    $ kn broker describe <broker_name>

    Example command using default broker

    $ kn broker describe default

    Example output

    Name:         default
    Namespace:    default
    Annotations:  eventing.knative.dev/broker.class=MTChannelBasedBroker, eventing.knative.dev/creato ...
    Age:          22s
    
    Address:
      URL:    http://broker-ingress.knative-eventing.svc.cluster.local/default/default
    
    Conditions:
      OK TYPE                   AGE REASON
      ++ Ready                  22s
      ++ Addressable            22s
      ++ FilterReady            22s
      ++ IngressReady           22s
      ++ TriggerChannelReady    22s

10.1.2.3. Deleting a broker that was created by injection

Brokers created by injection, by using a namespace label or trigger annotation, are not deleted permanently if a developer removes the label or annotation. A user with cluster administrator permissions must manually delete these brokers.

Procedure

  1. Remove the eventing.knative.dev/injection=enabled label from the namespace:

    $ oc label namespace <namespace> eventing.knative.dev/injection-

    Removing the annotation prevents Knative from recreating the broker after you delete it.

  2. Delete the broker from the selected namespace:

    $ oc -n <namespace> delete broker <broker_name>

Verification

  • Use the oc command to get the broker:

    $ oc -n <namespace> get broker <broker_name>

    Example command

    $ oc -n default get broker default

    Example output

    No resources found.
    Error from server (NotFound): brokers.eventing.knative.dev "default" not found

10.1.3. Filtering events using triggers

Using triggers enables you to filter events from the broker for delivery to event sinks.

Prerequisites

Before you can use triggers, you will need:

  • Knative Eventing and kn installed.
  • An available broker, either the default broker or one that you have created.

    You can create the default broker either by following the instructions on Using brokers with Knative Eventing, or by using the --inject-broker flag while creating a trigger. Use of this flag is described later in this section.

  • An available event consumer, such as a Knative service.

10.1.3.1. Creating a trigger using the Developer perspective

After you have created a broker, you can create a trigger in the web console Developer perspective.

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 are in the Developer perspective.
  • 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.
  • You have created a broker and a Knative service or other event sink to connect to the trigger.

Procedure

  1. In the Developer perspective, navigate to the Topology page.
  2. Hover over the broker that you want to create a trigger for, and drag the arrow. The Add Trigger option is displayed.

    Create a trigger for the broker
  3. Click Add Trigger.
  4. Select your sink as a Subscriber from the drop-down list.
  5. Click Add.

Verification

  • After the subscription has been created, it is represented as a line that connects the broker to the service in the Topology view:

    Trigger in the Topology view

10.1.3.2. Deleting a trigger using the Developer perspective

You can delete triggers in the web console Developer perspective.

Prerequisites

  • To delete a trigger using the Developer perspective, ensure that you have logged in to the web console.

Procedure

  1. In the Developer perspective, navigate to the Topology page.
  2. Click on the trigger that you want to delete.
  3. In the Actions context menu, select Delete Trigger.

    Delete a trigger

10.1.3.3. Creating a trigger using kn

You can create a trigger by using the kn trigger create command.

Procedure

  • Create a trigger:

    $ kn trigger create <trigger_name> --broker <broker_name> --filter <key=value> --sink <sink_name>

    Alternatively, you can create a trigger and simultaneously create the default broker using broker injection:

    $ kn trigger create <trigger_name> --inject-broker --filter <key=value> --sink <sink_name>

    By default, triggers forward all events sent to a broker to sinks that are subscribed to that broker. Using the --filter attribute for triggers allows you to filter events from a broker, so that subscribers will only receive a subset of events based on your defined criteria.

10.1.3.4. Listing triggers using kn

The kn trigger list command prints a list of available triggers.

Procedure

  1. To print a list of available triggers, enter the following command:

    $ kn trigger list

    Example output:

    NAME    BROKER    SINK           AGE   CONDITIONS   READY   REASON
    email   default   ksvc:edisplay   4s    5 OK / 5     True
    ping    default   ksvc:edisplay   32s   5 OK / 5     True

  2. Optional: Print a list of triggers in JSON format:

    $ kn trigger list -o json
10.1.3.4.1. Describing a trigger using kn

You can use the kn trigger describe command to print information about a trigger.

Procedure

  • To print information about a trigger, enter the following command:

    $ kn trigger describe <trigger_name>

    Example output

    Name:         ping
    Namespace:    default
    Labels:       eventing.knative.dev/broker=default
    Annotations:  eventing.knative.dev/creator=kube:admin, eventing.knative.dev/lastModifier=kube:admin
    Age:          2m
    Broker:       default
    Filter:
      type:       dev.knative.event
    
    Sink:
      Name:       edisplay
      Namespace:  default
      Resource:   Service (serving.knative.dev/v1)
    
    Conditions:
      OK TYPE                  AGE REASON
      ++ Ready                  2m
      ++ BrokerReady            2m
      ++ DependencyReady        2m
      ++ Subscribed             2m
      ++ SubscriberResolved     2m

10.1.3.4.2. Filtering events using triggers

In the following trigger example, only events with attribute type: dev.knative.samples.helloworld will reach the event consumer.

$ kn trigger create <trigger_name> --broker <broker_name> --filter type=dev.knative.samples.helloworld --sink ksvc:<service_name>

You can also filter events using multiple attributes. The following example shows how to filter events using the type, source, and extension attributes.

$ kn trigger create <trigger_name> --broker <broker_name> --sink ksvc:<service_name> \
--filter type=dev.knative.samples.helloworld \
--filter source=dev.knative.samples/helloworldsource \
--filter myextension=my-extension-value
10.1.3.4.3. Updating a trigger using kn

You can use the kn trigger update command with certain flags to quickly update attributes of a trigger.

Procedure

  • Update a trigger:

    $ kn trigger update <trigger_name> --filter <key=value> --sink <sink_name> [flags]
    • You can update a trigger to filter exact event attributes that match incoming events. For example, using the type attribute:

      $ kn trigger update mytrigger --filter type=knative.dev.event
    • You can remove a filter attribute from a trigger. For example, you can remove the filter attribute with key type:

      $ kn trigger update mytrigger --filter type-
    • You can use the --sink parameter to change the event sink of a trigger:

      $ kn trigger update <trigger_name> --sink ksvc:my-event-sink
10.1.3.4.4. Deleting a trigger using kn

Procedure

  • Delete a trigger:

    $ kn trigger delete <trigger_name>

Verification

  1. List existing triggers:

    $ kn trigger list
  2. Verify that the trigger no longer exists:

    Example output

    No triggers found.

10.2. Event delivery workflows using channels

Events can be sent from a source to a sink by using channels and subscriptions for event delivery.

Channel workflow overview

Channels are custom resources that define a single event-forwarding and persistence layer.

After events have been sent to a channel, these events can be sent to multiple Knative services, or other sinks, by using a subscription.

The default configuration for channel instances is defined in the default-ch-webhook config map. Developers can create their own channels directly by instantiating a supported Channel object.

10.2.1. Supported channel types

Currently, OpenShift Serverless only supports InMemoryChannel kind channels for development use, as part of the Knative Eventing Technology Preview.

The following are limitations of InMemoryChannel channels:

  • No event persistence is available. If a pod goes down, events on that pod are lost.
  • InMemoryChannel channels do not implement event ordering, so two events that are received in the channel at the same time can be delivered to a subscriber in any order.
  • If a subscriber rejects an event, there are no re-delivery attempts. Instead, the rejected event is sent to a deadLetterSink object if this exists, or is dropped.

10.2.1.1. Using the default development channel configuration

When you install Knative Eventing, the following default-ch-webhook config map is created automatically in the knative-eventing namespace:

apiVersion: v1
kind: ConfigMap
metadata:
  name: default-ch-webhook
  namespace: knative-eventing
data:
  default-ch-config: |
    clusterDefault:
      apiVersion: messaging.knative.dev/v1
      kind: InMemoryChannel
    namespaceDefaults:
      some-namespace:
        apiVersion: messaging.knative.dev/v1
        kind: InMemoryChannel

This config map can specify either a cluster-wide default channel implementation, or a namespace-specific default channel implementation. Configuring a namespace-specific default overrides any cluster-wide settings.

After you create a Channel object, a mutating admission webhook adds a set of spec.channelTemplate properties for the Channel object based on the default channel implementation.

Example Channel object with spec.channelTemplate properties

apiVersion: messaging.knative.dev/v1
kind: Channel
metadata:
  name: example-channel
  namespace: default
spec:
  channelTemplate:
    apiVersion: messaging.knative.dev/v1
    kind: InMemoryChannel

The channel controller then creates the backing channel instance based on the spec.channelTemplate configuration.

Note

The spec.channelTemplate properties cannot be changed after creation, because they are set by the default channel mechanism rather than by the user.

When this mechanism is used, two objects are created: a generic channel, and an InMemoryChannel channel.

The generic channel acts as a proxy that copies its subscriptions to the InMemoryChannel channel, and sets its status to reflect the status of the backing InMemoryChannel channel.

Because the channel in this example is created in the default namespace, the channel uses the cluster default, which is InMemoryChannel.

10.2.2. Creating a development channel

Procedure

You can create a channel using the cluster default configuration by completing the following procedure.

  1. Create a Channel object.

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

      apiVersion: messaging.knative.dev/v1
      kind: Channel
      metadata:
        name: example-channel
        namespace: default
    2. Apply the YAML file by entering:

      $ oc apply -f <filename>

10.2.3. Creating a subscription

You can create a Subscription object to connect a channel to a sink. In the following procedure, the example sink is a Knative service named error-handler.

Procedure

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

    apiVersion: messaging.knative.dev/v1beta1
    kind: Subscription
    metadata:
      name: my-subscription 1
      namespace: default
    spec:
      channel: 2
        apiVersion: messaging.knative.dev/v1beta1
        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.
  2. Apply the YAML file:

    $ oc apply -f <FILENAME>