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Chapter 5. Triggers

5.1. Triggers overview

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

Broker event delivery overview

If you are using a Knative broker for Apache Kafka, you can configure the delivery order of events from triggers to event sinks. See Configuring event delivery ordering for triggers.

5.1.1. Configuring event delivery ordering for triggers

If you are using a Kafka broker, you can configure the delivery order of events from triggers to event sinks.

Prerequisites

  • The OpenShift Serverless Operator, Knative Eventing, and Knative Kafka are installed on your OpenShift Container Platform cluster.
  • Kafka broker is enabled for use on your cluster, and you have created a Kafka broker.
  • 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 installed the OpenShift (oc) CLI.

Procedure

  1. Create or modify a Trigger object and set the kafka.eventing.knative.dev/delivery.order annotation: 

    apiVersion: eventing.knative.dev/v1
kind: Trigger
metadata:
  name: <trigger_name>
  annotations:
     kafka.eventing.knative.dev/delivery.order: ordered
...

    The supported consumer delivery guarantees are:

    unordered
    An unordered consumer is a non-blocking consumer that delivers messages unordered, while preserving proper offset management.
    ordered

    An ordered consumer is a per-partition blocking consumer that waits for a successful response from the CloudEvent subscriber before it delivers the next message of the partition.

    The default ordering guarantee is unordered.

  2. Apply the Trigger object: 

    $ oc apply -f <filename>

5.1.2. Next steps

5.2. Creating triggers

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

Broker event delivery overview

5.2.1. Creating a trigger by using the Administrator perspective

Using the OpenShift Container Platform web console provides a streamlined and intuitive user interface to create a trigger. After Knative Eventing is installed on your cluster and you have created a broker, you can create a trigger by using the web console.

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 broker.
  • 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 Broker tab, select the Options menu kebab for the broker that you want to add a trigger to.
  3. Click Add Trigger in the list.
  4. In the Add Trigger dialogue box, select a Subscriber for the trigger. The subscriber is the Knative service that will receive events from the broker.
  5. Click Add.

5.2.2. Creating a trigger by using the Developer perspective

Using the OpenShift Container Platform web console provides a streamlined and intuitive user interface to create a trigger. After Knative Eventing is installed on your cluster and you have created a broker, you can create a trigger by using the web console.

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 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.
  3. Click Add Trigger.
  4. Select your sink in the Subscriber list.
  5. Click Add.

Verification

  • After the subscription has been created, you can view it in the Topology page, where it is represented as a line that connects the broker to the event sink.

Deleting a trigger

  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.

5.2.3. Creating a trigger by using the Knative CLI

You can use the kn trigger create command to create a trigger.

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 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.

5.3. List triggers from the command line

Using the Knative (kn) CLI to list triggers provides a streamlined and intuitive user interface.

5.3.1. Listing triggers by using the Knative CLI

You can use the kn trigger list command to list existing triggers in your cluster.

Prerequisites

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

Procedure

  1. Print a list of available triggers: 

    $ 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

5.4. Describe triggers from the command line

Using the Knative (kn) CLI to describe triggers provides a streamlined and intuitive user interface.

5.4.1. Describing a trigger by using the Knative CLI

You can use the kn trigger describe command to print information about existing triggers in your cluster by using the Knative CLI.

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 trigger.

Procedure

  • Enter the 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

5.5. Connecting a trigger to a sink

You can connect a trigger to a sink, so that events from a broker are filtered before they are sent to the sink. A sink that is connected to a trigger is configured as a subscriber in the Trigger object’s resource spec.

Example of a Trigger object connected to an Apache Kafka sink

apiVersion: eventing.knative.dev/v1
kind: Trigger
metadata:
  name: <trigger_name> 1
spec:
...
  subscriber:
    ref:
      apiVersion: eventing.knative.dev/v1alpha1
      kind: KafkaSink
      name: <kafka_sink_name> 2

1
The name of the trigger being connected to the sink.
2
The name of a KafkaSink object.

5.6. Filtering triggers from the command line

Using the Knative (kn) CLI to filter events by using triggers provides a streamlined and intuitive user interface. You can use the kn trigger create command, along with the appropriate flags, to filter events by using triggers.

5.6.1. Filtering events with triggers by using the Knative CLI

In the following trigger example, only events with the attribute type: dev.knative.samples.helloworld are sent to the event sink: 

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

You can also filter events by 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

5.7. Updating triggers from the command line

Using the Knative (kn) CLI to update triggers provides a streamlined and intuitive user interface.

5.7.1. Updating a trigger by using the Knative CLI

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

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

  • 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 <trigger_name> --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 <trigger_name> --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

5.8. Deleting triggers from the command line

Using the Knative (kn) CLI to delete a trigger provides a streamlined and intuitive user interface.

5.8.1. Deleting a trigger by using the Knative CLI

You can use the kn trigger delete command to delete a trigger.

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

  • 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.

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