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The Event Registry maintains a catalog of the event types that can be consumed from the different Brokers. It introduces a new EventType CRD in order to persist the event type’s information in the cluster’s data store.
Before you begin
- Read about the Broker and Trigger objects.
- Be familiar with the CloudEvents spec, particularly the Context Attributes section.
- Be familiar with the Eventing sources.
Discovering events with the registry
Using the registry, you can discover the different types of events you can consume from the Brokers’ event meshes. The registry is designed for use with the Broker/Trigger model and aims to help you create Triggers.
To see the event types available to subscribe to, enter the following command:
kubectl get eventtypes -n <namespace>
Below, we show an example output of executing the above command using the
default namespace in a testing cluster. We will address the question of how
this registry was populated in a later section.
NAME TYPE SOURCE SCHEMA BROKER DESCRIPTION READY REASON dev.knative.source.github.push-34cnb dev.knative.source.github.push https://github.com/knative/eventing default True dev.knative.source.github.push-44svn dev.knative.source.github.push https://github.com/knative/serving default True dev.knative.source.github.pullrequest-86jhv dev.knative.source.github.pull_request https://github.com/knative/eventing default True dev.knative.source.github.pullrequest-97shf dev.knative.source.github.pull_request https://github.com/knative/serving default True dev.knative.kafka.event-cjvcr dev.knative.kafka.event /apis/v1/namespaces/default/kafkasources/kafka-sample#news default True dev.knative.kafka.event-tdt48 dev.knative.kafka.event /apis/v1/namespaces/default/kafkasources/kafka-sample#knative-demo default True google.pubsub.topic.publish-hrxhh google.pubsub.topic.publish //pubsub.googleapis.com/knative/topics/testing dev False BrokerIsNotReady
NOTE: This assumes that the Sources, emitting those events reference a
broker as their sink.
We can see that there are seven different EventTypes in the registry of the
default namespace. Let’s pick the first one and see what the EventType yaml
kubectl get eventtype dev.knative.source.github.push-34cnb -o yaml
Omitting irrelevant fields:
apiVersion: eventing.knative.dev/v1 kind: EventType metadata: name: dev.knative.source.github.push-34cnb namespace: default labels: eventing.knative.dev/sourceName: github-sample spec: type: dev.knative.source.github.push source: https://github.com/knative/eventing schema: description: broker: default status: conditions: - status: "True" type: BrokerExists - status: "True" type: BrokerReady - status: "True" type: Ready
From a consumer standpoint, the fields that matter the most are the
fields as well as the
name is advisory (i.e., non-authoritative), and we typically generate it
generateName) to avoid naming collisions (e.g., two EventTypes listening to
pull requests on two different Github repositories). As
generateName are needed for consumers to create Triggers, we defer their
discussion for later on.
status, its main purpose it to tell consumers (or cluster operators)
whether the EventType is ready for consumption or not. That readiness is based
on the Broker being ready. We can see from the example output that the PubSub
EventType is not ready, as its
dev Broker isn’t.
Let’s talk in more details about the
type: is authoritative. This refers to the CloudEvent type as it enters into the event mesh. It is mandatory. Event consumers can (and in most cases would) create Triggers filtering on this attribute.
source: refers to the CloudEvent source as it enters into the event mesh. It is mandatory. Event consumers can (and in most cases would) create Triggers filtering on this attribute.
schema: is a valid URI with the EventType schema. It may be a JSON schema, a protobuf schema, etc. It is optional.
description: is a string describing what the EventType is about. It is optional.
brokerrefers to the Broker that can provide the EventType. It is mandatory.
Subscribing to events
Now that you know what events can be consumed from the Brokers’ event meshes, you can create Triggers to subscribe to particular events.
Here are a few example Triggers that subscribe to events using exact matching on
source, based on the above registry output:
Subscribes to GitHub pushes from any source.
apiVersion: eventing.knative.dev/v1 kind: Trigger metadata: name: push-trigger namespace: default spec: broker: default filter: attributes: type: dev.knative.source.github.push subscriber: ref: apiVersion: serving.knative.dev/v1 kind: Service name: push-service
As per the registry output above, only two sources exist for that particular type of event (knative’s eventing and serving repositories). If later on new sources are registered for GitHub pushes, this trigger will be able to consume them.
Subscribes to GitHub pull requests from knative’s eventing repository.
apiVersion: eventing.knative.dev/v1 kind: Trigger metadata: name: gh-knative-eventing-pull-trigger namespace: default spec: broker: default filter: attributes: type: dev.knative.source.github.pull_request source: https://github.com/knative/eventing subscriber: ref: apiVersion: serving.knative.dev/v1 kind: Service name: gh-knative-eventing-pull-service
Subscribes to Kafka messages sent to the knative-demo topic
apiVersion: eventing.knative.dev/v1 kind: Trigger metadata: name: kafka-knative-demo-trigger namespace: default spec: broker: default filter: attributes: type: dev.knative.kafka.event source: /apis/v1/namespaces/default/kafkasources/kafka-sample#knative-demo subscriber: ref: apiVersion: serving.knative.dev/v1 kind: Service name: kafka-knative-demo-service
Subscribes to PubSub messages from GCP’s knative project sent to the testing topic
apiVersion: eventing.knative.dev/v1 kind: Trigger metadata: name: gcp-pubsub-knative-testing-trigger namespace: default spec: broker: dev filter: attributes: source: //pubsub.googleapis.com/knative/topics/testing subscriber: ref: apiVersion: serving.knative.dev/v1 kind: Service name: gcp-pubsub-knative-testing-service
Note that events won’t be able to be consumed by this Trigger’s subscriber until the Broker becomes ready.
Populating the registry
Now that we know how to discover events using the registry and how we can leverage that information to subscribe to events of interest, let’s move on to the next topic: How do we actually populate the registry in the first place?
In order to populate the registry, a cluster configurator can manually register the EventTypes. This means that the configurator can simply apply EventTypes yaml files, just as with any other Kubernetes resource:
kubectl apply -f <event_type.yaml>
As Manual Registration might be tedious and error-prone, we also support automatic registration of EventTypes. The creation of the EventTypes is done upon instantiation of an Event Source. We currently support automatic registration of EventTypes for the following Event Sources:
Let’s look at an example, in particular, the KafkaSource sample we used to populate the registry in our testing cluster. Below is what the yaml looks like.
apiVersion: sources.knative.dev/v1beta1 kind: KafkaSource metadata: name: kafka-sample namespace: default spec: bootstrapServers: - my-cluster-kafka-bootstrap.kafka:9092 topics: - knative-demo - news sink: apiVersion: eventing.knative.dev/v1 kind: Broker name: default
If you are interested in more information regarding configuration options of a KafkaSource, please refer to the KafKaSource sample.
For this discussion, the relevant information from the yaml above are the
topics. We observe that the
sinkis of kind
Broker. We currently only support automatic creation of EventTypes for Sources instances that point to Brokers. Regarding
topics, this is what we use to generate the EventTypes
sourcefield, which is equal to the CloudEvent source attribute.
kubectl applythis yaml, the KafkaSource
kafka-source-samplewill be instantiated, and two EventTypes will be added to the registry (as there are two topics). You can see that in the registry example output from the previous sections.
To get started, install Knative Eventing if you haven’t yet, and try experimenting with different Event Sources in your Knative cluster.
- Installing Knative in case you haven’t already done so.
- Getting started with eventing in case you haven’t read it.
- Knative code samples is a useful resource to better understand some of the Event Sources (remember to point them to a Broker if you want automatic registration of EventTypes in the registry).
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