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Broker and Trigger Delivery

In Complex Delivery with reply example, we see how an Event Source can fan out a message to multiple Services using Channels and Subscriptions. A Service can also reply to an event with another event for further delivery with other Channels and Subscriptions.

This model works but it's hard to maintain multiple Channels and Subscriptions. It also does not have the concept for filtering, so Services have to filter all messages themselves.

A simpler model to work with is Broker and Trigger. Broker combines channel, reply, and filter functionality into a single resource. Trigger provides declarative filtering of all events.

In the Broker and Trigger model, the Complex Delivery with reply example can be done as follows:

Broker and Trigger Delivery

In this example, only Service2 sends a reply but any Service can reply back. Replies are sent back into the Broker and filtered again.

Broker

Make sure there's a Broker in the default namespace by following instructions in Broker Creation page.

Source

Create a PingSource to target the Broker. Define source.yaml.

Create the source:

kubectl apply -f source.yaml

cronjobsource.sources.eventing.knative.dev/source created

Services

Create Knative services that will subscribe to the Broker.

Define service1.yaml and service3.yaml which are services that log out received messages.

Define another service2.yaml for the second service logs the received message and replies back with another CloudEvent with type dev.knative.samples.hifromknative. You can check out the source in event-display-with-reply folder.

Create services:

kubectl apply -f service1.yaml -f service2.yaml -f service3.yaml

service.serving.knative.dev/service1 created
service.serving.knative.dev/service2 created
service.serving.knative.dev/service3 created

Triggers

Connect services to the Broker with Triggers.

Define trigger1.yaml. This filters ping events and sends them to service1.

Define trigger2.yaml for service2. It is the same as trigger1.yaml except it points to service2.

Define trigger3.yaml. This filters dev.knative.samples.hifromknative events and sends them to service3.

Create triggers:

kubectl apply -f trigger1.yaml -f trigger2.yaml -f trigger3.yaml

trigger.eventing.knative.dev/trigger1 created
trigger.eventing.knative.dev/trigger2 created
trigger.eventing.knative.dev/trigger3 created

Under the covers, triggers creates the subscriptions:

kubectl get subscription

NAME                                                    READY
default-trigger1-8e05f7c3-702e-49b4-96f8-c46764aff45c   True
default-trigger2-6fdd4bf3-992e-4edd-ac57-466235c2a09c   True
default-trigger3-399562ed-65ec-4dba-bef2-0bb85d84081d   True

Verify

Check running pods:

kubectl get pods

NAME                                                              READY STATUS    RESTARTS   AGE
cronjobsource-source-8653ad6d-2499-4b95-91a7-98fdee8f841d-wh6rk   1/1     Running   0          16m
service1-5mf7x-deployment-6bcdb4df9-pnhc4                         2/2     Running   0          5m19s
service2-fd752-deployment-9d589f7b7-m7r8j                         2/2     Running   0          5m19s
service3-hh6c7-deployment-7455c845cd-2ktr4                        2/2     Running   0          65s

Check the logs of the services.

In service1, you should see messages from the CronJobSource:

kubectl logs service1-5mf7x-deployment-6bcdb4df9-pnhc4 -c user-container

info: event_display.Startup[0]
      Event Display received event: {"message":"Hello world from cron!"}

In service2, you should see messages from the CronJobSource and outgoing replies:

kubectl logs service2-fd752-deployment-9d589f7b7-m7r8j -c user-container

info: event_display_with_reply.Startup[0]
      Received CloudEvent
      Id: 2eac9a1d-6f14-4343-a391-8f0fa504258d
      Source: /apis/v1/namespaces/default/cronjobsources/source
      Type: dev.knative.cronjob.event
      Data: {"message":"Hello world from cron!"}
info: event_display_with_reply.Startup[0]
      Replying with CloudEvent
      Id: a38095ae-7816-46a7-8464-2943e26bf66a
      Source: urn:knative/eventing/samples/hello-world
      Type: dev.knative.samples.hifromknative
      Data: "This is a Knative reply!"

In service3, you should see the replied messages:

kubectl logs service3-hh6c7-deployment-7455c845cd-2ktr4 -c user-container

info: event_display.Startup[0]
      Event Display received event: "This is a Knative reply!"