Knative Quickstart CloudEvents Example

Simply put, the official Knative quickstart tutorial has quite a few gaps explaining things and it took me a while to piece things together. How can your app reply events to the broker?

This example deploys two services (cloudevents-player and our custom app kn-app) with two triggers. If you send an CloudEvent with type my-event from cloudevents-player, kn-app will reply an CloudEvent with type app-event which will in turn be sent back to cloudevents-player.

The app is written in Golang using CloudEvents SDK, modified from the second official serving example. Both incoming and outputing events has a JSON payload with a single message field:

{
  "message": "something"
}

Run This Example

Create Knative Quickstart Environment

Install Docker and download binaries of kn, kn-quickstart and kind.

Start the demo environment:

./kn quickstart kind

Generate and Upload kn-app Image

docker login
docker build . -t <your docker hub name>/kn-app -f Dockerfile
docker push <your docker hub name>/kn-app

Then modify the image source in kn-app.yaml so that it can find your image:

containers:
- image: <your docker hub name>/kn-app:latest

Deploy Resources in Knative

kubectl apply -f kn-app.yaml

Wait for Services Ready

./kn service list

Send and Receive Events

Open the URL of CloudEvents player (for example, http://cloudevents-player.default.127.0.0.1.sslip.io) and send an event with type my-event. The message is a JSON object with one field message. You should see a second event with type app-event appear shortly.

The event with ID 42 is the one replied by the app. You can modify the eveent ID, source and message in the code.

K_SINK vs. Direct Reply

In kn-app.yaml the my-app has a environment variable K_SINK, which will tell the code where is the event target. Here we set it with the URL of example-broker, which is the built-in broker in the Knative quickstart environment:

http://broker-ingress.knative-eventing.svc.cluster.local/default/example-broker

If K_SINK is not set (target is empty) and you send an direct CloudEvent-compatible HTTP request to the app, the code will reply an event directly as a HTTP response. This also works if you run the app as a local Docker container.