Hello World - Python

A simple web app written in Python that you can use to test knative eventing. It shows how to consume a CloudEvent in Knative eventing, and optionally how to respond back with another CloudEvent in the http response, by adding the Cloud Eventing headers outlined in the Cloud Events standard definition.

We will deploy the app as a Kubernetes Deployment along with a Kubernetes Service. However, you can also deploy the app as a Knative Serving Service.

Follow the steps below to create the sample code and then deploy the app to your cluster. You can also download a working copy of the sample, by running the following commands:

# Clone the relevant branch version such as "release-0.13"
git clone -b "{{< branch >}}" https://github.com/knative/docs knative-docs
cd knative-docs/docs/eventing/samples/helloworld/helloworld-python

Before you begin

  • A Kubernetes cluster with Knative Eventing installed.
  • Docker installed and running on your local machine, and a Docker Hub account configured (we’ll use it for a container registry).

Recreating the sample code

  1. Create a new file named helloworld.py and paste the following code. This code creates a basic web server which listens on port 8080:

    from flask import Flask, request, make_response
import uuid

app = Flask(__name__)

@app.route('/', methods=['POST'])
def hello_world():
    app.logger.warning(request.data)
    # Respond with another event (optional)
    response = make_response({
        "msg": "Hi from helloworld-python app!"
    })
    response.headers["Ce-Id"] = str(uuid.uuid4())
    response.headers["Ce-specversion"] = "0.3"
    response.headers["Ce-Source"] = "knative/eventing/samples/hello-world"
    response.headers["Ce-Type"] = "dev.knative.samples.hifromknative"
    return response

if __name__ == '__main__':
    app.run(debug=True, host='0.0.0.0', port=8080)

  2. Add a requirements.txt file containing the following contents:

    Flask==1.1.1

  3. In your project directory, create a file named Dockerfile and copy the code block below into it. For detailed instructions on dockerizing a Go app, see Deploying Go servers with Docker.

    FROM python:alpine3.7

COPY . /app

WORKDIR /app

RUN pip install -r requirements.txt

EXPOSE 8080

ENTRYPOINT [ "python" ]

CMD [ "helloworld.py" ]

  4. Create a new file, sample-app.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username.

    # Namespace for sample application with eventing enabled
apiVersion: v1
kind: Namespace
metadata:
  name: knative-samples
  labels:
       eventing.knative.dev/injection: enabled
---
# Helloworld-python app deploment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: helloworld-python
  namespace: knative-samples
spec:
  replicas: 1
  selector:
    matchLabels: &labels
      app: helloworld-python
  template:
    metadata:
      labels: *labels
    spec:
      containers:
        - name: helloworld-python
          image: docker.io/{username}/helloworld-python
          imagePullPolicy: IfNotPresent
---
# Service that exposes helloworld-python app.
# This will be the subscriber for the Trigger
  kind: Service
  apiVersion: v1
  metadata:
    name: helloworld-python
    namespace: knative-samples
  spec:
    selector:
      app: helloworld-python
    ports:
    - protocol: TCP
      port: 80
      targetPort: 8080
---
# Knative Eventing Trigger to trigger the helloworld-python service
apiVersion: eventing.knative.dev/v1
kind: Trigger
metadata:
  name: helloworld-python
  namespace: knative-samples
spec:
  broker: default
  filter:
    attributes:
      type: dev.knative.samples.helloworld
      source: dev.knative.samples/helloworldsource
  subscriber:
    ref:
      apiVersion: v1
      kind: Service
      name: helloworld-python

Building and deploying the sample

Once you have recreated the sample code files (or used the files in the sample folder) you’re ready to build and deploy the sample app.

  1. Use Docker to build the sample code into a container. To build and push with Docker Hub, run these commands replacing {username} with your Docker Hub username:

    # Build the container on your local machine
docker build -t {username}/helloworld-python .

# Push the container to docker registry
docker push {username}/helloworld-python

  2. After the build has completed and the container is pushed to Docker Hub, you can deploy the sample application into your cluster. Ensure that the container image value in sample-app.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

    kubectl apply --filename sample-app.yaml
    1. Above command created a namespace knative-samples and labelled it with knative-eventing-injection=enabled, to enable eventing in the namespace. Verify using the following command: 
      kubectl get ns knative-samples --show-labels
    2. It deployed the helloworld-python app as a K8s Deployment and created a K8s service names helloworld-python. Verify using the following command. 
      kubectl --namespace knative-samples get deployments helloworld-python
kubectl --namespace knative-samples get svc helloworld-python
    3. It created a Knative Eventing Trigger to route certain events to the helloworld-python application. Make sure that Ready=true 
      kubectl --namespace knative-samples get trigger helloworld-python

Send and verify CloudEvents

After you have deployed the application, and have verified that the namespace, sample application and trigger are ready, you can send a CloudEvent.

Send CloudEvent to the Broker

You can send an HTTP request directly to the Knative broker if the correct CloudEvent headers are set.

  1. Deploy a curl pod and SSH into it 
    kubectl --namespace knative-samples run curl --image=radial/busyboxplus:curl -it
  2. Run the following in the SSH terminal 
    curl -v "default-broker.knative-samples.svc.cluster.local" \
-X POST \
-H "Ce-Id: 536808d3-88be-4077-9d7a-a3f162705f79" \
-H "Ce-specversion: 0.3" \
-H "Ce-Type: dev.knative.samples.helloworld" \
-H "Ce-Source: dev.knative.samples/helloworldsource" \
-H "Content-Type: application/json" \
-d '{"msg":"Hello World from the curl pod."}'

exit

Verify that event is received by helloworld-python app

Helloworld-python app logs the context and the msg of the above event, and replies back with another event.

  1. Display helloworld-python app logs 
    kubectl --namespace knative-samples logs -l app=helloworld-python --tail=50

    You should see something similar to:

    Event received. Context: Context Attributes,
  specversion: 0.3
  type: dev.knative.samples.helloworld
  source: dev.knative.samples/helloworldsource
  id: 536808d3-88be-4077-9d7a-a3f162705f79
  time: 2019-10-04T22:35:26.05871736Z
  datacontenttype: application/json
Extensions,
  knativearrivaltime: 2019-10-04T22:35:26Z
  knativehistory: default-kn2-trigger-kn-channel.knative-samples.svc.cluster.local
  traceparent: 00-971d4644229653483d38c46e92a959c7-92c66312e4bb39be-00

Hello World Message "Hello World from the curl pod."
Responded with event Validation: valid
Context Attributes,
  specversion: 0.2
  type: dev.knative.samples.hifromknative
  source: knative/eventing/samples/hello-world
  id: 37458d77-01f5-411e-a243-a459bbf79682
Data,
  {"msg":"Hi from Knative!"}

Try the CloudEvent attributes in the curl command and the trigger specification to understand how triggers work.

Verify reply from helloworld-python app

The helloworld-python app replies with an event type type= dev.knative.samples.hifromknative, and source source=knative/eventing/samples/hello-world. The event enters the eventing mesh through the broker, and can be delivered to event sinks using a trigger

  1. Deploy a pod that receives any CloudEvent and logs the event to its output.

    kubectl --namespace knative-samples apply --filename - << END
# event-display app deploment
apiVersion: apps/v1
kind: Deployment
metadata:
  name: event-display
  namespace: knative-samples
spec:
  replicas: 1
  selector:
    matchLabels: &labels
      app: event-display
  template:
    metadata:
      labels: *labels
    spec:
      containers:
        - name: helloworld-python
          image: gcr.io/knative-releases/knative.dev/eventing-contrib/cmd/event_display
---
# Service that exposes event-display app.
# This will be the subscriber for the Trigger
kind: Service
apiVersion: v1
metadata:
  name: event-display
  namespace: knative-samples
spec:
  selector:
    app: event-display
  ports:
    - protocol: TCP
      port: 80
      targetPort: 8080
END

  2. Create a trigger to deliver the event to the above service

    kubectl --namespace knative-samples apply --filename - << END
apiVersion: eventing.knative.dev/v1
kind: Trigger
metadata:
  name: event-display
  namespace: knative-samples
spec:
  broker: default
  filter:
    attributes:
      type: dev.knative.samples.hifromknative
      source: knative/eventing/samples/hello-world
  subscriber:
    ref:
      apiVersion: v1
      kind: Service
      name: event-display
END

  3. Send a CloudEvent to the Broker

  4. Check the logs of event-display service

    kubectl --namespace knative-samples logs -l app=event-display --tail=50

    You should see something similar to:

      cloudevents.Event
  Validation: valid
  Context Attributes,
    specversion: 0.3
    type: dev.knative.samples.hifromknative
    source: knative/eventing/samples/hello-world
    id: 8a7384b9-8bbe-4634-bf0f-ead07e450b2a
    time: 2019-10-04T22:53:39.844943931Z
    datacontenttype: application/json
  Extensions,
    knativearrivaltime: 2019-10-04T22:53:39Z
    knativehistory: default-kn2-ingress-kn-channel.knative-samples.svc.cluster.local
    traceparent: 00-4b01db030b9ea04bb150b77c8fa86509-2740816590a7604f-00
  Data,
    {
      "msg": "Hi from helloworld- app!"
    }

Note: You could use the above approach to test your applications too.

Removing the sample app deployment

To remove the sample app from your cluster, delete the service record:

kubectl delete --filename sample-app.yaml