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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 "mkdocs" 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
    ---
    # A default broker
    apiVersion: eventing.knative.dev/v1
    kind: Broker
    metadata:
      name: default
      namespace: knative-samples
      annotations:
        # Note: you can set the eventing.knative.dev/broker.class annotation to change the class of the broker.
        # The default broker class is MTChannelBasedBroker, but Knative also supports use of the other class.
        eventing.knative.dev/broker.class: MTChannelBasedBroker
    spec: {}
    ---
    # Helloworld-python app deployment
    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
    apiVersion: v1
    kind: Service
    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 -f sample-app.yaml
    
  3. The previous command creates a namespace knative-samples and labels it with knative-eventing-injection=enabled, to enable eventing in the namespace. Verify using the following command:

    kubectl get ns knative-samples --show-labels
    
  4. 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
    
  5. It created a Knative Eventing Trigger to route certain events to the helloworld-python application. Make sure that Ready=true:

    kubectl -n 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 command in the SSH terminal:

    curl -v "broker-ingress.knative-eventing.svc.cluster.local/knative-samples/default" \
    -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.

    1. Copy the YAML below into a file:

      # 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/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
      
    2. Apply the YAML file by running the command:

      kubectl apply -f <filename>.yaml
      
      Where <filename> is the name of the file you created in the previous step.

  2. Create a trigger to deliver the event to the previously created service.

    1. Copy the YAML below into a file:

      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
      
    2. Apply the YAML file by running the command:

      kubectl apply -f <filename>.yaml
      
      Where <filename> is the name of the file you created in the previous step.

  3. Send a CloudEvent to the Broker

  4. Check the logs of event-display Service:

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

    Example output:

        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!"
          }
    

Removing the sample app deployment

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

kubectl delete -f sample-app.yaml
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