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:

  1. # Clone the relevant branch version such as "release-0.13"
  2. git clone -b "{{< branch >}}" https://github.com/knative/docs knative-docs
  3. 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():

    1. app.logger.warning(request.data)
    2. # Respond with another event (optional)
    3. response = make_response({
    4.     "msg": "Hi from helloworld-python app!"
    5. })
    6. response.headers["Ce-Id"] = str(uuid.uuid4())
    7. response.headers["Ce-specversion"] = "0.3"
    8. response.headers["Ce-Source"] = "knative/eventing/samples/hello-world"
    9. response.headers["Ce-Type"] = "dev.knative.samples.hifromknative"
    10. return response

    if name == ‘main‘:

    1. app.run(debug=True, host='0.0.0.0', port=8080)
  1. ```

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

    1. Flask==1.1.1

  2. 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.

    1. FROM python:alpine3.7
    3. COPY . /app
    5. WORKDIR /app
    7. RUN pip install -r requirements.txt
    9. EXPOSE 8080
    11. ENTRYPOINT [ "python" ]
    13. CMD [ "helloworld.py" ]

  3. 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.

    1. # Namespace for sample application with eventing enabled
    2. apiVersion: v1
    3. kind: Namespace
    4. metadata:
    5.   name: knative-samples
    6.   labels:
    7.        eventing.knative.dev/injection: enabled
    8. ---
    9. # Helloworld-python app deploment
    10. apiVersion: apps/v1
    11. kind: Deployment
    12. metadata:
    13.   name: helloworld-python
    14.   namespace: knative-samples
    15. spec:
    16.   replicas: 1
    17.   selector:
    18.     matchLabels: &labels
    19.       app: helloworld-python
    20.   template:
    21.     metadata:
    22.       labels: *labels
    23.     spec:
    24.       containers:
    25.         - name: helloworld-python
    26.           image: docker.io/{username}/helloworld-python
    27.           imagePullPolicy: IfNotPresent
    28. ---
    29. # Service that exposes helloworld-python app.
    30. # This will be the subscriber for the Trigger
    31.   kind: Service
    32.   apiVersion: v1
    33.   metadata:
    34.     name: helloworld-python
    35.     namespace: knative-samples
    36.   spec:
    37.     selector:
    38.       app: helloworld-python
    39.     ports:
    40.     - protocol: TCP
    41.       port: 80
    42.       targetPort: 8080
    43. ---
    44. # Knative Eventing Trigger to trigger the helloworld-python service
    45. apiVersion: eventing.knative.dev/v1
    46. kind: Trigger
    47. metadata:
    48.   name: helloworld-python
    49.   namespace: knative-samples
    50. spec:
    51.   broker: default
    52.   filter:
    53.     attributes:
    54.       type: dev.knative.samples.helloworld
    55.       source: dev.knative.samples/helloworldsource
    56.   subscriber:
    57.     ref:
    58.       apiVersion: v1
    59.       kind: Service
    60.       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:

    1. # Build the container on your local machine
    2. docker build -t {username}/helloworld-python .
    4. # Push the container to docker registry
    5. 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:

    1. 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:

      1. 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.

      1. kubectl --namespace knative-samples get deployments helloworld-python
      2. 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

      1. 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

    1. kubectl --namespace knative-samples run curl --image=radial/busyboxplus:curl -it

  2. Run the following in the SSH terminal

    1. curl -v "default-broker.knative-samples.svc.cluster.local" \
    2. -X POST \
    3. -H "Ce-Id: 536808d3-88be-4077-9d7a-a3f162705f79" \
    4. -H "Ce-specversion: 0.3" \
    5. -H "Ce-Type: dev.knative.samples.helloworld" \
    6. -H "Ce-Source: dev.knative.samples/helloworldsource" \
    7. -H "Content-Type: application/json" \
    8. -d '{"msg":"Hello World from the curl pod."}'
    10. 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

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

    You should see something similar to:

    1. Event received. Context: Context Attributes,
    2.   specversion: 0.3
    3.   type: dev.knative.samples.helloworld
    4.   source: dev.knative.samples/helloworldsource
    5.   id: 536808d3-88be-4077-9d7a-a3f162705f79
    6.   time: 2019-10-04T22:35:26.05871736Z
    7.   datacontenttype: application/json
    8. Extensions,
    9.   knativearrivaltime: 2019-10-04T22:35:26Z
    10.   knativehistory: default-kn2-trigger-kn-channel.knative-samples.svc.cluster.local
    11.   traceparent: 00-971d4644229653483d38c46e92a959c7-92c66312e4bb39be-00
    13. Hello World Message "Hello World from the curl pod."
    14. Responded with event Validation: valid
    15. Context Attributes,
    16.   specversion: 0.2
    17.   type: dev.knative.samples.hifromknative
    18.   source: knative/eventing/samples/hello-world
    19.   id: 37458d77-01f5-411e-a243-a459bbf79682
    20. Data,
    21.   {"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. kubectl --namespace knative-samples apply --filename - << END
    2. # event-display app deploment
    3. apiVersion: apps/v1
    4. kind: Deployment
    5. metadata:
    6.   name: event-display
    7.   namespace: knative-samples
    8. spec:
    9.   replicas: 1
    10.   selector:
    11.     matchLabels: &labels
    12.       app: event-display
    13.   template:
    14.     metadata:
    15.       labels: *labels
    16.     spec:
    17.       containers:
    18.         - name: helloworld-python
    19.           image: gcr.io/knative-releases/knative.dev/eventing-contrib/cmd/event_display
    20. ---
    21. # Service that exposes event-display app.
    22. # This will be the subscriber for the Trigger
    23. kind: Service
    24. apiVersion: v1
    25. metadata:
    26.   name: event-display
    27.   namespace: knative-samples
    28. spec:
    29.   selector:
    30.     app: event-display
    31.   ports:
    32.     - protocol: TCP
    33.       port: 80
    34.       targetPort: 8080
    35. END

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

    1. kubectl --namespace knative-samples apply --filename - << END
    2. apiVersion: eventing.knative.dev/v1
    3. kind: Trigger
    4. metadata:
    5.   name: event-display
    6.   namespace: knative-samples
    7. spec:
    8.   broker: default
    9.   filter:
    10.     attributes:
    11.       type: dev.knative.samples.hifromknative
    12.       source: knative/eventing/samples/hello-world
    13.   subscriber:
    14.     ref:
    15.       apiVersion: v1
    16.       kind: Service
    17.       name: event-display
    18. END

  3. Send a CloudEvent to the Broker

  4. Check the logs of event-display service

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

    You should see something similar to:

    1.   cloudevents.Event
    2.   Validation: valid
    3.   Context Attributes,
    4.     specversion: 0.3
    5.     type: dev.knative.samples.hifromknative
    6.     source: knative/eventing/samples/hello-world
    7.     id: 8a7384b9-8bbe-4634-bf0f-ead07e450b2a
    8.     time: 2019-10-04T22:53:39.844943931Z
    9.     datacontenttype: application/json
    10.   Extensions,
    11.     knativearrivaltime: 2019-10-04T22:53:39Z
    12.     knativehistory: default-kn2-ingress-kn-channel.knative-samples.svc.cluster.local
    13.     traceparent: 00-4b01db030b9ea04bb150b77c8fa86509-2740816590a7604f-00
    14.   Data,
    15.     {
    16.       "msg": "Hi from helloworld- app!"
    17.     }

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:

  1. kubectl delete --filename sample-app.yaml