Hello World - Python

This guide describes the steps required to create the helloworld-python sample app and deploy it to your cluster.

The sample app reads a TARGET environment variable, and prints Hello ${TARGET}!. If TARGET is not specified, World is used as the default value.

You can also download a working copy of the sample, by running the following commands:

git clone -b "main" https://github.com/knative/docs knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-python

Prerequisites

  • A Kubernetes cluster with Knative installed and DNS configured. Follow the installation instructions.
  • Docker installed and running on your local machine, and a Docker Hub account configured.
  • (optional) The Knative CLI client kn can be used to simplify the deployment. Alternatively, you can use kubectl, and apply resource files directly.

Build

  1. Create a new directory and cd into it:

    mkdir app
cd app

  2. Create a file named app.py and copy the code block below into it:

    import os

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello_world():
   target = os.environ.get('TARGET', 'World')
   return 'Hello {}!\n'.format(target)

if __name__ == "__main__":
   app.run(debug=True,host='0.0.0.0',port=int(os.environ.get('PORT', 8080)))

  3. In your project directory, create a file named Dockerfile and copy the code block below into it. See official Python docker image for more details.

    # Use the official lightweight Python image.
# https://hub.docker.com/_/python
FROM python:3.7-slim

# Allow statements and log messages to immediately appear in the Knative logs
ENV PYTHONUNBUFFERED True

# Copy local code to the container image.
ENV APP_HOME /app
WORKDIR $APP_HOME
COPY . ./

# Install production dependencies.
RUN pip install Flask gunicorn

# Run the web service on container startup. Here we use the gunicorn
# webserver, with one worker process and 8 threads.
# For environments with multiple CPU cores, increase the number of workers
# to be equal to the cores available.
CMD exec gunicorn --bind :$PORT --workers 1 --threads 8 --timeout 0 app:app

  4. Create a .dockerignore file to ensure that any files related to a local build do not affect the container that you build for deployment.

    Dockerfile
README.md
*.pyc
*.pyo
*.pyd
__pycache__

NOTE: Use Docker to build the sample code into a container. To build and push to Docker Hub or container registry of your choice, run these commands replacing {username} with your Docker Hub username or the URL of the container registry.

  1. Use Docker to build the sample code into a container, then push the container to the Docker registry:

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

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

Deploying the app

  1. After the build has completed and the container is pushed to Docker Hub, you can deploy the app into your cluster.

    1. Create a new file, service.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username or with the URL provided by your container registry

      apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld-python
  namespace: default
spec:
  template:
    spec:
      containers:
        - image: docker.io/{username}/helloworld-python
          env:
            - name: TARGET
              value: "Python Sample v1"

    Ensure that the container image value in service.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

    kubectl apply --filename service.yaml

    With kn you can deploy the service with

    kn service create helloworld-python --image=docker.io/{username}/helloworld-python --env TARGET="Python Sample v1"

    This will wait until your service is deployed and ready, and ultimately it will print the URL through which you can access the service.

    During the creation of your service, Knative performs the following steps:

    • Creates a new immutable revision for this version of the app.
    • Network programming to create a route, ingress, service, and load balance for your app.
    • Automatically scales your pods up and down, including scaling down to zero active pods.

Verification

  1. Run one of the followings commands to find the domain URL for your service.

    Note: If your URL includes example.com then consult the setup instructions for configuring DNS (e.g. with xip.io), or using a Custom Domain.

    kubectl get ksvc helloworld-python  --output=custom-columns=NAME:.metadata.name,URL:.status.url

    Example:

    NAME                      URL
helloworld-python    http://helloworld-python.default.1.2.3.4.xip.io

    kn service describe helloworld-python -o url

    Example:

    http://helloworld-python.default.1.2.3.4.xip.io

  2. Now you can make a request to your app and see the result. Replace the URL below with the URL returned in the previous command.

    Example:

    curl http://helloworld-python.default.1.2.3.4.xip.io
Hello Python Sample v1!

# Even easier with kn:
curl $(kn service describe helloworld-python -o url)

    Note: Add -v option to get more detail if the curl command failed.

Removing

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

kubectl delete --filename service.yaml

kn service delete helloworld-python