Containerizing an application

In this section you’ll take some source code, verify it runs locally, and then create a Docker image of the application. The sample application used is a very simple Flask web application; if you want to test it locally, you’ll need Python installed. Otherwise, you can skip to the "Create a Dockerfile" section.

Get the application code

Use git to clone the repository to your local machine:

git clone https://github.com/JasonHaley/hello-python.git

Change to the app directory:

cd hello-python/app

from flask import Flask
app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello from Python!"

if __name__ == "__main__":
    app.run(host='0.0.0.0')

Run locally

Manually run the installer and application using the following commands:

pip install -r requirements.txt
python main.py

This will start a development web server hosting your application, which you will be able to see by navigating to http://localhost:5000. Because port 5000 is the default port for the development server, we didn’t need to specify it.

Create a Dockerfile

Now that you have verified the source code works, the first step in containerizing the application is to create a Dockerfile.

In the hello-python/app directory, create a file named Dockerfile with the following contents and save it:

FROM python:3.7

RUN mkdir /app
WORKDIR /app
ADD . /app/
RUN pip install -r requirements.txt

EXPOSE 5000
CMD ["python", "/app/main.py"]

This file is a set of instructions Docker will use to build the image. For this simple application, Docker is going to:

Create an image

At your command line or shell, in the hello-python/app directory, build the image with the following command:

docker build -f Dockerfile -t hello-python:latest .

This will perform those seven steps listed above and create the image. To verify the image was created, run the following command:

docker image ls

The application is now containerized, which means it can now run in Docker and Kubernetes!

Running in Docker

Before jumping into Kubernetes, let’s verify it works in Docker. Run the following command to have Docker run the application in a container and map it to port 5001:

docker run -p 5001:5000 hello-python

Now navigate to http://localhost:5001, and you should see the “Hello from Python!” message.

More info

Running in Kubernetes

You are finally ready to get the application running in Kubernetes. Because you have a web application, you will create a service and a deployment.

First verify your kubectl is configured. At the command line, type the following:

kubectl version

kubectl config use-context docker-for-desktop

Now you are working with Kubernetes! You can see the node by typing:

kubectl get nodes

Now let’s have it run the application. Create a file named deployment.yaml and add the following contents to it and then save it:

apiVersion: v1
kind: Service
metadata:
  name: hello-python-service
spec:
  selector:
    app: hello-python
  ports:
  - protocol: "TCP"
    port: 6000
    targetPort: 5000
  type: LoadBalancer

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: hello-python
spec:
  selector:
    matchLabels:
      app: hello-python
  replicas: 4
  template:
    metadata:
      labels:
        app: hello-python
    spec:
      containers:
      - name: hello-python
        image: hello-python:latest
        imagePullPolicy: Never
        ports:
        - containerPort: 5000

This YAML file is the instructions to Kubernetes for what you want running. It is telling Kubernetes the following:

• You want a load-balanced service exposing port 6000
• You want four instances of the hello-python container running

Use kubectl to send the YAML file to Kubernetes by running the following command:

kubectl apply -f deployment.yaml

You can see the pods are running if you execute the following command:

kubectl get pods

Now navigate to http://localhost:6000, and you should see the “Hello from Python!” message.

That’s it! The application is now running in Kubernetes!

More Info

Summary

In this walk-through, we containerized an application, and got it running in Docker and in Kubernetes. This simple application only scratches the surface of what’s possible (and what you’ll need to learn).

Next steps

If you are just getting started and this walk-through was useful to you, then the following resources should be good next steps for you to further expand your Kubernetes knowledge:

How to enable Kubernetes in Docker Desktop

Once you have Docker Desktop installed, open the Settings:

Select the Kubernetes menu item on the left and verify that the Enable Kubernetes is checked. If it isn’t, check it and click the Apply button at the bottom right: