Containerizing Flask

As we have discussed previously, Docker containers are critical to packaging a (e.g. Flask) application along with all of its dependencies, isolating it from other applications and services, and deploying it consistently and reproducibly and platform-agnostically.

Here, we will walk through the process of containerizing Flask with Docker, then curling it as a containerized microservice. After going through this module, students should be able to:

• Assemble the different components needed for a containerized microservice into on directory

• Establish and document requirements (e.g. dependencies, Python packages) for the project

• Build and run in the background a containerized Flask microservice

• Map ports on the ISP server to ports inside a container, and curl the correct ports to return a response from the microservice

Organize Your App Directory

First, create a “web” directory, and change directories to it:

[isp02]$ mkdir web
[isp02]$ cd web

Then, create a new app.py (or copy an existing one) into this folder. It should have the following contents:

from flask import Flask

app = Flask(__name__)

@app.route('/', methods = ['GET'])
def hello_world():
    return 'Hello, world!\n'

@app.route('/<name>', methods = ['GET'])
def hello_name(name):
    return 'Hello, {}!\n'.format(name)

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

Establish Requirements

Python uses a specific file called requirements.txt to capture the required libraries and packages for a project. For our example here, create a file called requirements.txt and add the following line:

Flask==1.1.2

Build a Docker Image

As we saw in a previous section, we write up the recipe for our application install process in a Dockerfile. Create a file called Dockerfile for our Flask microservice and add the following lines:

FROM ubuntu:latest
RUN apt-get update -y
RUN apt-get install -y python-pip python-dev build-essential
COPY . /app
WORKDIR /app
RUN pip install -r requirements.txt
ENTRYPOINT ["python"]
CMD ["app.py"]

Here we see usage of the Docker ENTRYPOINT and RUN instructions, which essentially specify a default command (python app.py) that should be run when an instance of this image is instantiated.

Save the file and build the image with the following command:

[isp02]$ docker build -t <username>/flask-helloworld:latest .

Run a Docker Container

To create an instance of your image (a “container”), use the following command:

[isp02]$ docker run --name "give-your-container-a-name" -d -p <your port number>:5000 <username>/flask-helloworld:latest"

For example:

[isp02]$ docker run --name "charlies-helloworld-flask-app" -d -p 5050:5000 charlie/flask-helloworld:latest"

The -d flag detaches your terminal from the running container - i.e. it runs the container in the background. The -p flag maps a port on the ISP server (5050, in the above case) to a port inside the container (5000, in the above case). In the above example, the Flask app was set up to use the default port inside the container (5000), and we can access that through our specified port on ISP (5050).

Check to see that things are up and running with:

[isp02]$ docker ps -a

The list should have a container with the name you gave it, an UP status, and the port mapping that you specified.

If the above is not found in the list of running containers, try to debug with the following:

[isp02]$ docker logs "your-container-name"
-or-
[isp02]$ docker logs "your-container-number"

Access Your Microservice

Now for the payoff - you can curl your REST API / Flask microservice by hitting the correct port on the ISP server. Following the example above, which was using port 5050:

[isp02]$ curl localhost:5050/
Hello, world!
[isp02]$ curl localhost:5050/Charlie
Hello, Charlie!

Clean Up

Finally, don’t forget to stop your running container and remove it.

[isp02]$ docker ps -a | grep charlie
60be6788d73d   charlie/flask-helloworld:latest     "python app.py"   4 minutes ago   Up 4 minutes   0.0.0.0:5050->5000/tcp   charlies-helloworld-flask-app
[isp02]$ docker stop 60be6788d73d
60be6788d73d
[isp02]$ docker rm 60be6788d73d
60be6788d73d

EXERCISE

Note

This exercise will be reflected in Homework 03, part C.

Containerize your Dr. Moreau apps! Create a route that creates one random animal. Post a link to your route to Slack. Have another classmate hit your route, and build an animal.