Dockerize Your Flask Application

3 min readApr 8, 2019

In this blog, we will write a Dockerfile to dockerize a simple python flask app.

Prerequisites:

Python 3.6
Flask
docker

What Is Docker?

Docker is a tool which helps to create, deploy, and run applications by using containers. Docker provides developers and operators with a friendly interface to build, ship, and run containers on any environment. To install docker follow the link https://www.docker.com/get-started

Containers are a group of processes run in isolation, these processes are running in a shared kernel.

Step 1|| Creating a flask app

from flask import Flask, render_template
app = Flask(__name__)@app.route('/')
def hello_world():
    return render_template('index.html')if __name__ == '__main__':
    app.run(debug=True,host='0.0.0.0')

Folder Structure:

src
  ____ app.py
  ____ templates
        ____ index.html
  ____ requirements.txt
  ____ Dockerfile

requirements.txt

Flask==0.12

templates/index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>dockerize flask app</title>
</head>
<body>
    <h1> Hello World! </h1>
</body>
</html>

Step 2 || Creating a Dockerfile

Dockerfile consists of a set of instructions to create docker image of the application.

1. FROM python:3.6.1-alpine
2. WORKDIR /project
3. ADD . /project
4. RUN pip install -r requirements.txt
5. CMD ["python","app.py"]

Note: Each of these lines of Dockerfile is a layer and each layer contains only the delta, or changes from the layers before it.

Line by line description:

1. FROM python:3.6.5-alpine

Note: FROM allows us to initialize the build over a base image. In our case, we are using a python:3.6.5-alpine image. (Alpine is a small Linux Distribution (~ 5MB).) It is heavily used by Docker apps because of its small size. In short, we are using a Linux environment with python 3.6.5 for our app.

2. WORKDIR /project

Note: We create a work directory called project where the "present working directory" will be set.

3. ADD . /project

Copy everything in the current directory (our server code and configurations) into the project directory.

4. RUN pip install -r requirements.txt

Note:The RUN command executes commands needed to set up your image for your application, such as installing packages, editing files, making directory or changing file permissions.

5. CMD ["python","app.py"]

CMD is the command that is executed when you start a container. Here, you are using CMD to run your Python applcation. There can be only one CMD per Dockerfile. If you specify more than one CMD, then the last CMD will take effect.

Note: Layers that change frequently, such as copying source code into the image, should be placed near the bottom of the file to take full advantage of the Docker layer cache.

Step 3 || Build the docker image

Run the following command to create the docker image from src directory, Pass in the -t parameter to name your image python-hello-world.

$ docker image build -t python-hello-world .

Verify that your image shows in your image list:

$ docker image ls

Step 4 || Run the docker container

$ docker run -p 5001:5000 -d python-hello-world

The -p flag maps a port running inside the container to your host. In this case, we're mapping the Python app running on port 5000 inside the container to port 5001 on your host.

Step 5 || Access The application from the host machine

Navigate to http://localhost:5001 in a browser to see the results.You should see "Hello World!" in your browser.

# Check the log output of the container

If you want to see logs from your application, you can use the docker container logs command. By default, docker container logs prints out what is sent to standard out by your application

$ docker container lsYou should see your container id by running this command.
$ docker container logs [container id]
output should be look like : * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)
172.17.0.1 - - [28/Jun/2017 19:35:33] "GET / HTTP/1.1" 200 -

# Stop a docker container

$ docker container stop [container id]Tip: You need to enter only enough digits of the ID to be unique. Three digits is typically adequate.

You can also use the names that you specified before to stop a container

# Remove the stopped containers

The following command removes any stopped containers, unused volumes and networks, and dangling images:

$ docker system prune

hooa your application is dockerized!!

Dockerize Your Flask Application

Written by Tasnuva Zaman

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