On this page you build a simple Python web application running on Docker Compose. The application uses the Flask framework and increments a value in Redis. While the sample uses Python, the concepts demonstrated here should be understandable even if you're not familiar with it.
Make sure you have already installed both Docker Engine and Docker Compose. You don't need to install Python, it is provided by a Docker image.
Create a directory for the project:
$ mkdir composetest
$ cd composetest
With your favorite text editor create a file called app.py
in your project
directory.
from flask import Flask
from redis import Redis
app = Flask(__name__)
redis = Redis(host='redis', port=6379)
@app.route('/')
def hello():
redis.incr('hits')
return 'Hello World! I have been seen %s times.' % redis.get('hits')
if __name__ == "__main__":
app.run(host="0.0.0.0", debug=True)
Create another file called requirements.txt
in your project directory and
add the following:
flask
redis
These define the applications dependencies.
In this step, you build a new Docker image. The image contains all the dependencies the Python application requires, including Python itself.
In your project directory create a file named Dockerfile
and add the
following:
FROM python:2.7
ADD . /code
WORKDIR /code
RUN pip install -r requirements.txt
CMD python app.py
This tells Docker to:
.
into the path /code
in the image./code
.python app.py
For more information on how to write Dockerfiles, see the Docker user guide and the Dockerfile reference.
Build the image.
$ docker build -t web .
This command builds an image named web
from the contents of the current
directory. The command automatically locates the Dockerfile
, app.py
, and
requirements.txt
files.
Define a set of services using docker-compose.yml
:
Create a file called docker-compose.yml in your project directory and add the following:
version: '2'
services:
web:
build: .
ports:
- "5000:5000"
volumes:
- .:/code
depends_on:
- redis
redis:
image: redis
This Compose file defines two services, web
and redis
. The web service:
Dockerfile
in the current directory./code
inside the container allowing you to modify the code without having to rebuild the image.The redis
service uses the latest public Redis image pulled from the Docker Hub registry.
From your project directory, start up your application.
$ docker-compose up
Pulling image redis...
Building web...
Starting composetest_redis_1...
Starting composetest_web_1...
redis_1 | [8] 02 Jan 18:43:35.576 # Server started, Redis version 2.8.3
web_1 | * Running on http://0.0.0.0:5000/
web_1 | * Restarting with stat
Compose pulls a Redis image, builds an image for your code, and start the services you defined.
http://0.0.0.0:5000/
in a browser to see the application running.If you're using Docker on Linux natively, then the web app should now be
listening on port 5000 on your Docker daemon host. If http://0.0.0.0:5000
doesn't resolve, you can also try http://localhost:5000
.
If you're using Docker Machine on a Mac, use docker-machine ip MACHINE_VM
to get
the IP address of your Docker host. Then, open http://MACHINE_VM_IP:5000
in a
browser.
You should see a message in your browser saying:
Hello World! I have been seen 1 times.
The number should increment.
If you want to run your services in the background, you can pass the -d
flag
(for "detached" mode) to docker-compose up
and use docker-compose ps
to
see what is currently running:
$ docker-compose up -d
Starting composetest_redis_1...
Starting composetest_web_1...
$ docker-compose ps
Name Command State Ports
-------------------------------------------------------------------
composetest_redis_1 /usr/local/bin/run Up
composetest_web_1 /bin/sh -c python app.py Up 5000->5000/tcp
The docker-compose run
command allows you to run one-off commands for your
services. For example, to see what environment variables are available to the
web
service:
$ docker-compose run web env
See docker-compose --help
to see other available commands. You can also install command completion for the bash and zsh shell, which will also show you available commands.
If you started Compose with docker-compose up -d
, you'll probably want to stop
your services once you've finished with them:
$ docker-compose stop
At this point, you have seen the basics of how Compose works.