multi-container-applications.md

Multi-container Applications with Compose

docker-compose is a tool used for defining and running multi-container applications. To use compose we have to:

• create Dockerfile with description for each service
• create docker-compose.yml file with description of how different services come together
• run docker-compose up to start the whole application

We're going to modify our flask app so that it keeps track of all visits to the website. We will store number of visits in redis store.

Redis is a data structure server. It is open-source, networked, in-memory, and stores keys with optional durability.

Let us create count-hello.py file:

#!/usr/bin/env python

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)

We will define python dependencies in requirements.txt

flask
redis

Description of flask image goes into new Dockerfile

FROM python:2.7
ADD . /app
WORKDIR /app
RUN pip install -r requirements.txt
CMD ["python", "count-hello.py"]

This time our starting image is ptyhon:2.7 intstead of ubuntu, this way we avoid installing python and can move immediately to installing python modules.

We're now going to build the image like we did before

$ docker build -t web .
$ docker images
REPOSITORY              TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
web                     latest              1b11b4e28507        6 seconds ago       682.7 MB

We will use docker-compose to define our complex application. The instructions should be stored in docker-compose.yml file:

web:
  build: .
  ports:
   - "5000:5000"
  volumes:
   - .:/code
  links:
   - redis
redis:
  image: redis

It defines web container which:

• is built from Dockerfile in same directory
• binds port 5000 in the container to port 5000 on the host
• mounts cwd (or $PWD) as /code inside the container
• links to other container called redis

It also defines redis container that will be buld from redis official image in dockerhub.

To start compose application run docker-compose up. To see report of running compose defined contaiers run docker-compose ps from the directory with docker-compose.yml file.

$ docker-compose up
$ docker-compose ps
        Name                      Command               State           Ports
--------------------------------------------------------------------------------------
composeflask_redis_1   /entrypoint.sh redis-server      Up      6379/tcp
composeflask_web_1     /bin/sh -c python count-hel ...   Up      0.0.0.0:5000->5000/tcp