20_docker_compose.md

Docker Compose

Compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services. Then, with a single command, you create and start all the services from your configuration.

Using Compose is basically a three-step process:

1. Define your app’s environment with a Dockerfile so it can be reproduced anywhere.
2. Define the services that make up your app in docker-compose.yml so they can be run together in an isolated environment.
3. Run docker-compose up and Compose starts and runs your entire app.

A docker-compose.yml looks like this:

version: '3'
services:
  dockerapp:
    build: .
    ports:
    - "5000:5000"
    depends_on:
    - redis
  redis:
    image: redis

Understanding the docker-compose.yml file

• We specify the Docker Compose version using the version: '3' variable. Here we are using version 3.
• Next we mention the services we want to run using Docker Compose. In our application there are two services dockerapp(flask api) and redis server.
• Within each service we mention the instructions for running that service.
• The first instruction build instruction mentions the path that contains the dockerfile to be used for building the dockerapp image.
• The second instruction is the ports instruction. Its similar to -p argument in docker client. The port format is host_port:container_port.
• Third instruction is depends_on. Docker compose will start services in dependency order mentioned in the depends_on section. Since we mention that our dockerapp depends on redis docker-compose will start redis first.
• Since the redis official image is available we only mention the image instruction to specify which image to use.

**Note: Version 2 docker compose has added a docker-network feature which allows a docker container to communicate with other container using it's name and hence we need not specify the link. **

Finally we can start the containers using docker-compose up command in the folder that contains the docker-compose.yml file.