Docker Compose

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Docker Compose is a tool for defining and running multi-container Docker applications.

It uses YAML files to configure application’s services.

Service

Services are really just “containers in production.”. A Service runs one image and it defines ports, volumes, resource limitation, restart policy and replicas.

Installation

Instruction to install Compose is here

To install in Linux

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# download latest version of Docker Compose
sudo curl -L "https://github.com/docker/compose/releases/download/1.22.0/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose

# apply executable permissions to binary
sudo chmod +x /usr/local/bin/docker-compose

# test installation
docker-compose --version

Simple Example

Here is the example from Get Started with Docker Compose

Files Structure

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composetest/
├── app.py
├── docker-compose.yml
├── Dockerfile
└── requirements.txt

app.py

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import time

import redis
from flask import Flask

app = Flask(__name__)
cache = redis.Redis(host='redis', port=6379)

def get_hit_count():
    retries = 5
    while True:
        try:
            return cache.incr('hits')
        except redis.exceptions.ConnectionError as exc:
            if retries == 0:
                raise exc
            retries -= 1
            time.sleep(0.5)


@app.route('/')
def hello():
    count = get_hit_count()
    return 'Hello World! I have been seen {} times.\n'.format(count)

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

requirements.txt

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flask
redis

Dockerfile

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FROM python:3.4-alpine
ADD . /code
WORKDIR /code
RUN pip install -r requirements.txt
CMD ["python", "app.py"]

docker-compose.yml - defines two service, web and redis.

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version: '3'
services:
  web:
    build: .
    ports:
      - "5000:5000"
    volumes:
      - .:/code
  redis:
    image: "redis:alpine"

use docker-compose up -d to start the services

Common docker-compose Commands

docker-compose up -d is the most used command to start service in the background

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$ docker-compose up -d
Starting composetest_web_1   ... done
Starting composetest_redis_1 ... done

The default compose file to be used is docker-compose.yml, you can set alternative file using -f option.

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docker-compose -f docker-compose.dev.yml  up -d

use docker-compose ps to list containers. If a container is not UP, you can usually use docker log command to see if there is error starting the container.

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$ docker-compose ps
       Name                      Command               State           Ports
-------------------------------------------------------------------------------------
composetest_redis_1   docker-entrypoint.sh redis ...   Up      6379/tcp
composetest_web_1     python app.py                    Up      0.0.0.0:5000->5000/tcp

Use docker-compose build command to rebuild the containers. This is useful after a change to a service

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$ docker-compose build

use docker-compose stop to stop the containers without removing them. You can use docker-compose start to start the services again

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$ docker-compose stop
Stopping composetest_web_1   ... done
Stopping composetest_redis_1 ... done

use docker-compose down to stop containers and removes containers, networks created by up. use -v to remove volumes and use --rmi to remove image

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$ docker-compose down -v --rmi all
Stopping composetest_web_1   ... done
Stopping composetest_redis_1 ... done
Removing composetest_web_1   ... done
Removing composetest_redis_1 ... done
Removing network composetest_default
Removing image composetest_web
Removing image redis:alpine

Print out log messages from all container.

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docker-compose logs -f --tail=0

Use docker help command to display the help menu.

For all the command reference, see docker-compose command-line Reference

docker-compose.yml file

Compose file has a version number. current version is version 3.

You can include the version number at the top of docker-compose.yml

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version: '3'

Service Configuration

Then services list all the services to create.

build - usually a path to a build context. It can also specify context and optionally Dockerfile and args.

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version: '3'
services:
  web:
    build: .

image - Specify the image to start the container from.

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image: "redis:alpine"

environment - sets environment variable

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environment:
  - RACK_ENV=development
  - SHOW=true
  - SESSION_SECRET

ports - specify ports(Host:Container)

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ports:
  - "5000:5000"
  - "1234:1234"

volumes

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volumes:
  - .:/code

depends_on - sets dependency between services.

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services:
  web:
    build: .
    depends_on:
      - db
      - redis
  redis:
    image: redis
  db:
    image: postgres

Restart - sets restart policy. Default is “no”

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restart: "no"
restart: always
restart: on-failure
restart: unless-stopped

Volume Configuration

you can create named volumes in compose file

Example

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version: "3"

services:
  db:
    image: db
    volumes:
      - data-volume:/var/lib/db
  backup:
    image: backup-service
    volumes:
      - data-volume:/var/lib/backup/data

volumes:
  data-volume:

Network Configuration

by default, Compose set up a bridge network for your app. You can customize it.

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networks:
    my_network:
        driver: bridge

Other Examples

Running Postgres and Adminer

Two services will be started. postgres database and adminer

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# Use postgres/example user/password credentials
version: '3.1'

services:
  db:
    image: postgres
    restart: always
    environment:
      POSTGRES_PASSWORD: example

  adminer:
    image: adminer
    restart: always
    ports:
      - 5555:8080

example from https://hub.docker.com/_/postgres

Wordpress

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version: '3'

services:
  db:
    image: mysql:5.7
    volumes:
      - db_data:/var/lib/mysql
    restart: always
    environment:
      MYSQL_DATABASE: exampledb
      MYSQL_USER: exampleuser
      MYSQL_PASSWORD: examplepass
      MYSQL_RANDOM_ROOT_PASSWORD: '1'

  web:
    image: wordpress
    depends_on:
      - db
    volumes:
      - wordpress_files:/var/www/html
    ports:
      - "80:80"
    restart: always
    environment:
      WORDPRESS_DB_HOST: db:3306
      WORDPRESS_DB_USER: exampleuser
      WORDPRESS_DB_PASSWORD: examplepass
      WORDPRESS_DB_NAME: exampledb

volumes:
  wordpress_files:
  db_data:

Reference