Introduction to Docker Compose

Docker is an amazing containerisation solution that can help you ship your code seamlessly, irrespective of the environment it runs on. Docker provides the ability to package and run an application in a loosely isolated environment called a container.

The backbone of delivering a Docker container is docker-compose. As a developer, you can use the docker-compose command to design and run multi-container Docker applications.

Using docker-compose involves 2 steps,

1. Create a docker-compose.yml file defining the services that will form your app
2. Run docker-compose up to create and start your app

Before you begin, you must have Docker and Docker-Compose installed on your machine.

docker-compose takes a .yml file and executes the “recipe” stored within it. By default it looks for a docker-compose.yml file in the current directory. A typical docker-compose.yml file looks like,

version: '3'
services:
  sql-server:
    image: mcr.microsoft.com/mssql/server
    restart: always
    environment:
      ACCEPT_EULA: y
      SA_PASSWORD: <your_password>
  adminer:
    image: adminer
    restart: always
    ports:
      - 8080:8080

Here, we have 2 containers running in our app, namely, SQL Server and Adminer.

The .yml file above creates a Docker container running Microsoft SQL Server. The “image” specifies what image Docker needs to download for the container. You can find a complete list of all Docker images available at Docker Hub.

The containers in the app can access each other via the names. For eg. from the above .yml file, to access SQL Server you may use sql-server:1433.

Docker hub images come with certain environment variables that you can configure to your requirement, for eg. in the above .yml file we have configured a service account password. Explore the documentation of the Docker image being used to understand what configuration options it makes available to you.

Main sections of a docker-compose file,

Descriptions for the most commonly used sections in a docker-compose file

Container level options,

Configuration options available for a container in docker-compose

A few commonly used docker-compose commands,

Create & Start containers

docker-compose up

List the containers

docker-compose ps

Delete a stopped container

docker-compose rm

Specify a .yml file

docker-compose -f docker-compose.yml

Full list of options available with docker-compose

Define and run multi-container applications with Docker.

Usage:
  docker-compose [-f <arg>...] [options] [COMMAND] [ARGS...]
  docker-compose -h|--help

Options:
  -f, --file FILE             Specify an alternate compose file
                              (default: docker-compose.yml)
  -p, --project-name NAME     Specify an alternate project name
                              (default: directory name)
  --verbose                   Show more output
  --log-level LEVEL           Set log level (DEBUG, INFO, WARNING, ERROR, CRITICAL)
  --no-ansi                   Do not print ANSI control characters
  -v, --version               Print version and exit
  -H, --host HOST             Daemon socket to connect to

  --tls                       Use TLS; implied by --tlsverify
  --tlscacert CA_PATH         Trust certs signed only by this CA
  --tlscert CLIENT_CERT_PATH  Path to TLS certificate file
  --tlskey TLS_KEY_PATH       Path to TLS key file
  --tlsverify                 Use TLS and verify the remote
  --skip-hostname-check       Don't check the daemon's hostname against the
                              name specified in the client certificate
  --project-directory PATH    Specify an alternate working directory
                              (default: the path of the Compose file)
  --compatibility             If set, Compose will attempt to convert deploy
                              keys in v3 files to their non-Swarm equivalent

Commands:
  build              Build or rebuild services
  bundle             Generate a Docker bundle from the Compose file
  config             Validate and view the Compose file
  create             Create services
  down               Stop and remove containers, networks, images, and volumes
  events             Receive real time events from containers
  exec               Execute a command in a running container
  help               Get help on a command
  images             List images
  kill               Kill containers
  logs               View output from containers
  pause              Pause services
  port               Print the public port for a port binding
  ps                 List containers
  pull               Pull service images
  push               Push service images
  restart            Restart services
  rm                 Remove stopped containers
  run                Run a one-off command
  scale              Set number of containers for a service
  start              Start services
  stop               Stop services
  top                Display the running processes
  unpause            Unpause services
  up                 Create and start containers
  version            Show the Docker-Compose version information

That concludes this introduction to docker-compose

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