Compose file

The compose file is a YAML file that is used for the Docker application to define services, networks, volumes, secrets, and configs. By default docker-compose.yaml(preferred) or docker-compose.yml is used. You can define your own custom compose file and use it as compose file also.

Similar to passing command-line options to docker run, a service definition contains a configuration that will be applied to each container launched for that service. As with docker network create and docker volume create, network and volume definitions are comparable.

The same as with docker run, the options supplied in the Dockerfile (such as CMD, EXPOSE, VOLUME, and ENV) are taken into account by default; you do not need to supply them again in docker-compose.yml.

For example, you have a database and web which are a part of your application. All of your services rely on an image with which we create a container. And there may be many options; and how these options are configured will be in docker-compose.yaml or compose.yaml or in your custom compose file.

Benefits

• Single host deployment: You can run everything on a single piece of hardware.
• Quick and easy configuration: It uses YAML scripts, so it’s easy to configure
• Security: Since all the containers are isolated, it reduces the threat landscape
• High productivity: Reduces the time it takes to perform tasks.

Example

Sample docker-compose.yml file

services:
  frontend:
    image: cloudyfox/webapp
    build: ./webapp

  backend:
    image: cludyfox/database
    build:
      context: backend
      dockerfile: ../backend.Dockerfile

  custom:
    build: ~/custom

Here,

• cloudyfox/webapp docker image is built using webapp sub-directory within compose file as a docker build context. If no Dockerfile is in this directory, an error will be thrown.
• cloudyfox/database docker image is built using backend sub-directory. backend.Dockerfile is used to define build steps.
• Another docker image is built using custom directory as a docker context.

On push, two docker images cloudyfox/webapp and cloudyfox/database are pushed to the registry. While custom service image is skipped as there is no image attribute.