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Dependencies

Updated Mar 11, 2022 ·

Overview​

Dependencies in Docker Compose define the order in which resources, such as containers, start up. Some containers require others to be available first to function properly.

Example: A web application with three resources.

  • Database container: Runs PostgreSQL and must start first to avoid errors.
  • Python application: Manages communication between the web and database.
  • Nginx container: Starts last to serve web content.

depends_on​

The depends_on attribute specifies basic dependencies between resources.

services:
  database:
    image: postgres
  python_app:
    image: python-app
    depends_on:
      - database
  nginx:
    image: nginx
    depends_on:
      - python_app

Key points:

  • Dependencies can be chained or listed per resource.
  • Resource order in the compose.yaml file doesn’t matter once dependencies are defined.

Shutting Down Applications​

When shutting down, resources stop in reverse order. Using the previous example:

  • Nginx stops first.
  • Then Python application is removed.
  • Finally, PostgreSQL shuts down when no other containers need it.

Other Dependency Options​

Docker Compose provides additional attributes for managing dependencies.

  • condition attribute: Controls when a dependency is considered ready.

    • service_started: Default behavior, starts normally.
    • service_completed_successfully: Resource must run to completion (e.g., setup or validation tasks).
    • service_healthy: Resource passes a healthcheck before being ready.

  • Healthchecks:

    • Defined methods, like verifying a webpage or TCP port.
    • Specified in Dockerfiles or Compose files.
    • Not covered in this course but usable with existing configurations.

Troubleshooting Tools​

Docker Compose includes tools for diagnosing issues in applications.

  • docker compose logs:

    • Collects output from all resources.
    • Example: docker compose logs web for specific resource logs.

  • docker compose top:

    • Shows the status of resources in an application.
    • Example: Displays resources in the composetest application.