Difference between links and depends_on in docker_compose.yml

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According to the Docker Compose's compose-file documentation:

  • depends_on - Express dependency between services.
  • links - Link to containers in another service and also express dependency between services in the same way as depends_on.

I don't understand the purpose of linking to other containers so the difference between two options still seems quite difficult for me.

It would be much easier if there is an example, but I can't find any.

I noticed, when I link container B with container A then container B will be "pingable" inside container A's shell.

I ran ping B inside container A's bash and got result like this (just for reference, image from the Internet)

enter image description here

dockerfile docker-compose docker
4 Answers

The post needs an update after the links option is deprecated.

Basically, links is no longer needed because its main purpose, making container reachable by another by adding environment variable, is included implicitly with network. When containers are placed in the same network, they are reachable by each other using their container name and other alias as host.

For docker run, --link is also deprecated and should be replaced by a custom network. 

docker network create mynet
docker run -d --net mynet --name container1 my_image
docker run -it --net mynet --name container1 another_image

depends_on expresses start order (and implicitly image pulling order), which was a good side effect of links.

user_49806510

I think that the answers for this question need updating based on the new Docker compose specification introduced first in v1.27.0, which now allows for a long-form of depends_on:

https://github.com/compose-spec/compose-spec/blob/master/spec.md#long-syntax-1

In this long form, you can specify that you want to wait for a service to be either started, healthy, or completed successfully.

Docker compose knows that a service is healthy if you produce a health_check on that service:

https://github.com/compose-spec/compose-spec/blob/master/spec.md#healthcheck

I'd recommend to read the examples in the documentation for more details, see links above!

For a quick example, this is what I used in a compose file for integration tests:

services:
  cloud-broker:
    image: my.docker.registry/activemq-artemis:latest
    healthcheck:
      test: ["CMD-SHELL", "wget http://localhost:8161/ --delete-after --tries=3 2> /dev/null"]
      interval: 10s
      timeout: 5s
      retries: 5

  postgresql:
    image: postgres
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 10s
      timeout: 5s
      retries: 5
    environment:
      POSTGRES_PASSWORD: "<my-secret>"
      POSTGRES_USER: "postgres"
      POSTGRES_DB: "postgres"
  
  # This service calls a script to create an empty database and the service-user
  postgresql-setup:
    image: postgres
    depends_on:
      postgresql:
        condition: service_healthy
    restart: "no"
    volumes:
      - "./scripts:/scripts"
    environment:
      PGPASSWORD: "<my-secret>"
    entrypoint: "psql -U postgres -d postgres -h postgresql -f /scripts/create-db.sql"

  my-business-service:
    image: my.docker.registry/my-business-service:latest
    depends_on:
      cloud-broker:
        condition: service_healthy
      postgresql-setup:
        condition: service_completed_successfully
user_35882310
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