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1. Overview

Docker containers run as isolated processes in our system. However, we usually want them to communicate and transfer information from one to another.

In this tutorial, we'll see the difference between Docker links and depends_on with some practical examples using Docker Compose.

2. Docker Compose depends_on

depends_on is a Docker Compose keyword to set the order in which services must start and stop.

For example, suppose we want our web application, which we'll build as a web-app image, to start after our Postgres container. Let's have a look at the docker-compose.yml file:

services:
  db:
    image: postgres:latest
    environment:
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=postgres
    ports:
      - 5432:5432
  web-app:
    image: web-app:latest
    ports:
      - 8080:8080
    depends_on:
      - db

Docker will pull the images and run the containers based on the given dependencies. So, in this case, the Postgres container is the first in the queue to run.

However, there are limitations because depends_on doesn't explicitly wait for dependencies to be ready.

Let's imagine our web application needs to run some migrations scripts at startup. If the database isn't accepting connections, although the Postgres service has started correctly, we can't execute any script.

However, we can avoid this if we control the startup or shutdown order using specific tools or our own managed scripting.

3. Docker Compose links

links instructs Docker to link containers over a network. When we link containers, Docker creates environment variables and adds containers to the known hosts list so they can discover each other.

We'll check out a simple Docker example running a Postgres container and link it to our web application.

First, let's run our Postgres container:

docker run -d --name db -p 5342:5342 postgres:latest 

Then, we link it to our web application:

docker run -d -p 8080:8080 --name web-app --link db 

Let's convert our example to Docker Compose:

services:
  db:
    image: postgres:latest
    environment:
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=postgres
    ports:
      - 5432:5432
  web-app:
    images: web-app:latest
    ports:
      - 8080:8080
    links:
      - db

4. Docker Compose network

We can find Docker links still in use. However, Docker Compose deprecates it since version 2 because of the introduction of the network.

This way, we can link applications with complex networking, for example, overlay networks.

However, in a standalone application, we can typically use a bridge as the default when we don't specify a network.

Let's remove links and replace it with the network while also adding a volume and environment variables for the database:

services:
  db:
    image: postgres:latest
    restart: always
    environment:
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=postgres
    ports:
      - 5432:5432
    volumes: 
      - db:/var/lib/postgresql/data
    networks:
      - mynet

  web-app:
    image:web-app:latest
    depends_on:
      - db
    networks:
      - mynet
    ports:
      - 8080:8080
    environment:
      DB_HOST: db
      DB_PORT: 5432
      DB_USER: postgres
      DB_PASSWORD: postgres
      DB_NAME: postgres

networks:
  mynet:
    driver: bridge

volumes:
  db:
    driver: local

5. Difference between Docker links and depends_on

Although they involve expressing dependencies, Docker links and depends_on have different meanings.

While depends_on indicates the order in which the services must start and stop, the links keyword handles the communication of containers over a network.

Furthermore, depends_on is a Docker Compose keyword, while we can similarly use links as a legacy feature of Docker.

6. Conclusion

In this article, we've seen the difference between Docker links and depends_on with Docker Compose examples.

depends_on tells Docker the order of running containers while links, or the network in newer versions of Docker Compose, sets a connection for containers over a network.

As always, we can find the source docker-compose.yml files of our examples over on GitHub.

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