How to check that the `db` and `redis` services are running before starting the `web` service in a `docker-compose.yml` file.

Question

Consider the following content in a `docker-compose.yml` :
```yaml
services:
  web:
    image: nginx
    depends_on:
      - db
      - redis
  db:
    image: postgres
  redis:
    image: redis
```

In this file, what does the `depends_on` field do under the `web` service?

Accepted Answer

It checks that the `db` and `redis` services are running before starting the `web` service.

Answer

It copies the `db` and `redis` configuration files into the `web` service.

Answer

It creates network links between the `web` service and the `db` and `redis` services.

Answer

It installs `db` and `redis` databases in the `web` service.

Question from the Docker compose test

How to check that the `db` and `redis` services are running before starting the `web` service in a `docker-compose.yml` file.