How to connect Docker web app container to Docker

Question

I'm practicing making a Golang web app that interacts with a PostgreSQL database, each running on their own container.

I'm running the containers with docker-compose up

But I seem to be failing on getting the postgres container properly set up.

For brevity, links to Dockerfiles and other settings files are on this gist (let me know if you want it here instead).

version: '2'
services:
  web_app:
    build: dockerfiles/web_app
    ports:
      - "9000:9000"
    volumes:
      - .:/go/src/gitlab.com/repo/web_app
    # links might be replaced by depends_on.
    # links:
    #   - db
    depends_on:
      - db
    # tty and stdin_open cause docker-compose to disconnect from docker-machine after 60sec.
    # A fix is on the way.
    # tty: true
    # stdin_open: true
  db:
    build: dockerfiles/db
    volumes:
      - data:/var/lib/postgresql/data
volumes:
  data: {}

docker-compose up works fine. But when the application tries to open a database connection with:

var pgConf string = "user=web_app dbname=web_app sslmode=verify-full password=password"

db, err := sql.Open("postgres", pgConf)

I get the following error from docker compose:

Error creating new user:  dial tcp [::1]:5432: getsockopt: connection refused

What can I do to make both containers talk to each other?

Thank you in advance.

Answer 1

When using the docker-compose v2, it's not needed to create links between services. Docker 1.9 and 1.10 allows you to connect to other containers on the same (custom) network through their name.

You should be able to connect using either the name of the service or the name of the container as a hostname. Given that the name of the container is generated by docker-compose, this is not really convenient to use, so for that reason, docker-compose also adds an alias with the service name to each container.

Take this very simple example. I've used an Nginx container for convenience, but the same should apply to your situation;

version: '2'
services:
  web_app:
    image: nginx
  db:
    image: nginx

First start the project (assuming;

$ docker-compose --project-name=test up -d
Creating network "test_default" with the default driver
Creating test_db_1
Creating test_web_app_1

Then ping the "db" service from inside the test_web_app_1 container:

$ docker exec -it test_web_app_1 ping -c 2 db
PING db (172.18.0.2): 56 data bytes
64 bytes from 172.18.0.2: icmp_seq=0 ttl=64 time=0.108 ms
64 bytes from 172.18.0.2: icmp_seq=1 ttl=64 time=0.243 ms
--- db ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.108/0.175/0.243/0.068 ms

If you inspect the test_db_1 container, you can see that docker-compose automatically added a "db" alias for the test_db_1 container;

$ docker inspect test_db_1

Gives: (just the NetworkSettings.Networkspart)

"Networks": {
    "test_default": {
        "IPAMConfig": null,
        "Links": null,
        "Aliases": [
            "db",
            "002b1875e61f"
        ],
        "NetworkID": "0f9e2cddeca79e5a46c08294ed61dee273828607f99014f6410bda887626be70",
        "EndpointID": "a941ab95586a8fdafc5075f9c5c44d745f974e5790ef6048b9e90115a22fb31f",
        "Gateway": "172.18.0.1",
        "IPAddress": "172.18.0.2",
        "IPPrefixLen": 16,
        "IPv6Gateway": "",
        "GlobalIPv6Address": "",
        "GlobalIPv6PrefixLen": 0,
        "MacAddress": "02:42:ac:12:00:02"
    }
}