How to get hostname from one container to another using docker compose? - go

Question

I have two docker containers. One backend, and the other db (postgres). They both are linked. How do I utilise the backend environment HOST variable in the golang docker container?
From my understanding, both containers have their own IP addresses. I cannot use 'localhost' in the golang container because postgres isn't on localhost, but in an isolated container.
version: "3.7"
services:
backend:
image: golang:1.16
build: ./
working_dir: /app
volumes:
- ./backend/:/app
environment:
HOST: db
command: go run main.go
ports:
- 8080:8080
depends_on:
- db
db:
image: postgres
restart: always
environment:
POSTGRES_USER: gorm
POSTGRES_PASSWORD: gorm
POSTGRES_DB: gorm
ports:
- 9920:9920
I've tried researching how to access this variable as well as check Docker tutorials/documentation, but haven't found a solution.

Answer 1

Docker compose does DNS resolution. You should be able to access your database by name.
Remove:
environment:
HOST: db
Correct the postgres port to 5432:
db:
...
ports:
- 5432:5432
You must be able to connect like so:
db := pg.Connect(&pg.Options{
Addr: "db:5432",
User: "gorm",
Database: "gorm",
Password: "gorm",
})
As for environment variables, you can declare and access them like this:
backend:
environment:
POSTGRES_USER: gorm
...
os.Getenv("POSTGRES_USER")

Answer 2

Docker compose will create a network for your containers where they can communicate and reach each other.
You can make a simple change to your docker compose file by adding a name to your services which will make sure they get the same name each time.
version: "3.7"
services:
backend:
image: golang:1.16
build: ./
container_name: backend
working_dir: /app
volumes:
- ./backend/:/app
command: go run main.go
ports:
- "8080:8080"
depends_on:
- db
db:
image: postgres
restart: always
container_name: db
environment:
POSTGRES_USER: gorm
POSTGRES_PASSWORD: gorm
POSTGRES_DB: gorm
ports:
- "9920:5432"
The default port for Postgres is 5432, so I mapped it to your 9920. Then, you can access the db container from the backend container by specifying:
db:9920