How to pass container ip as ENV to other container in docker-compose file

Question

this is my docker-compose file:

version: '3.0'
services:
 app-web:
  restart: always
  build: ./web
  environment:
    PG_HOST: $(APP_DB_IP)
    PG_PORT: 5432
  ports:
   - "8081:8080"
  links: 
   - app-db

 app-db:
  build: ./db
  expose: 
   - "5432"

  volumes:
   - /var/lib/postgresql/data

I want to pass to app-web the ip of app-db (Postgresql in this case) as ENV var so it can connect to the DB smoothly... any idea on how to achieve it?

Answer 1

You actually don't need to do any of this, since you're already using the links feature in Docker Compose. Just get rid of the PG_HOST variable and use the app-db hostname:

services:
 app-web:
  restart: always
  build: ./web
  environment:
    PG_PORT: 5432
  ports:
   - "8081:8080"
  links: 
   - app-db

Since you included the app-db entry under links, you can simply use app-db as a hostname in your app-web container. Docker will set up a hostname mapping in the app-web container that resolves the app-db hostname to the database container's IP address.

You can verify that by running the following, which will try to ping the app-db container from the app-web container:

docker-compose exec app-web bash -c "ping app-db"

This should show output from the ping command showing the resolved IP address of the app-db container, for example like this:

PING app-db (172.19.0.2): 56 data bytes
64 bytes from 172.19.0.2: icmp_seq=0 ttl=64 time=0.055 ms
64 bytes from 172.19.0.2: icmp_seq=1 ttl=64 time=0.080 ms
64 bytes from 172.19.0.2: icmp_seq=2 ttl=64 time=0.098 ms

Press ctrl+c to stop the ping command.

Like shown in the other answer, if you still want to pass in the hostname (which is probably a good idea, just in case you ever want to point to a different database), you can just use app-db as a value:

services:
 app-web:
  restart: always
  build: ./web
  environment:
    PG_HOST: app-db
    PG_PORT: 5432
  ports:
   - "8081:8080"
  links: 
   - app-db