Unable to forward/map port internaly in Docker-Compose - docker

Question

I am currently having an app with 3 different databases (it's for a test). I have the following docker image:
Dockerfile
FROM golang:1.15
WORKDIR /myapp
# Download wait for it tool.
ADD https://raw.githubusercontent.com/vishnubob/wait-for-it/master/wait-for-it.sh /wait-for-it
RUN chmod +x /wait-for-it
and the following docker-compose.yml
version: '3.7'
services:
app:
build:
context: .
dockerfile: Dockerfile.dev
volumes:
- .:/app
command: sh -c "/wait-for-it postgres:10001 -- /wait-for-it oracle:10000 -- /wait-for-it mongodb:10002"
depends_on:
- oracle
- mongodb
- postgres
ports:
- "8080:8080"
oracle:
image: chameleon82/oracle-xe-10g:latest
ports:
- "10000:8080"
expose:
- 10000
postgres:
image: postgres:9.6-alpine
ports:
- "10001:5432"
environment:
- POSTGRES_HOST_AUTH_METHOD=trust
- POSTGRES_DB=testdb
expose:
- 10001
mongodb:
image: mongo:latest
ports:
- "10002:27017"
expose:
- 10002
The thing is, as you saw, my app listens on 8080, but so Oracle does. I know I can change my app port, but still, I would like to switch Oracle to another port. I am trying to achieve it from port mapping, but I do feel that it only works for the host machine, not for a use from within the docker-compose, am I wrong?

Answer 1

Think of services within a Docker Compose network (i.e. app, oracle) as distinct hosts. Each is addressable by its service name, i.e. app should refer to the oracle service by this name (oracle).
The port mapping allows you to expose (and map) service ports within a Docker Compose network to (possibly different) host ports. This is commonly because, the host has a singular dimension of port spaces (0...65535) whereas each service within the Docker Compose network has a port space. 2 services (e.g. http1 and http2) may each use port e.g. 8080 but there's only one 8080 on the host and so, to expose each of these services on your host, one would have to yield; one could also be on the host's 8080 but the other would need to be elsewhere, perhaps 8081.
In your case, e.g. oracle runs on 8080 within the Docker Compose network and is exposed on the host's port 10000. As far as the app service is concerned, this service is available as oracle:8080 (8080 not 10000) within the Docker Compose network.
The expose syntax is purely documentary and has no functional effect.
Responding to comments
If I run your Compose script as-is, it does not work. This is expected because e.g. postgres is available on 5432 within the Compose network not on 10001
docker-compose logs app
Attaching to 63690852_app_1
app_1 | wait-for-it: waiting 15 seconds for postgres:10001
app_1 | wait-for-it: timeout occurred after waiting 15 seconds for postgres:10001
app_1 | wait-for-it: waiting 15 seconds for oracle:10000
app_1 | wait-for-it: timeout occurred after waiting 15 seconds for oracle:10000
app_1 | wait-for-it: waiting 15 seconds for mongodb:10002
app_1 | wait-for-it: timeout occurred after waiting 15 seconds for mongodb:10002
If I correct the ports:
command: sh -c "/wait-for-it postgres:5432 -- /wait-for-it oracle:8080 -- /wait-for-it mongodb:27017"
It works as expected:
docker-compose logs app
Attaching to 63690852_app_1
app_1 | wait-for-it: waiting 15 seconds for postgres:5432
app_1 | wait-for-it: postgres:5432 is available after 0 seconds
app_1 | wait-for-it: waiting 15 seconds for oracle:8080
app_1 | wait-for-it: oracle:8080 is available after 8 seconds
app_1 | wait-for-it: waiting 15 seconds for mongodb:27017
app_1 | wait-for-it: mongodb:27017 is available after 0 seconds