Unable to access postgres in docker from web app in another container - postgresql

Question

I have a samle app I'm using docker-compose to run locally on my machine. The web app is in one container and the db (postgres) in another.
I am having an connection issue that I can't work through.
docker-compose
version: '3.8'
services:
db:
image: postgres
environment:
POSTGRES_PASSWORD: 'password'
POSTGRES_DB: 'postgres'
POSTGRES_USER: 'postgres'
volumes:
- ./postgres-db:/var/lib/postgresql/data
ports:
- '5432:5432'
app:
build:
context: .
dockerfile: app/Dockerfile
restart: always
environment:
APP_FRONTEND_PORT: '8080'
DB_PORT: '5433'
DB_HOST: 'db'
ports:
- '8080:8080'
depends_on:
- 'db'
volumes:
postgres-db:
Dockerfile
FROM golang:latest
WORKDIR /scratch
COPY . .
RUN CGO_ENABLED=0 GOOS=linux go build -o /bin/frontend ./...
FROM alpine:latest
RUN apk --no-cache add ca-certificates
WORKDIR /go/bin/
COPY --from=build /bin/frontend /go/bin/frontend
ENTRYPOINT ["/go/bin/frontend"]
Both containers are running and I'm able to log into the running postgres container and postgres us running.
When I try to run a update from the US I get a 500 error and it does not seem like the app container can communicate with the db container. I'm not sure what I'm missing
client side error when trying to make a call to update date:
encountered err: failed to begin transaction: failed to connect to `host=db user=postgres database=postgres`: dial error (dial tcp 172.29.0.2:5433: connect: connection refused)
docker ps yeilds:
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
19eeed869434 sample_app "/go/bin/frontend" 48 minutes ago Up 48 minutes 0.0.0.0:8080->8080/tcp, :::8080->8080/tcp sample_app_1
84804f00c751 postgres "docker-entrypoint.s…" 48 minutes ago Up 48 minutes 0.0.0.0:5432->5432/tcp, :::5432->5432/tcp sample_app_db_1
$

Answer 1

As per stated in https://docs.docker.com/network/bridge/, you need to put both services into a user-defined bridge network for them to refer to each other by the container names. Here is how to do it inside docker-compose.yml:
Define a custom bridge network:
networks:
some-name:
driver: bridge
Put both services into that network:
services:
db:
image: postgres
environment:
POSTGRES_PASSWORD: 'password'
POSTGRES_DB: 'postgres'
POSTGRES_USER: 'postgres'
volumes:
- ./postgres-db:/var/lib/postgresql/data
ports:
- '5432:5432'
networks:
- some-name
app:
build:
context: .
dockerfile: app/Dockerfile
restart: always
environment:
APP_FRONTEND_PORT: '8080'
DB_PORT: '5433'
DB_HOST: 'db'
ports:
- '8080:8080'
depends_on:
- 'db'
networks:
- some-name
Force specific container names especially the one being referred to by the other, otherwise docker-compose will add prefix and suffix to the service name as the container name like sample_app_db_1:
services:
db:
container_name: db
image: postgres
environment:
POSTGRES_PASSWORD: 'password'
POSTGRES_DB: 'postgres'
POSTGRES_USER: 'postgres'
volumes:
- ./postgres-db:/var/lib/postgresql/data
ports:
- '5432:5432'
networks:
- some-name