Sqlalchemy does not see Postgresql db - python-3.x

Question

I develop a web-app using Flask under Python3. I have a problem with connecting to postgresql via sqlalchemy.
Dockerfile:
FROM ubuntu:20.04
ENV LANG en_US.UTF-8
RUN apt update
RUN apt install -y python3-pip python3.8-dev
RUN apt install default-libmysqlclient-dev -y
RUN apt install libpq-dev -y
WORKDIR /app
ADD ./requirements.txt /app/requirements.txt
RUN pip3 install psycopg2
RUN --mount=type=cache,target=/root/.cache/pip pip3 install -r requirements.txt
RUN pip3 install -r requirements.txt
COPY . /app
ENV PYTHONPATH "${PYTHONPATH}:/app/src"
CMD ["gunicorn", "-c", "python:config.gunicorn", "--worker-class", "aiohttp.worker.GunicornWebWorker", "--pythonpath", "/app/src/", "src.main:app"]
docker-compose.yml:
services:
backend:
environment:
TZ: 'UTC'
volumes:
- ./src:/app/src
build:
context: .
restart: always
ports:
- '80:5000'
depends_on:
- postgres
links:
- postgres:postgres
postgres:
restart: always
image: postgres:latest
volumes:
- /var/lib/postgresql
environment:
POSTGRES_PASSWORD: 1234
POSTGRES_USER: postgres
POSTGRES_DB: postgres
POSTGRES_INITDB_ARGS: "-A md5"
ports:
- "5432:5432"
Postgresql logs:
2022-08-03 14:21:46.773 UTC [1] LOG: starting PostgreSQL 14.4 (Debian 14.4-1.pgdg110+1) on aarch64-unknown-linux-gnu, compiled by gcc (Debian 10.2.1-6) 10.2.1 20210110, 64-bit
2022-08-03 14:21:46.776 UTC [1] LOG: listening on IPv4 address "0.0.0.0", port 5432
2022-08-03 14:21:46.776 UTC [1] LOG: listening on IPv6 address "::", port 5432
2022-08-03 14:21:46.781 UTC [1] LOG: listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
2022-08-03 14:21:46.787 UTC [27] LOG: database system was shut down at 2022-08-03 14:20:52 UTC
2022-08-03 14:21:46.794 UTC [1] LOG: database system is ready to accept connections
2022-08-03 14:21:50.342 UTC [1] LOG: received fast shutdown request
2022-08-03 14:21:50.344 UTC [1] LOG: aborting any active transactions
2022-08-03 14:21:50.347 UTC [1] LOG: background worker "logical replication launcher" (PID 33) exited with exit code 1
2022-08-03 14:21:50.347 UTC [28] LOG: shutting down
2022-08-03 14:21:50.365 UTC [1] LOG: database system is shut down
Python app logs:
sqlalchemy.exc.OperationalError: (psycopg2.OperationalError) could not connect to server: Connection refused
Is the server running on host "0.0.0.0" and accepting
TCP/IP connections on port 5432?
As you can see postgresql is running on 0.0.0.0:5432 and sqlalchemy tries to connect to 0.0.0.0:5432. Therefore, it is not very clear what it does not like.

Answer 1

If I remember correctly, when connecting to another service within a docker compose network, you have to connect using the service name as the host name - so postgres:5432 in your case.