connect to postgres container from flask app inside container

Question

I have my flask app app.py:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

APP = Flask(__name__)
DB = SQLAlchemy()

if __name__ == '__main__':
    APP.config.from_mapping(
        SQLALCHEMY_DATABASE_URI='postgres://postgres:[email protected]:5432',
        SQLALCHEMY_TRACK_MODIFICATIONS=False
    )
    DB.init_app(APP)
    DB.create_all(app=APP)
    APP.run(use_reloader=False, host='0.0.0.0', port='5000')

and I have a Dockerfile for it:

FROM python:3.6-alpine

RUN apk update && apk add postgresql-dev gcc python3-dev musl-dev

WORKDIR /root

COPY app.py .

RUN pip3 install Flask==1.0.2
RUN pip3 install psycopg2-binary==2.7.6.1
RUN pip3 install Flask-SQLAlchemy==2.3.2

CMD ["python3", "app.py"]

I run:

docker build . --tag flaskapp:1
docker run -d -p 5432:5432 -e POSTGRES_PASSWORD=password --name database postgres
docker run --rm -p 5000:5000 flaskapp:1

I then get an exception which points out:

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?

How do I fix this?

ok so I actually figured out that --network=host solves this problem. It may cause other problems but its better than not working for the moment — Theo Walton
– Theo Walton, Commented Nov 30, 2018 at 21:33

programmerq · Accepted Answer · 2018-11-30 21:34:13Z

You have specified 0.0.0.0 as the IP address to connect to, which doesn't make sense. 0.0.0.0 is the "Any Address". You probably saw a message that postgres was listening on 0.0.0.0, which is where you got it from. In the context of a server listening on 0.0.0.0, it means that it is listening on all ipv4 interfaces. See https://en.wikipedia.org/wiki/0.0.0.0 for more information about the special 0.0.0.0 address and what it means.

If you want to connect to the postgres service, then you would need to use a valid ip address or dns name of where it is running.

In Docker, if you have multiple named containers connected to the same user-defined network, you can make use of the built-in service discovery mechanism that Docker ships with.

Here's a modified set of commands to run to take advantage of this:

docker network create mynet
docker run -d -p 5432:5432 -e POSTGRES_PASSWORD=password --net mynet --name database postgres
docker run --rm -p 5000:5000 --net mynet flaskapp:1

Be sure to change your code to connect to postgres://postgres:password@database:5432 instead of postgres://postgres:[email protected]:5432

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