Docker-compose: App container can't connect to Postgres

Question

Using docker-compose, I'm trying to view a Flask app from manage.py runserver that connects with a postgres image but I get the following error:

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? 

MY COMPOSE FILE

version: '3.1'
services:
  app:
    build: .
    ports:
      - 5000:5000
    volumes:
      - /Users/sammy/Projects/my_flask_app:/my_flask_app
    container_name: my_flask_app_1
    depends_on:
      - db
    entrypoint: ["python", "manage.py", "runserver"]
  db:
    image: postgres:10
    restart: always
    container_name: my_app_db_1
    environment:
      POSTGRES_USER: ubuntu
      POSTGRES_PASSWORD: pickles
      POSTGRES_DB: db
    ports:
      - 5432:5432

FROM MY APP'S settings.py FILE

POSTGRES = {
    'user': "ubuntu",
    'pw': "pickles",
    'db': "db",
    'host': os.getenv('IP', '0.0.0.0'),
    'port': '5432',
}
SQLALCHEMY_DATABASE_URI = 'postgresql://%(user)s:\
%(pw)s@%(host)s:%(port)s/%(db)s' % POSTGRES

It builds and runs the app well as I can visit the site and see the Werkzeug traceback interpreter. I'm failing either on the configuration of the Postgres container itself or my Flask's connection to it.

• Are the environmental variables correct?
• Do I need to specify a volume for the DB? If so, why? I haven't understood the point of that other than tucking the username/pass into environmental variables.

Answer 1

Your host in the settings.py file should be "db". compose exposes hosts on an internal network with the same host names as your services.

POSTGRES = {
    'user': "ubuntu",
    'pw': "pickles",
    'db': "db",
    'host': "db",
    'port': '5432',
}