[FIXED] Docker with postgresql in flask web application (part 2)

Issue

I am building a Flask application in Python. I'm using SQLAlchemy to connect to PostgreSQL.

In the flask application, I'm using this to connect SQLAlchemy to PostgreSQL

engine = create_engine('postgresql://postgres:[mypassword]@db:5432/employee-manager-db')

And this is my docker-compose.yml

version: '3.8'
services:
  backend:
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - 8000:8000
    volumes:
      - .:/app
    links:
      - db:db
    depends_on:
      - pgadmin

  db:
    image: postgres:14.5
    restart: always
    volumes:
      - .dbdata:/var/lib/postgresql
    hostname: postgres
    environment:
      POSTGRES_PASSWORD: [mypassword]
      POSTGRES_DB: employee-manager-db

  pgadmin:
    image: 'dpage/pgadmin4'
    restart: always
    environment:
      PGADMIN_DEFAULT_EMAIL: [myemail]
      PGADMIN_DEFAULT_PASSWORD: [mypassword]
    ports:
      - "5050:80"
    depends_on:
      - db

I can do "docker build -t employee-manager ." to build the image. However, when I do "docker run -p 5000:5000 employee-manager" to run the image, I get an error saying

conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: could not translate host name "db" to address: Try again

Does anybody know how to fix this? Thank you so much for your help

Solution

Your containers are on different networks and that is why they don't see each other.

When you run docker-compose up, docker-compose creates a separate network and puts all the services defined inside docker-compose.yml on that network. You can see that with docker network ls.

When you run a container with docker run, it is attached to the default bridge network, which is isolated from other networks.

There are several ways to fix this, but this one will serve you in many other scenarios:

Run docker container ls and identify the name or ID of the db container that was started with docker-compose

Then run your container with:

# ID_or_name from the previous point
docker run -p 5000:5000 --network container:<ID_or_name> employee-manager

This attached the new container to the same network as your database container.

Other ways include creating a network manually and defining that network as default in the docker-compose.yml. Then you can use docker run --network <network_name> ... to attach other containers to that network.

Answered By - Mihai