Docker – Unknown server host ‘db’ when migrating in Python

So, if you’re getting that “Unknown server host ‘db'” error in your Python and Docker setup, it’s likely causing some migration headaches. Let’s tackle this issue step by step.

First off, peek into your Docker Compose file. Make sure it’s got a ‘db’ service set up properly. Here’s a snippet to guide you:

version: '3'
services:
  db:
    image: your_database_image
    # Add other database config options
  app:
    image: your_python_app_image
    depends_on:
      - db
    # Add other app config options

Note that your app service should depend on the ‘db’ service.

Next, double-check your Python app’s database connection settings. The hostname should match the service name in your Docker Compose file (‘db’ in this case).

# Example database connection settings
DATABASES = {
   'default': {
       'ENGINE': 'django.db.backends.postgresql',
       'NAME': 'your_database_name',
       'USER': 'your_database_user',
       'PASSWORD': 'your_database_password',
       'HOST': 'db',
       'PORT': '5432',
   }
}

Adjust these settings based on your specific database type and setup.

Now, the ‘depends_on’ in Docker Compose ensures ‘db’ starts before ‘app,’ but it doesn’t guarantee the database is ready to roll. You might need to add some logic in your app to patiently wait for the database to be good to go.

Also, check if your services share the same Docker network. If not, make sure they do:

version: '3'
services:
  db:
    image: your_database_image
    networks:
      - your_network
  app:
    image: your_python_app_image
    depends_on:
      - db
    networks:
      - your_network
networks:
  your_network:

Lastly, confirm your DNS game is strong inside the containers. Hop into your ‘app’ container and ping ‘db’:

docker exec -it your_app_container_name /bin/bash ping db

If this ping doesn’t fly, there might be a hiccup in DNS resolution.

Give these steps a go, and you should be on the road to fixing that pesky “Unknown server host ‘db'” issue during migrations in your Python/Docker dance.