Django will not connect to mysql container on port 3307 with 3307:3306 mapped in docker compose file

docker-compose.yaml

services:
  db:
    image: mysql:9.0.1
    container_name: db_mysql_container
    environment:
      MYSQL_DATABASE: backend
      MYSQL_USER: asdf
      MYSQL_PASSWORD: asdf
      MYSQL_ROOT_PASSWORD: asdf
    ports:
      - '3307:3306'
  api:
    build: .
    container_name: django_container
    command: bash -c "pip install -q -r requirements.txt &&
      python manage.py migrate &&
      python manage.py runserver 0.0.0.0:8000"
    volumes:
      - .:/app
    ports:
      - '8000:8000'
    depends_on:
      - db

django settings

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        'NAME': "backend",
        'USER': "asdf",
        'PASSWORD': "asdf",
        'HOST': "db",
        'PORT': "3307",
    }
}

The rest of my django settings file is what the default is. The whole django project is just what you start out with from the django-admin startproject command, plus the mysqlclient dependency.

Here is my Dockerfile, nothing special going on:

# Use an official Python runtime as a parent image
FROM python:3.12

# Set environment variables
ENV PYTHONDONTWRITEBYTECODE 1
ENV PYTHONUNBUFFERED 1

# Set the working directory
WORKDIR /app

# Install dependencies
COPY requirements.txt /app/

# Copy the project code into the container
COPY . /app/

requirements.txt:

asgiref==3.8.1
Django==5.1
mysqlclient==2.2.4
sqlparse==0.5.1

When I bring up the db, wait, and then bring up the api, I get this:

django.db.utils.OperationalError: (2002, "Can't connect to server on 'db' (115)")

If I change my docker-compose.yaml to this:

services:
  db:
    image: mysql:9.0.1
    container_name: db_mysql_container
    environment:
      MYSQL_DATABASE: backend
      MYSQL_USER: asdf
      MYSQL_PASSWORD: asdf
      MYSQL_ROOT_PASSWORD: asdf
      MYSQL_TCP_PORT: 3307
    ports:
      - '3307:3307'
  api:
    build: .
    container_name: django_container
    command: bash -c "pip install -q -r requirements.txt &&
      python manage.py migrate &&
      python manage.py runserver 0.0.0.0:8000"
    volumes:
      - .:/app
    ports:
      - '8000:8000'
    depends_on:
      - db

it works. It seems like I should be able to run mysql in the container on port 3306 and then have docker send traffic sent to my host 3307 port to the container's 3306 port.

Solution

Your containers communicate directly to each other over the docker internal network on their "inside" ports. When you do a port exposure like "outside:inside" the outside port is only relevant to traffic targeting the host system. Your containers do not use the host ports to talk to each other.

That's why the 3707:3707 mapping works when the application is configured to connect using port 3707 -- because the inside port is using port 3707.

In the first example with 3707:3706, you would need to configure the application to connect on port 3706, since that's the inside port.