I’m trying to run a FastAPI application with a MySQL database in a Docker environment, but I’m encountering a connection error. The application is unable to connect to the database.
Here’s the error I’m getting:

Copysqlalchemy.exc.OperationalError: (pymysql.err.OperationalError) (2003, "Can't connect to MySQL server on 'db' ([Errno -2] Name or service not known)")
(Background on this error at: https://sqlalche.me/e/20/e3q8)

docker-compose.yml:

services:
  backend:
    build: .
    ports:
      - "8001:8000"
    networks:
      - music-app
    volumes:
      - .:/app
    env_file:
      - .env
    environment:
      - DATABASE_URL=${BACKEND_DATABASE_URL}
      - MYSQL_ROOT_PASSWORD=${MYSQL_ROOT_PASSWORD}
    depends_on:
      - db
    command: bash -c "alembic upgrade head && uvicorn app.main:app --host 0.0.0.0 --port 8000 --reload"

  db:
    image: mysql:8.0
    volumes:
      - mysql_data:/var/lib/mysql
    env_file:
      - .env
    environment:
      - MYSQL_ROOT_PASSWORD=${MYSQL_ROOT_PASSWORD}
      - MYSQL_DATABASE=${MYSQL_DATABASE}
    ports:
      - "3307:3306"
    networks:
      - music-app
    healthcheck:
      test:
        [
          "CMD-SHELL",
          "mysqladmin ping -h 127.0.0.1 -u root -p$$MYSQL_ROOT_PASSWORD",
        ]
      timeout: 20s
      retries: 10

volumes:
  mysql_data:

networks:
  music-app:
    driver: bridge

In my alembic/env.py, I’m trying to connect to the database using mysql+pymysql as the dialect.
I’ve already tried:

Checking environment variables
Verifying network settings
Setting up a healthcheck for the database

What could be causing this issue, and how can I resolve it?

I’ve tried the following

Checked environment variables:

Verified that the BACKEND_DATABASE_URL in the .env file is correctly set with the appropriate
format: mysql://:[password]@[host]:[port]/[database_name].

Verified network settings:

Confirmed both backend and db services are on the same music-app network in docker-compose.yml.
Expected: Services to be able to communicate with each other.

Despite these attempts, I’m still unable to establish a connection between the FastAPI backend and the MySQL database. I expected the application to start successfully and be able to communicate with the database.