Docker Compose映射3307:3306时Django无法连接MySQL容器

问题场景 #

Docker Compose配置主机3307端口映射到MySQL容器3306端口，但Django无法连接数据库；修改MySQL容器内部监听端口为3307后可正常运行，希望了解该问题的原因及让MySQL保持默认3306端口的正确配置方法。

初始相关配置 #

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数据库配置（settings.py） #

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

Dockerfile #

# 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

错误信息 #

启动容器后，Django抛出连接错误：

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

临时可行配置（修改MySQL容器端口） #

调整MySQL容器内部监听端口后可正常运行，修改后的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
      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

问题原因 #

Docker Compose创建的内部网络中，容器之间直接通过容器内部端口通信，而非主机映射的端口。初始配置中Django的PORT设为3307（主机映射端口），但MySQL容器内部实际监听的是默认3306端口，导致连接不匹配失败。临时配置修改了MySQL内部监听端口为3307，虽然能解决问题，但完全没必要改动MySQL的默认端口设置。

正确配置方法 #

只需调整Django的数据库配置，将PORT改为MySQL容器内部的默认端口3306即可，docker-compose.yaml的端口映射3307:3306保持不变：

修改后的Django数据库配置 #

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.mysql',
        'NAME': "backend",
        'USER': "asdf",
        'PASSWORD': "asdf",
        'HOST': "db",
        'PORT': "3306",  # 改为容器内部默认端口
    }
}

额外优化：确保MySQL服务就绪 #

depends_on仅保证容器启动顺序，不确保MySQL服务已完全就绪，建议添加等待脚本避免启动时的连接失败：

1. 修改api服务的command：

command: bash -c "pip install -q -r requirements.txt &&
  while ! nc -z db 3306; do sleep 1; done &&
  python manage.py migrate &&
  python manage.py runserver 0.0.0.0:8000"

2. 在Dockerfile中添加netcat工具：

RUN apt-get update && apt-get install -y netcat-traditional && rm -rf /var/lib/apt/lists/*

内容的提问来源于stack exchange，提问作者Matt Bliese