# django/db/backends/mysql/base.py

class DatabaseWrapper(BaseDatabaseWrapper):
    vendor = 'mysql'
	...
	...
	...
    def get_new_connection(self, conn_params):
	    # 每次查询都会重新建立连接
        return Database.connect(**conn_params)
	...
	...
	...

# django/db/backends/base/base.py

class BaseDatabaseWrapper:
    """Represent a database connection."""
    # Mapping of Field objects to their column types.
    data_types = {}
	...
	...
	...

    def _close(self):
        if self.connection is not None:
            with self.wrap_database_errors:
                # 每次查询完又要调用 close 关闭连接
                return self.connection.close()
	...
	...
	...

...
DATABASES = {
    'default': {
        'ENGINE': 'db_pool.mysql',     # 重写 mysql 连接库实现连接池
        'NAME': 'devops',
        'USER': 'devops',
        'PASSWORD': 'devops',
        'HOST': '192.168.223.111',
        'PORT': '3306',
        # 'CONN_MAX_AGE': 600,    # 如果使用 db_pool.mysql 绝对不能设置此参数，否则会造成使用连接后不会快速释放到连接池，从而造成连接池阻塞
        # 数据库连接池大小，mysql 总连接数大小为：连接池大小 * 服务进程数
        'DB_POOL_SIZE': 3,     # 默认 5 个
        'OPTIONS': {
            'init_command': "SET sql_mode='STRICT_TRANS_TABLES'",
         },
    }
}
...

db_pool/
├── __init__.py
└── mysql
    ├── base.py
    └── __init__.py

# -*- coding: utf-8 -*-
from django.core.exceptions import ImproperlyConfigured
import queue
import threading

try:
    import MySQLdb as Database
except ImportError as err:
    raise ImproperlyConfigured(
        'Error loading MySQLdb module.\n'
        'Did you install mysqlclient?'
    ) from err

from django.db.backends.mysql.base import *
from django.db.backends.mysql.base import DatabaseWrapper as _DatabaseWrapper

DEFAULT_DB_POOL_SIZE = 5


class DatabaseWrapper(_DatabaseWrapper):
    """
    使用此库时绝对不能设置 CONN_MAX_AGE 连接参数，否则会造成使用连接后不会快速释放到连接池，从而造成连接池阻塞
    """
    connect_pools = {}
    pool_size = None
    mutex = threading.Lock()

    def get_new_connection(self, conn_params):
        with self.mutex:
            # 获取 DATABASES 配置字典中的 DB_POOL_SIZE 参数
            if not self.pool_size:
                self.pool_size = self.settings_dict.get('DB_POOL_SIZE') or DEFAULT_DB_POOL_SIZE
            if self.alias not in self.connect_pools:
                self.connect_pools[self.alias] = ConnectPool(conn_params, self.pool_size)
            return self.connect_pools[self.alias].get_connection()

    def _close(self):
        with self.mutex:
            # 覆盖掉原来的 close 方法，查询结束后连接释放回连接池
            if self.connection is not None:
                with self.wrap_database_errors:
                    return self.connect_pools[self.alias].release_connection(self.connection)


class ConnectPool(object):
    def __init__(self, conn_params, pool_size):
        self.conn_params = conn_params
        self.pool_size = pool_size
        self.count = 0
        self.connects = queue.Queue()

    def get_connection(self):
        if self.count < self.pool_size:
            self.count = self.count + 1
            return Database.connect(**self.conn_params)
        conn = self.connects.get()
        try:
            # 检测连接是否有效，去掉性能更好，但建议保留
            conn.ping()
        except Exception:
            conn = Database.connect(**self.conn_params)
        return conn

    def release_connection(self, conn):
        self.connects.put(conn)

def get_connection(self):
        if len(self.connects) < self.pool_size:
            new_connect = Database.connect(**self.conn_params)
            self.connects.append(new_connect)
            return new_connect
        index = random.randint(0, self.pool_size - 1)   # 随机返回连接池中的连接
        try:
            # 检测连接是否有效，去掉性能更好，但建议保留
            self.connects[index].ping()
        except Exception:
            self.connects[index] = Database.connect(**self.conn_params)
        return self.connects[index]