最近我们的Django项目供Java Sofa应用进行tr调用时, 经常会出现一个异常: django.db.utils.OperationalError: (2006, 'MySQL server has gone away'). 本文记录了分析, 本地重现与解决此问题的全过程.

原因分析:

Django在1.6引入长链接(Persistent connections)的概念, 可以在一个HTTP请求中一直用同一个连接对数据库进行读写操作.

但我们的应用对数据库的操作太不频繁了, 两次操作数据库的间隔大于MySQL配置的超时时间(默认为8个小时), 导致下一次操作数据库时的connection过期失效.

Our databases have a 300-second (5-minute) timeout on inactive connections. That means, if you open a connection to the database, and then you don’t do anything with it for 5 minutes, then the server will disconnect, and the next time you try to execute a query, it will fail.

重现问题:

设置mysql wait_timeout为10s

在macOS上的mysql配置文件路径: /usr/local/etc/my.cnf

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# Default Homebrew MySQL server config

[mysqld]

# Only allow connections from localhost

bind-address = 127.0.0.1

wait_timeout = 10

interactive_timeout = 10

重启mysql:

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➜ ~ brew services restart mysql

Stopping `mysql`... (might take a while)

==> Successfully stopped `mysql` (label: homebrew.mxcl.mysql)

==> Successfully started `mysql` (label: homebrew.mxcl.mysql)

检查wait_timeout的值是否已被更新.

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mysql> show variables like '%wait_timeout%';

+--------------------------+----------+

| Variable_name | Value |

+--------------------------+----------+

| innodb_lock_wait_timeout | 50 |

| lock_wait_timeout | 31536000 |

| wait_timeout | 10 |

+--------------------------+----------+

3 rows in set (0.00 sec)

重现exception:

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>>> XXX.objects.exists()

True

>>> import time

>>> time.sleep(15)

>>> XXX.objects.exists()

True

>>> XXX.objects.exists()

...

django.db.utils.OperationalError: (2013, 'Lost connection to MySQL server during query')

>>> XXX.objects.exists()

...

django.db.utils.OperationalError: (2006, 'MySQL server has gone away')

有意思的一个点是, sleep 10s 之后, 第一次操作数据库, 会出现(2013, 'Lost connection to MySQL server during query’)异常. 之后再操作数据库, 才会抛出(2006, 'MySQL server has gone away’)异常.

解决问题:

第一个最暴力的方法就是增加mysql的wait_timeout让mysql不要太快放弃连接. 感觉不太靠谱, 因为不能杜绝这种Exception的发生.

第二个办法就是手动把connection直接关闭:

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>>> Alarm.objects.exists()

True

>>> from django.db import connection

>>> connection.close()

>>> time.sleep(10)

>>> Alarm.objects.exists()

True

>>>

发现不会出现(2006, 'MySQL server has gone away’)异常了, 但总感觉还是不够优雅.

最终决定在客户端(Django), 设置超时时间(CONN_MAX_AGE: 5)比mysql服务端(wait_timeout = 10)小:

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DATABASES = {

'default': {

'ENGINE': 'django.db.backends.mysql',

'CONN_MAX_AGE': 5,

}

}

但很奇怪没有生效??? 看了源代码, 发现只有在request_started(HTTP request)和request_finished的时候, 在close_if_unusable_or_obsolete才用到CONN_MAX_AGE并去验证时间关闭connection.

具体代码见: python3.6/site-packages/django/db/__init__.py#64

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# Register an event to reset transaction state and close connections past

# their lifetime.

def close_old_connections(**kwargs):

for conn in connections.all():

conn.close_if_unusable_or_obsolete()

signals.request_started.connect(close_old_connections)

signals.request_finished.connect(close_old_connections)

而我的代码是处理一个任务而不是HTTP请求, 所以不会触发这个signal. 于是我写了一个装饰器, 在任务的开始和结束的时候, 关闭所有数据库连接.

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from django.db import connections

# ref: django.db.close_old_connections

def close_old_connections():

for conn in connections.all():

conn.close_if_unusable_or_obsolete()

def handle_db_connections(func):

def func_wrapper(request):

close_old_connections()

result = func(request)

close_old_connections()

return result

return func_wrapper

# ------割-------

@handle_db_connections

def process_trsbrain_request(request):

...

ps. CONN_MAX_AGE默认其实为0, 意味着默认在http请求和结束时会关闭所有数据库连接.

其他:

django.db中connection和connections的区别???

connection对应的是默认数据库的连接, 用代码表示就是connections[DEFAULT_DB_ALIAS]

connections对应的是setting.DATABASES中所有数据库的connection