优雅地处理"MySQL已经消失"

Question

我正在用Python编写一个小型数据库适配器,主要是为了好玩.我试图让代码优雅地从MySQL连接"消失",即wait_timeout超出的情况中恢复.我已经准备好了wait_timeout,10所以我可以尝试一下.

这是我的代码:

def select(self, query, params=[]):
        try:
            self.cursor = self.cxn.cursor()
            self.cursor.execute(query, params)
        except MySQLdb.OperationalError, e:
            if e[0] == 2006:
                print "We caught the exception properly!"
                print self.cxn
                self.cxn.close()
                self.cxn = self.db._get_cxn()
                self.cursor = self.cxn.cursor()
                self.cursor.execute(query, params)
                print self.cxn

        return self.cursor.fetchall()

接下来我等待十秒钟并尝试提出请求.这就是CherryPy的样子:

[31/Dec/2009:20:47:29] ENGINE Bus STARTING
[31/Dec/2009:20:47:29] ENGINE Starting database pool...
[31/Dec/2009:20:47:29] ENGINE POOL Connecting to MySQL...
[31/Dec/2009:20:47:29] ENGINE POOL Connecting to MySQL...
[31/Dec/2009:20:47:29] ENGINE POOL Connecting to MySQL...
[31/Dec/2009:20:47:29] ENGINE POOL Connecting to MySQL...
[31/Dec/2009:20:47:29] ENGINE POOL Connecting to MySQL...
[31/Dec/2009:20:47:29] ENGINE Started monitor thread '_TimeoutMonitor'.
[31/Dec/2009:20:47:29] ENGINE Started monitor thread 'Autoreloader'.
[31/Dec/2009:20:47:30] ENGINE Serving on 0.0.0.0:8888
[31/Dec/2009:20:47:30] ENGINE Bus STARTED
We caught the exception properly!   <====================================== Aaarg!
<_mysql.connection open to 'localhost' at 1ee22b0>
[31/Dec/2009:20:48:25] HTTP Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/CherryPy-3.1.2-py2.6.egg/cherrypy/_cprequest.py", line 606, in respond
cherrypy.response.body = self.handler()
  File "/usr/local/lib/python2.6/dist-packages/CherryPy-3.1.2-py2.6.egg/cherrypy/_cpdispatch.py", line 25, in __call__
    return self.callable(*self.args, **self.kwargs)
  File "adp.py", line 69, in reports
    page.sources = sql.GetSources()
  File "/home/swoods/dev/adp/sql.py", line 45, in __call__
    return getattr(self.formatter.cxn, parsefn)(sql, sql_vars)
  File "/home/swoods/dev/adp/database.py", line 96, in select
    self.cursor.execute(query, params)
  File "/usr/lib/pymodules/python2.6/MySQLdb/cursors.py", line 166, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/pymodules/python2.6/MySQLdb/connections.py", line 35, in defaulterrorhandler
    raise errorclass, errorvalue
OperationalError: (2006, 'MySQL server has gone away')

[31/Dec/2009:20:48:25] HTTP 
Request Headers:
  COOKIE: session_id=e14f63acc306b26f14d966e606612642af2dd423
  HOST: localhost:8888
  CACHE-CONTROL: max-age=0
  ACCEPT: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
  ACCEPT-CHARSET: ISO-8859-1,utf-8;q=0.7,*;q=0.3
  USER-AGENT: Mozilla/5.0 (X11; U; Linux x86_64; en-US) AppleWebKit/532.5 (KHTML, like     Gecko) Chrome/4.0.249.43 Safari/532.5
  CONNECTION: keep-alive
  Remote-Addr: 127.0.0.1
  ACCEPT-LANGUAGE: en-US,en;q=0.8
  ACCEPT-ENCODING: gzip,deflate
127.0.0.1 - - [31/Dec/2009:20:48:25] "GET /reports/1 HTTP/1.1" 500 1770 "" "Mozilla/5.0 (X11; U; Linux x86_64; en-US) AppleWebKit/532.5 (KHTML, like Gecko) Chrome/4.0.249.43 Safari/532.5"

为什么不工作？我清楚地捕获异常,重新生成连接和光标,但它仍然无法正常工作.它与MySQLdb如何获得连接有关吗？

Answer 1

从代码中看不到,但我的猜测是该db._get_cxn()方法正在进行某种连接池并返回现有的连接对象而不是创建一个新的连接对象.是否没有电话可以db刷新现有的无用连接？(你真的应该调用一个内部_预定的方法吗？)

为了防止MySQL has gone away我通常喜欢使用上一次使用它的连接保留时间戳.然后在尝试再次使用它之前,我会查看时间戳并关闭/丢弃连接,如果它是在几个小时前最后一次使用的话.这节省了使用a包装每个可能的查询try...except OperationalError...try again.