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Gracefully handling "MySQL has gone away"

Tags:

python

mysql

I'm writing a small database adapter in Python, mostly for fun. I'm trying to get the code to gracefully recover from a situation where the MySQL connection "goes away," aka wait_timeout is exceeded. I've set wait_timeout at 10 so I can try this.

Here's my code:

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()

Next I wait ten seconds and try to make a request. Here's what CherryPy looks like:

[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"

Why doesn't this work?? I clearly catch the exception, regenerate both the connection and the cursor, but it still doesn't work. Is it related to how MySQLdb gets the connections?

like image 807
Sean Woods Avatar asked Jan 01 '10 01:01

Sean Woods


1 Answers

Can't see from the code, but my guess would be that the db._get_cxn() method is doing some kind of connection pooling and returning the existing connection object instead of making a new one. Is there not a call you can make on db to flush the existing useless connection? (And should you really be calling an internal _-prefixed method?)

For preventing MySQL has gone away I generally prefer to keep a timestamp with the connection of the last time I used it. Then before trying to use it again I look at the timestamp and close/discard the connection if it was last used more than a few hours ago. This saves wrapping every possible query with a try...except OperationalError...try again.

like image 100
bobince Avatar answered Sep 30 '22 18:09

bobince