[Twisted-Python] Consensus on how to handle "MySQL server has gone away"
p.mayers at imperial.ac.uk
Wed Oct 3 04:28:06 EDT 2012
On 10/02/2012 06:09 PM, E S wrote:
> I have a service running that occasionally connects to a MySQL
> database. When there is no activity on it for some time, I eventually
> get the the error
> _mysql_exceptions.OperationalError: (2006, 'MySQL server has gone away')
> I have found a few posts on this group related to this issue
> as well as some tickets on the twisted homepage:
> I have also seen this referenced as a potential fix:
> I currently have the cp_reconnect parameter set to True, but it does
> not appear to be doing the job. I don't really see much consensus on
> how to properly handle this issue. Some people seem to think that the
> cp_reconnect parameter should take care of it for you, other people
> say that cp_reconnect is only part of the solution and that you have
> to write your own error handling.
For what it's worth - I think adbapi is seriously sub-optimal in this
regard. We have continual low-level problems with Twisted apps getting
stuck due to hung/dead ConnectionPool. And if you forget cp_reconnect,
well you are basically committing suicide. Your Twisted app will need a
In particular - it's not clear to my why CP isn't using "cp_good_sql" to
probe a connection *before* starting the transaction, and to
close/re-open it transparently if it has died and cp_reconnect==1.
Instead, the only place the "good" SQL is run is *after* a rollback, so
the next N transactions into the pool (where N is the number of threads)
all fail, because they don't get as far as "rollback".
I think the behaviour it should be aiming for is clear:
1. Test each connection with "good_sql" before beginning the user
2. If execeptions occur inside the user interaction, either at cursor
methods like execute, or connection methods like commit, then:
1. rollback - if *this* raises an exception, throw the conn away
2. propagate the original exception upwards unchanged (maybe
wrapped, maybe not)
cp_reconnect should be the default.
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