Pylons error - 'MySQL server has gone away'

Question

I'm using Pylons (a python framework) to serve a simple web application, but it seems to die from time to time, with this in the error log: (2006, 'MySQL server has gone away')

I did a bit of checking, and saw that this was because the connections to MySQL were not being renewed. This shouldn't be a problem though, because the sqlalchemy.pool_recycle in the config file should automatically keep it alive. The default was 3600, but I dialed it back to 1800 because of this problem. It helped a bit, but 3600 should be fine according to the docs. The errors still happen semi-regularly. I don't want to lower it too much though and DOS my own database :).

Maybe something in my MySQL config is goofy? Not sure where to look exactly.

Other relevant details:

  Python 2.5
  Pylons: 0.9.6.2 (w/ sql_alchemy)
  MySQL: 5.0.51

Answer 1

I think I fixed it. It's turns out I had a simple config error. My ini file read:

sqlalchemy.default.url = [connection string here]
sqlalchemy.pool_recycle = 1800

The problem is that my environment.py file declared that the engine would only map keys with the prefix: sqlalchemy.default so pool_recycle was ignored.

The solution is to simply change the second line in the ini to:

sqlalchemy.default.pool_recycle = 1800

Answer 2

You might want to check MySQL's timeout variables:

show variables like '%timeout%';

You're probably interested in wait_timeout (less likely but possible: interactive_timeout). On Debian and Ubuntu, the defaults are 28800 (MySQL kills connections after 8 hours), but maybe the default for your platform is different or whoever administrates the server has configured things differently.

AFAICT, pool_recycle doesn't actually keep the connections alive, it expires them on its own before MySQL kills them. I'm not familiar with pylons, but if causing the connections to intermittently do a SELECT 1; is an option, that will keep them alive at the cost of basically no server load and minimal network traffic. One final thought: are you somehow managing to use a connection that pylons thinks it has expired?