MySQL Connection Timeout and SQLAlchemy Connection Pool Recycling
Recently a Pyramid web app surprised me with some odd behaviour. The app uses SQLAlchemy to interface with a MySQL database. The first request of the day would always fail with a traceback that ended with some sort of database connection failure, though not always the same error. This was happening in the development environment where I was the only one sending requests to the server. Hitting refresh, or sending another request resulted in the expected response, and things would continue to work that way - until the next morning...
The subtitle gives lots of hints about what was going on, so if you're still reading at this point I'll assume that you're still as puzzled as I was.
It took a surprising number of tries on Google before I found this section of the SQLAlchemy docs which explains all. To summarize, the default configuration of MySQL drops connections on which there has been no activity for 8 hours. SQLAlchemy provides the pool_recycle parameter for its engine creation functions as a way of working around that behaviour. (Although, as noted in the SQLAlchemy Engine creation API docs full description of pool_recycle, the behaviour is also configurable at the MySQLDB connection, and database configuration levels too.)
Since my Pyramid app uses the sqlalchemy.engine_from_config function, all I had to do was add:
sqlalchemy.pool_recycle = 3600
to my development.ini and production.ini config files, and ... problem solved.
P.S. Sorry - I couldn't resist the Bee Gees reference in the title. It's an age thing...