Try connection pre-ping

[rod-glover]

We are having trouble with connections being dropped by our proxy (or some other culprit), leading to an error in the backend. SQLAlchemy connection pre-ping is one possible "external" answer to that problem. However, note the following from that document:

It is critical to note that the pre-ping approach does not accommodate for connections dropped in the middle of transactions or other SQL operations. If the database becomes unavailable while a transaction is in progress, the transaction will be lost and the database error will be raised. While the Connection object will detect a “disconnect” situation and recycle the connection as well as invalidate the rest of the connection pool when this condition occurs, the individual operation where the exception was raised will be lost, and it’s up to the application to either abandon the operation, or retry the whole transaction again.

There is an alternative; see #221 .