MySQL - Lost connection to MySQL server during query

[panakouris]

I finally managed to reproduce this issue mentioned also in #265, and found a possible solution to this. The issue is that after some period of inactivity from the time I was getting the driver with tc.get_driver() to the time I was interacting with the database (for example with driver.get_datasets()), I keep getting this error message OperationalError: (2013, 'Lost connection to MySQL server during query'). From a small test I performed, I saw that the period of inactivity is not always the same, most of the times between 5 and 20 seconds, which means that the reason for the dropped connection is not some hardcoded timeout. The solution that I have tested and seems to work fine is setting pool_pre_ping=True when creating the sqlalchemy engine https://github.com/DHI-GRAS/terracotta/blob/2e8bcdb718bd911569248c0cd1482a29c0ae24a1/terracotta/drivers/relational_meta_store.py#L87 .
From SQLAlchemy docs The “pre ping” feature will normally emit SQL equivalent to “SELECT 1” each time a connection is checked out from the pool; if an error is raised that is detected as a “disconnect” situation, the connection will be immediately recycled, and all other pooled connections older than the current time are invalidated, so that the next time they are checked out, they will also be recycled before use.
I tested up to 400 seconds of inactivity between tc.get_driver() and driver.get_datasets(), and I haven't received any error. What do you think @Nick0693 and @dionhaefner ?

[dionhaefner]

Sounds reasonable to me.