These answers are a bit outdated. Google App Engine instances can connect to an external database server as per https://cloud.google.com/appengine/docs/php/using-third-party-databases

It's worth noting that the ability to connect externally requires that the account be a "paid account" a/k/a "billing enabled". See: https://cloud.google.com/appengine/docs/php/runtime#PHP_Functions_that_requires_billing_enabled

These answers are a bit outdated. Google App Engine instances can connect to external database servers.

The ability to connect externally requires that the account the App Engine is running under be a "paid account" a/k/a "billing enabled".

References:

-https://cloud.google.com/appengine/docs/php/using-third-party-databases

-https://cloud.google.com/appengine/docs/php/runtime#PHP_Functions_that_requires_billing_enabled