What does "FDProxy.Open error 2" mean when accessing CloudSQL from Go App Engine

Question

We sometimes get the error "FDProxy.Open error 2" back from CloudSQL when attempting to execute a query. I think that this means "connection refused because there are already too many open connections" but I'd love some official confirmation of this as I'm not positive. I'm also not sure if it always means this or if there are also other error states (e.g. write QPS too high) that will also trigger this error. If it matters, we are using the Go-MySQL-Driver although we have also seen it when using the mymysql driver.

Answer 1

We currently have a limit of 30 Cloud SQL connections per App Engine app instance (see the Warning from [1]) but that restriction can sometimes be applied to a total number of connections from a subset of those. This will be fixed in the App Engine 1.9.5 release when a strict per App Engine instance limiting will be rolled out. Sorry for the pain. :-(

[1] https://developers.google.com/appengine/docs/java/cloud-sql/#Java_Connect_to_your_database