Comment 12 for bug 1907250

One of the causes of a charm going into a "Failed to connect to MySQL" state is that a connection to the database failed when the db-router charm attempted to restart the db-router service. Currently the charm will only retry the connection in response to one return code from the mysql. The return code is 2013 which is "Message: Lost connection to MySQL server during query" *1. However, if the connection fails to be established in the first place then the error returned is 2003 "Can't connect to MySQL server on...".

*1 https://dev.mysql.com/doc/mysql-errors/8.0/en/client-error-reference.html