Autoscale workers on Digital Ocean - digital-ocean

Question

I have 3 Webservers. DB, Web and Worker. The worker is just processing sidekiq processes all day long.
As soon the queue is over 100.000 jobs, I want to have a second worker instance and I do struggle a little bit with the right thinking of how to do it. (and if the queue is above 300.000 I need 3 workers, on and on).
I take my Worker and make a snapshot.
Via Digital-Ocean::API I will create a new instance, based on that image.
As soon as the instance is booting it needs to update the code from the Git-Repository
I need to tell the Database Server that it is allowed to receive connections from this instance IP
as soon the the queue is below 20.000 i can kill my instance.
Is this the right way of doing or are there better ways of how to do ? Am i missing something?
Additional Question:
On DB i only have mysql and redis. no ruby or anything else. so there is also no rails to run. If my worker decides, to create another worker, the new one needs to have access to mysql. It seems like to be impossible to create some access from a remote machine and it looks I need to create the access from the db server.
mysql> show grants;
+-----------------------------------------------------------------------------------------+
| Grants for rails#162.243.10.147 |
+-----------------------------------------------------------------------------------------+
| GRANT ALL PRIVILEGES ON *.* TO 'rails'#'162.243.10.147' IDENTIFIED BY PASSWORD <secret> |
| GRANT ALL PRIVILEGES ON `followrado`.* TO 'rails'#'162.243.10.147' |
+-----------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)
mysql> CREATE USER 'rails'#'162.243.243.127' IDENTIFIED BY 'swag';
ERROR 1227 (42000): Access denied; you need (at least one of) the CREATE USER privilege(s) for this operation

Answer 1

Is this the right way of doing or are there better ways of how to do ? Am i missing something?
Yes - seems reasonable
as soon the the queue is below 20.000 i can kill my instance.
Maybe let it linger around for a while before killing it in case the queue goes up again
On DB i only have mysql and redis. no ruby or anything else. so there is also no rails to run. If my worker decides, to create another worker, the new one needs to have access to mysql. It seems like to be impossible to create some access from a remote machine and it looks I need to create the access from the db server.
Yes, you need to create access from the db server - in general the access is granted to the entire VPC cidr and not just single server IPs [more common when static instances] - specially if you plan to launch dynamic instances with constantly changing IPs