Your suggestion of splitting vertical layers of Parse Server, as in “you have the auth, (…) another the db, and in another the clouds” does not seems to make much sense in the context of Parse Server.
For example, if a Cloud Function that contains a query is executed, the request will go to the Cloud Code endpoint, validate through auth and then communicate with the DB. It involves all 3 of the layers you mentioned above.
What you could do it to split up horizontally. For example, if you have 2 Cloud Code functions, you could run function 1 only on Parse Server instance A, and function 2 only on Parse Server instance B. The request routing would determine to which server instance (or microservice) you are sending the request. That routing is outside the scope of Parse Server - naturally - because it needs to happen before the request reaches the server.
Through routing you could split Parse Server endpoints (
/hooks, etc) into different microservices, or if you have a more fine grained routing, you could split by geographic origin or request header fields. But again this is a feature of your cloud environment, not of Parse Server.