[eikenberry]

> Having two resources (such as DB and app) scale in concert can be exceedingly difficult.

This means your resources are too tightly coupled. If they are so tightly coupled that they need to scale together then they are not two resources and you should look into restructuring them into two actual resources or bind them more closely to make a single resource.

[xfitm3]

In my example of DB and backend: How could they be decoupled?

[eikenberry]

As far as runtime, applications and DBs are already decoupled. You have N application instances mapped to M database instances. Applications can usually scale pretty much with load. Databases vary wildly in how they scale and it depends on the DB type.

[analogj]

read only replicas, sharding, clustering, caching