Is using Drupal for a frontend and a non-Drupal API for a backend possible? For my design, I want to have Drupal handle all security and presentation.  My backend is an api that returns JSON.  All I could find showed Drupal as "headless," where Drupal was the back end.

I am trying to avoid building too much into Drupal and being dependent on it.

Is using Drupal for the frontend and having separate API for the back end possible?  It sounds like I have it backwards, but I do not to build everything in Drupal.  I want to do all the Database and Business logic outside of Drupal.

Edit:  This is for an Intranet where I hit many databases.
Drupal 7/8 can make use of Remote Entities which could be provided in a JSON format. But, what your describing seems needlessly complex. If you have a JSON based backend I would do something more simple like create the frontend using Javascript to talk to your backend, or maybe using a more generic php framework.

Attempting to use Drupal as a frontend and ignoring its data creation CRUD, Content Types and storage mechanisms -- sounds like big headache.