Imagine for a moment that our bookstore website is quite successful, so we think of building a mobile app to increase our market. Of course, we would want to use the same database that we use for our website, as we need to sync the books that people borrow or buy from both apps. We do not want to be in a position where two people buy the same last copy of a book!

Not only the database, but the queries used to get books, update them, and so on, have to be the same too, otherwise we would end up with unexpected behavior. Of course, one apparently easy option would be to replicate the queries in both codebases, but that has a huge maintainability problem. What if we change one single field of our database? We need to apply the same change to at least two different codebases. That does not seem to be useful at all.

Business logic plays an important role here too. Think of it as decisions you need to take that affect your business. In our case, that a premium customer is able to borrow...