Business logic is the most important part of software. It’s the reason the software is being implemented in the first place. A system’s user interface can be sexy and its database can be blazing fast and scalable. But if the software is not useful for the busi‐ ness, it’s nothing but an expensive technology demo.

As we saw in Chapter 2, not all business subdomains are created equal. Different sub‐ domains have different levels of strategic importance and complexity. This chapter begins our exploration of the different ways to model and implement business logic code. We will start with two patterns suited for rather simple business logic: transac‐ tion script and active record.