Learning Domain-Driven Design

Chapter 5. Implementing Simple Business Logic

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 business, it’s nothing but an expensive technology demo.

As we saw in Chapter 2, not all business subdomains are created equal. Different subdomains 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: transaction script and active record.

Transaction Script

Organizes business logic by procedures where each procedure handles a single request from the presentation.

Martin Fowler¹

A system’s public interface can be seen as a collection of business transactions that consumers can execute, as shown in Figure 5-1. These transactions can retrieve information managed by the system, modify it, or both. The pattern organizes the system’s business logic based on procedures, where each procedure implements an operation that is executed by the system’s consumer via its public interface. In effect, the system’s public operations are used as encapsulation boundaries.

Figure 5-1. Transaction script interface