As the name suggests, this layer is responsible for implementing and encapsulating the program’s business logic. This is the place where business decisions are imple‐ mented. As Eric Evans says,1 this layer is the heart of software.

This layer is where the business logic patterns described in Chapters 5–7 are imple‐ mented—for example, active records or a domain model (see Figure 8-3).

Figure 8-3. Business logic layer