This post is part of The Software Architecture Chronicles, a series of posts about Software Architecture. In them, I write about what I’ve learned on Software Architecture, how I think of it, and how I use that knowledge. The contents of this post might make more sense if you read the previous posts in this series.
Layering is a common practice to separate and organise code units by their role/responsibilities in the system.
In an object-oriented program, UI, database, and other support code often gets written directly into the business objects. Additional business logic is embedded in the behaviour of UI widgets and database scripts. This happens because it is the easiest way to make things work, in the short run.
When the domain-related code is diffused through such a large amount of other code, it becomes extremely difficult to see and to reason about. Superficial changes to the UI can actually change business logic. To change a business rule may require meticulous tracing of UI code, database code, or other program elements. Implementing coherent, model-driven objects becomes impractical. Automated testing is awkward. With all the technologies and logic involved in each activity, a program must be kept very simple or it becomes impossible to understand.
Eric Evans 2014, Domain-Driven Design Reference