We do not expect changes in this layer to affect the entities. These use cases orchestrate the flow of data to and from the entities, and direct those entities to use their enterprise wide business rules to achieve the goals of the use case. It encapsulates and implements all of the use cases of the system. The software in this layer contains application specific business rules. No operational change to any particular application should affect the entity layer. For example, you would not expect these objects to be affected by a change to page navigation, or security. They are the least likely to change when something external changes. They encapsulate the most general and high-level rules. If you don’t have an enterprise, and are just writing a single application, then these entities are the business objects of the application. It doesn’t matter so long as the entities could be used by many different applications in the enterprise. An entity can be an object with methods, or it can be a set of data structures and functions. EntitiesĮntities encapsulate Enterprise wide business rules. We don’t want anything in an outer circle to impact the inner circles. variables, or any other named software entity.īy the same token, data formats used in an outer circle should not be used by an inner circle, especially if those formats are generate by a framework in an outer circle. In particular, the name of something declared in an outer circle must not be mentioned by the code in the an inner circle. Nothing in an inner circle can know anything at all about something in an outer circle. This rule says that source code dependencies can only point inwards. The overriding rule that makes this architecture work is The Dependency Rule. In general, the further in you go, the higher level the software becomes. The concentric circles represent different areas of software. The diagram at the top of this article is an attempt at integrating all these architectures into a single actionable idea. In fact your business rules simply don’t know anything at all about the outside world. Your business rules are not bound to the database. You can swap out Oracle or SQL Server, for Mongo, BigTable, CouchDB, or something else. A Web UI could be replaced with a console UI, for example, without changing the business rules. The UI can change easily, without changing the rest of the system. The business rules can be tested without the UI, Database, Web Server, or any other external element. This allows you to use such frameworks as tools, rather than having to cram your system into their limited constraints. The architecture does not depend on the existence of some library of feature laden software. Each has at least one layer for business rules, and another for interfaces.Įach of these architectures produce systems that are: They all achieve this separation by dividing the software into layers. They all have the same objective, which is the separation of concerns. Though these architectures all vary somewhat in their details, they are very similar.
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