N-Layer Architecture

A N-Layer Architecture is an architectural style.

What is an Architectural Style?

An architectural style is a set of principles. Such principles intend to improve partitioning and promote design reuse by providing solutions to frequently recurring problems. It also provides a common language that abstracts the understanding of the problem away from the implementation, facilitating a higher level conversation.

A great example for the Web is the Model-View-Controller architecture. It doesn't matter the programming language of the implementation, because the principles are the same.

The simple graph presented above is enough to represent the basic idea of the system's architecture, facilitating communication and understanding.

Applying the design principle of replaceability, if a company decides to switch database engine, it is just a matter of switching the Model layer, instead of rewriting the whole system. Besides, if the implementation follows the architectural guidelines, other 2 important principles come into play: principle of least knowledge and abstraction, so that the controller and view should not be highly affected by a change in the Model layer.

Where MVC Falls Short

In Web development, MVC is the most famous and widely used architecture in the present days. It is adopted due to its simplicity and suitability, not only on the Web scenario.

However, the MVC architecture, as in ASP.NET MVC, falls short in some aspects:

Multipurpose controllers. Controllers are the core of the application, dealing with multiple responsibilities. Not only it implements the business logic, it also deals with manipulation and validation of data being transfered between the model and the view. This fact conflicts with the single responsibility principle, over sizing the controller.
Distributed services. If the application requires an API (implying multiple ways of presenting data), a whole new controller must be implemented, or the existing one must be extended (adding another purpose), also over sizing the controller.
Lack of an ORM interface. The controller access the Model's Object-Relational Mapper directly. A change in the ORM implies a reimplementation of the controller, breaking the abstraction and least knowledge principles.
Background and real time services. Require implementation inside the controller, over sizing it once again.

The points presented above don't imply that choosing MVC is a bad choice. On the contrary, it is an amazing choice. but not the best when the application increases in complexity. What is being proposed is an improvement in the current MVC architecture: the N-Layer architecture.

The N-Layer Architecture

N-Layer data applications are data applications that are separated into multiple layers, which clearly separate processing into discrete layers. This definition seems a little bit vague, but it is vague enough to fit MVC into it. Therefore, MVC is also a N-Layer architecture.

What is being proposed here is to extend the MVC architecture, as a N-Layer architecture, to fit the requirements of the application.

Originally, the MVC layers have this purpose:

Model. Layer that implements the logic for the application's data domain.
Controller. Layer that has components that handle user interaction, working with the model and selecting a View to be displayed.
View. Layer that displays the application's user interface (UI).

A suggested expansion would be:

Data Access Layer (DAL). Provides access to data stored in a persistent storage. A switch from MySQL to PostgreSQL would affect only this layer. A switch from Entity Framework to N-Hibernate as well.
Entity Objects. Encapsulates the business model. This is similar to the Model layer, where classes are defined to represent persistent data structure.
Business Logic Layer. The core of the application. Contains all business logic that is constantly used to modify the business' persistent data.
Business Façade Logic Layer. Provides a consistent interface that isolates the implementation of the Business Logic Layer from other layers.
Service Interfaces. A interface used as entry point for consumers to access the Business Logic Layer.
UI Controllers. Unlike MVC's Controller, the N-Layer Controller is used as a UI Controller. It manipulate data to be displayed by a View.
UI View. The same as the MVC View.

A generic graph of the N-Layer can be viewed bellow:

It is possible to see that the Controllers are View-specific. Interface and façade are added to the stack, making the implementation of the Business Logic Layer (Domain Layer) transparent to the layers above it.

It is also possible to notice a new aspect: cross-cutting layers (security, operations, communications). These are layers that are commonly used and accessed by all layers.