Layered Architecture organizes source code into layers, each responsible for a particular kind of work. It separates receiving requests, applying business rules and accessing data so that the code is easier to understand and maintain.
The layers can run within one application. Layers describe code responsibilities; placing parts on separate servers is a deployment concern addressed by N-Tier Architecture.
A web application often has three layers:
In traditional layering, presentation calls business logic, which calls data access. This keeps database queries and business rules out of the presentation code.
Consider a leave-management application: When an employee submits a request, presentation receives the dates and passes them to business logic. Business logic retrieves the remaining allowance through data access and checks the request.
If it is valid, the application saves the request for approval and returns a confirmation. If the allowance is insufficient, business logic rejects the request and presentation displays the reason. In this example, leave is deducted later during approval.
When the company changes its leave policy, developers update the business layer. Presentation and data storage can stay unchanged if the information exchanged remains the same.
Layered Architecture suits administration tools, HR applications and systems with many forms and familiar business workflows. For a larger system, divide the code into business modules and apply layers within each module, avoiding one enormous business layer that contains every feature.
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