Rockford describes n-tier architecture from both a logical and physical perspective. When most people talk about n-tier applications, they're talking about physical models in which the application is spread across multiple machines and different functions: a client, a web server, an application server, a database server, and so on.
Logical n-tier architecture (personally I call this a layered architecture) is about spreading different types of functionality. The most common logical separation is into a UI tier, a business tier, and a data tier that may exist on a single machine, or on three separate machines - the logical architecture doesn't define those details.
Rockford describes the Five Logical Tiers and the Roles They Provide:
Presentation - Renders display and collects user input
UI - Acts as an intermediary between the user and the business logic, taking use input and providing it to the business logic, then returning results to the user.
Business logic - Provides all business rules, validation, manipulation, processing, and security for the application.
Data access - Acts as an intermediary between the business logic and data management. Also encapsulates and contains all knowledge of data-access technologies (such as ADO.NET), databases, and data structures.
Data storage and management - Physically creates, retrieves, updates, and deletes data in a persistent data store.
Anchored object - When an anchored object is "passed" from one machine or process to another, only a pointer, or reference, is passed, not the object itself.
Unanchored object - When an unanchored object is "passed" from one machine or process to another, the object is physically copied and passed by value.
source: http://davidhayden.com/blog/dave/archive/2004/09/11/471.aspx
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