[This is preliminary documentation and subject to change]

# The New System Design

Today's client/server applications resemble their ancestors so little that they have been given a new name, the multitier application, also known as n-tier architecture. In this model, processing is distributed between the client and the server, and business logic is captured in a middle tier. Most systems will perform the following three main tasks, which correspond to three tiers, or layers, of the n-tier model:

Task | Description
User interface and navigation | Labeled Tier 1 in the following graphic, this layer comprises the entire user experience. Not only does this layer provide a graphical interface so that users can interact with the application, input data, and view the results of requests, it also manages the manipulation and formatting of data once the client receives it. In Web applications, the browser performs the tasks of this layer.
Business logic | Tier 2, between the interface and data services layers, is the domain of the distributed application developer. Business logic, which captures the rules that govern application processing, connects the user at one end with the data at the other. The functions that the rules govern closely mimic everyday business tasks, and can be a single task or a series of tasks.
Data services | Shown as Tier 3 in the following graphic, data services are provided by a structured (SQL, Oracle database) or unstructured (Microsoft® Exchange, Microsoft® Message Queuing) data store, which manages and provides access to the application data. A single application may enlist the services of one or more data stores.

The three-tier architecture isolates each major piece of functionality, so that the presentation is independent of the processing rules and business logic, which in turn is separate from the data. This model requires much more analysis and design up front, but greatly reduces maintenance costs and increases functional flexibility in the long run. The following diagram depicts the Microsoft technologies that service the various tiers in the new system design.

(The following words are used by Search; they are not intended to be displayed on the page.) MSMQ

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