Hosting Components

The primary function of the middle tier of the three-tier model is to provide an execution environment for the core business logic of an application-logic that is implemented in the form of components. This execution environment loads and unloads the components, handles inter-component communication, and provides standardized implementations of common services required by the components. Therefore, to understand how Windows Server 2003 and other application servers work, it is first necessary to understand the role of components. Specifically, in order to understand the role of components in Windows Server 2003 it is necessary to understand the following:

• The two component models supported by Windows Server 2003
• How additional services (such as message queuing) are provided to components
• The component improvements made in Windows Server 2003 compared with its predecessor, Windows 2000.

Component Models

A component is a piece of software that is designed to be reused or shared across multiple applications, that hides its internal implementation, and that presents its functionality through a well-defined API. Clients (either applications or other components) of a component make requests through the component’s API. For example, an order-entry component might present an interface that allows another component to supply a customer’s name, address, and zip code, and then have the component validate the zip code against the address. The exact way in which the component performs the validation is hidden and can even be changed over time without affecting other components that rely on it for zip code validation.