CML Architecture

The Corporate Media Library (CML) is a three-tier application. It is intended for use on an intranet, specifically the internal network of the Fitch & Mather (F & M) Corporation. However, most of the functionality of the CML application would also be available over the Internet. (One limitation is that to obtain full functionality, a library user must be a recipient listed in the Microsoft® Exchange Server directory store used by the CML. Those who are not can browse the library catalog but cannot check out items.) For more information, see Future Enhancements.

The CML's functionality works on three tiers:

• User-services tier
• Business-services tier
• Data-services tier

The following diagram shows where many of the most important technologies used for n-tier development operate within this three-tier model. Refer to this diagram when reading the rest of this topic, which discusses where the various elements of the CML are located.

Location of CML Elements

The various parts of the CML application are distributed over the three tiers as follows:

• The user-services tier is a rich Web client accessed through Microsoft Internet Explorer version 4.01 or later. The CML setup places a Component Object Model (COM) component and a Visual Basic® run-time dynamic-link library (DLL) on the user-services tier, which is also where the library user's browser runs. This is also where an RDS ActiveX® component used by the CML runs.
• Logic at the business-services tier runs on one of F & M's Web servers, powered by Internet Information Server (IIS) version 4.0 and Windows NT® Server version 4.0. Five of the CML custom-built COM components run on the business-services tier, which is physically located on the Web server. This middle tier is also the location of ISAPI and ASP, which processes the script that generates the CML's user interface (UI) and communicates with the business-logic code of the COM components. Because ASP is so important to the creation of script that defines UI elements, it can be argued that it belongs on the user-services tier. Nevertheless, this guide places it on the business-services tier.
• The data-services tier stores data and provides related services. Of greatest importance for the CML is a Microsoft SQL Server™ database containing the F & M catalog of library materials. Within this database also reside a number of stored procedures used by the CML. (This database could have been located on the same computer as IIS. For information on deciding where to place application components, see Deployment.) This layer also contains Microsoft Exchange Server and its directory, which is used by the CML to communicate with library users in various ways. Finally, Index Server (also on the data-services tier) is used by the CML to generate catalogs of text to enable rapid full-text search for library users.