Connecting the Three Tiers
So far you've learned that by using Windows DNA, you can split your application's services into three parts: user services components, business services components, and data services components. Now you will learn how you can tie the three parts together. In Windows DNA, this is done by exposing system and application services through the Component Object Model (COM).
The foundation of Windows is COM, which has become COM+ in Microsoft Windows 2000. (COM as used here refers to both COM and COM+.) COM provides a way for applications to communicate with COM objects. COM also enables one object to communicate with another object through method calling and property setting. How a COM object actually performs its services is not important. Objects hide or encapsulate the details of how they work. Because an object hides how it performs its services, you can change the way an object performs its services without breaking the application that uses the object.
If the services components in each tier are COM components, the communication between tiers will be COM method calls. To be specific, your user services components will call the methods of the business services components to request services from the business services tier, and your business services components will call the methods of the data services components to ask for services from the data services tier.