Visual Basic —

Tiers and Data

Tiered systems built with Visual Basic are essentially systems of class instances provided by a number of physical components. The components can be deployed on a number of machines interacting to produce system behavior. This means the class instances share their data with the system through class interface methods, properties, and events. Figure 12-5 illustrates how data and objects fit together at the simplest level.

Figure 12-5 Objects and their data.

A class is a template of functionality-it is a generic functional wrapper around data variables. It comes to life when you createan instance of it and give it some specific data to hold in its variables. It is possible to have data-only classes composed only of properties. It is equally possible to have functionality-only classes composed entirely of methods. In both cases a developer's first reaction to such classes should be to question their design. However, such classes are the exception rather than the rule.

Objects, State, and Data: The Buxom Server

In an object, the data can ultimately come from the two ends of a tiered system: the user (or some other form of) input, or some persistent data storage (usually a database). Some data is transient, ephemeral, short-lived stuff-the data equivalent of a mayfly. Most, however, requires storage and has persistence. It is typically a design goal of scalable COM-based systems that the objects they are made of are as stateless as possible. In other words, specific instances of a class (and thus specific sets of data) stay "live" and in memory for the briefest time. There are many good reasons for this. Principally this is so that we minimize the impact of idle instantiated objects on server system resources such as CPU and memory. At TMS, for any sizable system we typically would attempt to assess the likely impact of this on a server, as early as possible in the design process (see the scalability example spreadsheet in the samples directory for this chapter).

There is always a compromise to be made between the benefits of preinstantiation and the overhead of idle instances. Consider the case of the Buxom server-a publicly instantiable class provided by an ActiveX EXE server, sitting on a machine (The Bar) remote from its clients (thirsty punters like you and I). Its purpose is to stand as the software equivalent of a bartender in a system. Consider the process of ordering a round of drinks from an instance of the Buxom Server.