Although some of you may have developed VB applications in the past, chances are they were designed to run on single computers as monolithic applications. Distributed applications, on the other hand, are usually designed to be capable of running on two or more computers.
Client/Server Architecture
The most common model of a distributed application is the client/server application model:
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A client/server application runs the client processes separately from the server processes, usually on a different computer. The main points of a client/server application are:
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The client processes provide an interface for the user, and gather and present data, usually either on a screen on the user's computer or in a printed report. This part of the application is also called the user tier or the presentation layer.
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The server processes provide an interface with the data store. It is, for this reason, also called the data tier or the data layer.
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The logic that validates data, monitors security and permissions, and performs other business rules can be contained on either the client or the server, or split between the two.
This type of distributed application may be executed over more than one computer, although it doesn't have to be – both user tier and the data tier may be run on a single machine. The client/server model is often used with database programs where a front end program, often designed in Visual Basic or Microsoft Access, is used to present the user with data that is acquired from a database server.
A client/server application is an example of a 2-tier distributed application. The logic for client/server applications can also be designed to form a separate middle tier instead of running either in the client or the server or being split between the two applications. This middle tier is often called the business tier or the business logic layer. Client/server applications that are designed this way have three logical tiers but run in two physical tiers. The middle tier may be contained in either the client or the server.
The Fat and Thin Client Models
Client/server applications that are designed to run the user and business tiers of the application on the client, and the data tier on the server are known as fat client applications.
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On the other hand, client/server applications that are designed to run the user tier on the client and the business and data tiers on the server are known as thin client applications. As you can see, even though such applications may be intended to run on two computers, they may implement three logical tiers.
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