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VB6 Database

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Client Server Architecture

Client Server Architecture is a process involving a minimum of two independant entities, one is the client and the other is the server. The client makes a request to the server and the server services the requests an dthe resulting data is sent to the client. In this application, two separate applications operating independantly could work together to complete a task.

A well brought out implementation of this concept is the SQL based database Management System. The client server architecture is well illustrated in figure shown below. Here, unlike the File Server System, the requests that goes to the server is not merely a request for file in the form of disk input/output requests which are returned as series of disk blocks. Intead, instructions are communicated to an application running on the server, and the server executes those requests and sends a response to the client.

Two-Tier Client Server Model

In a two-tier model, a desktop machine operates as a client and a network server functions as a back-end database engine. The logic in a two-tier model is split between the two physical locations namely the server and the client. The client in a two-tier model is necessarily a 4GL like Visual Basic or PowerBuilder. The salient point for a two-tier application is that the business logic must physically reside either on the client or be implemented on the back-end within the DBMS in the form of triggers and stored procedures. Both triggers and procedures are stored as precompiled collections of SQL statements and control-of-flow statements.

In Visual Basic, using any of the data controls that provide a graphical link to the back-end data source creates a two-tier client/server relationship.

The advantages of this two-tier model are - data access is simplified, allowing very rapid development of applications. The GUI is bound directly to the data source and all the details of data manipulation are handled automatically. But there is a disadvantage in this, i.e. though the data access is simplified, it is less flexible due to which the user will not have complete control of interactions with the data source.

Limitations of Two=Tier Client Server Model

Not Scalable - Inability of a two-tier approach to grow beyond the physical boundaries of a client and server machine.

Unmanageable - Business rules cannot be encapsulated and deployed centrally because of which sharing common processes and reusing work becomes difficult.

Poor Performance - The graphical interface binding to the data consumes major resources on the client machine which results in poor performance and dissatisfied clients.

Tree-Tier Client/Server Model

An improved model for client server development resulted as an outcome of limited effectiveness of two-tiered client/server solutions. The three-tier client/server model is based on the ability to build partitioned applications. Partitioning an application breaks our code into logical components. Each tier is called a service. These services are shared between the client and the servers. The Service Model employed to design this architecture suggests that these components can be logically grouped into three tiers: User services, Business Services and Data Services. The service and its location is given below.


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