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Designing VB Data-Access Components for a Multitier Application

The role of the data-access component is central to the n-tier application model. In the standard client/server model, the components are divided into the following:
  • A client component, usually residing on local workstations and containing a user interface and business rules.

  • A server component that implements data access.

In the standard three-tier model, the components are as follows:

  • A client component containing the user interface services

  • A business logic component containing business rules

  • A data-access component

In an n-tier model, the components are as follows:

  • A client component containing the user interface services

  • One or more business logic components that implement various broad areas of business rules

  • One or more business data access components that mediate between the business logic components and the data-services component

  • A data-services component that provides data to the dataaccess components

In the context of VB6 development, a data-access component will typically be a COM component exposing one or more data-aware classes, as either of the following:

  • An ActiveX EXE (an out-of-process component) running on a server

  • An ActiveX DLL (an in-process component) running on a server under Microsoft Transaction Server

An important consideration for implementing individual data manipulation facilities is the decision about whether to locate them

  • With the data itself. In a SQL Server environment, for example, this would take the form of stored procedures or triggers.

  • In a separate data-access component, as discussed earlier.


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