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:
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
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
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.