Data Access and Transactions

Designing Custom Business Objects

The main application components on the middle tier are business objects that contain information such as application logic, business rules, and specialized data-access code, in order to retrieve information from underlying databases. Business objects can be created with Visual Basic, C++, or any other language that supports COM.

The life span of a business object is as long as the execution of the method call invoked by the client-side RDS.DataSpace object. Business objects are created and destroyed with each method call, so no interim state is maintained. Therefore, custom business objects designed for use with RDS need to be stateless (don’t use properties or static data). If state needs to be saved between invocations to the object, consider using the Shared Property Manager. A feature of Component Services, the Shared Property Manager simplifies programming of components that need to save state information or share state with other components. For more about working with business objects, see Transactional Components.

When designing business objects, keep the following in mind:

RDS can only marshal back one Recordset at a time. Beware of procedures that return two or more result sets, since they will require special handling by your business object. You can use the new MSDataShape provider to return hierarchical Recordset objects, but the data shape query language doesn’t support stored procedures at this time.
RDS cannot marshal user-defined data types. Return only basic data types (Integer, String, and so on), Recordset objects, or Variants. Variant arrays are also supported.
Pass and return ADOR recordsets, not ADODB.
Use BYVAL parameters whenever possible. BYREF (by reference) parameters require two network trips, so using the BYVAL keyword to pass parameters by value saves time. BYREF is the default behavior of Visual Basic, so you must type BYVAL wherever you need it.
Business objects created with the RDS.DataSpace object cannot access ASP objects, including Session or Application state.

You can use either RDS or ADO, or a combination of both technologies, in the same application. For example, your application can create a custom business object that uses ADO to manage server-side data, and that uses RDS to transfer remote data to the client tier, where the user can interact with it.

The next section discusses the issues involved in creating a server-side ADO application, using ASP and COM components.