HOWTO: Implement a Real-Time Application Using OLE

• Microsoft Visual Basic Professional and Enterprise Editions, 16-bit and 32-bit, for Windows, version 4.0

SUMMARY

Real-time applications are used to solve a wide range of business problems. For example, an application may have a connection to a data source where certain data elements are continuously changing. The application needs to be aware of these changes and needs to perform some actions accordingly. One solution would be for the application to continuously poll the data source. This would constitute a great performance hit to the application and will virtually suspend other application functions.

A better alternative would be to have a backend poll the data source independently and notify the application when changes take place. Such a model can be easily implemented using OLE automation. This article gives a broad description of how to do it.

MORE INFORMATION

The backend model is based on asynchronous notifications. The way to do this with OLE is to have the OLE client application pass an object reference to the OLE server. The server then calls a method of that object when it's time to notify the client. The server runs in the background doing periodic tests to see if the threshold conditions for notification are met.

Here's an example. The OLE client application exposes a Receiver object with a Notify method. The parameters of the Notify method can be variants, so that arrays of data can be passed efficiently. The OLE server application exposes a Notification object with a Register method. The parameters of the Register method specify the condition to check for, the frequency, and a reference to an instance of the OLE client's Receiver class.

When the client application is run, it creates an instance of its Receiver class. It then requests an instance of the Notification object from the server. The client calls the Register method of the Notification object, passing condition, frequency, and a reference to the Receiver object. The Register method returns immediately, and the client goes about its business.

The OLE server has some mechanism for executing periodic events, such as a timer on a hidden form. Whenever the timer event fires, the first thing it checks is to see if any new registrations have been made. These are added to its notification list. The timer event then performs whatever database operations are necessary to determine whether any client's notification conditions have been met.

If a client's notification condition is met, the OLE server calls the Notify method of the client's Receiver object, passing the data as one of the parameters.

This scenario becomes even more interesting when you add Remote Automation to the picture. If the workstations are running a Win32 operating system (Windows 95, Windows 98, or Windows NT), they can pass their Receiver object reference across the network to an OLE server running on a network computer. (This requires the Remote Automation Manager on the workstation.) The network computer can be dedicated to servicing notifications for a large number of workstations.

Designed carefully with a lot of RAM at the server, you could even locate the OLE server on the same computer as the SQL Server database, so that the database operations would be local.

REFERENCES

For more information, please see the following product documentation:

• Creating OLE Servers.



• Building Client/Server Applications.



• The CallBack sample application in \Samples\Remauto\Callback, Enterprise Edition only.

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