Q137097: HOWTO: Implement a Real-Time Application Using OLE

Article: Q137097
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:4.0
Operating System(s): 
Keyword(s): kbnokeyword kbVBp kbVBp400 kbGrpDSVB kbDSupport
Last Modified: 11-JAN-2001

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The information in this article applies to:

- Microsoft Visual Basic Professional Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Professional Edition, 32-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 16-bit, for Windows, version 4.0 
- Microsoft Visual Basic Enterprise Edition, 32-bit, for Windows, version 4.0 
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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, Windows NT, or Windows 2000), 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.

Additional query words:

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Keywords          : kbnokeyword kbVBp kbVBp400 kbGrpDSVB kbDSupport 
Technology        : kbVBSearch kbAudDeveloper kbVB400Search kbVB400 kbVB16bitSearch
Version           : WINDOWS:4.0
Issue type        : kbhowto

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