Exchange 2000 Server Service Pack 2 (SP2) introduces the ability for Microsoft Internet Explorer (IE) 5.0 and later clients to receive new-email notifications and calendar reminders through Outlook Web Access (OWA). Although Outlook clients typically have a permanent connection to the Exchange server, OWA is a stateless client working over the HTTP protocol. How can the server notify the client that new email has arrived or an appointment is due if the client isn't connected? In this article, I look at how the Web Distributed Authoring and Versioning (WebDAV) protocol's Subscribe and Poll methods support OWA email notifications and calendar reminders. (For more information about WebDAV, go to the WebDAV Resources Web site at http://www.webdav.org.) I use new-email notifications as the primary example, but you implement calendar reminders in a similar way.
Basic Functionality
When you use OWA to connect to your mailbox and browse to your Inbox folder, you're taking a snapshot view of the folder's contents. Consequently, you need to click Check for new messages to refresh and update the Inbox folder's and other folders' contents. This approach isn't ideal because it results in wasteful processing as users continually ask, "I wonder whether I have new email?" and click Check for new messages. New-email notification lets users cease pondering this question and provides the trigger to refresh the Inbox folder. . . .
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