Browser-Server Dialogue - Call Tracking Alternatives
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Fire-and-forget
Some calls need no tracking because they are low-priority uploads of information to the server. For example, a chat app might reasonably ignore the results of uploading the user's messages because a separate thread is continuously polling for all recent messages. Another example would be uploading information to support Predictive Fetch (Chapter 13), where the worst case is simply the lost opportunity of a performance optimization. Ajaxian.com featured an interesting article on optimizing for this kind of request (http://www. ajaxian.com/archives/2005/09/ajaxian_fire_an.html).
Global XMLHttpRequest
You may be able to get away with a single, global XMLHttpRequest under certain conditions and with care. For example, you can use a lock to ensure there's only one pending call at each time (which is really a special case of Call Tracking). However, you risk forcing the user to endure long waiting periods.
Metaphor
Call Tracking is like tagging self-addressed envelopes before you send them away, so you can track them as they return.
Acknowledgments This pattern was originally inspired by Richard Schwartz's caution against the familiar anti-pattern of working with a global XMLHttpRequest (http://smokey.rhs.com/web/blog/ PowerOfTheSchwartz.nsf/d6plinks/RSCZ-6CDPEX).
Please check back next week for the continuation of this article.
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 This article is excerpted from chapter 10 of the book Ajax Design Patterns, written by Michael Mahemoff (O'Reilly, 2006; ISBN: 0596101805). Check it out today at your favorite bookstore. Buy this book now.
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