Maybe I simplifed my case too much. Actually, I cannot ignore request completely. Let say there are 2 spreadsheet rows, each change in a row will trigger the changes of subtotal and grand total (yes, not just one destination). When user changes cells of rows, the grand total will be the problem if the response order is not guaranteed. I am thinking to implement an event queue in javascript but I am not sure whether it is the only way to go…

For the IE 6 only supports 2 connections at a time, is that means the request will be queued til the connections are free? If so, the asynchronous behavior of AJAX is limited and the responsiveness of the UI will be hurted.. am i right?

thanks

ray

Thanks for the kind words, and good luck with your project - sounds like a familiar scenario to me :-)

To answer your specific points:
(1) you’d need to specify in the second request that it should cancel the first one, as both are targetting the same destination cell. Use cell id as a uid maybe? Command pattern has some useful features for cancelling and undoing user actions which you might like to investigate.
(2) Internet Explorer typically only allows two http connections at a time, IIRC - can be altered with a registry hack, I think.

It is great that some like you and Eric giving us guidelines of how to use AJAX right! I have got some requirements from our boss that he wants to to use web browser to replace one of our critical products that is written in SmallTalk (fat client). In order to maintain the usability, we decide to adopt AJAX…. However, we found out the unknown in AJAX that I hope guru like u or Eric can give a light.

(1) Request Ordering
AJAX sends request asynchronously back to server, but we cannot guarantee the order of the requests being process and/or the order of the response being send back. Let say I have a spreadsheet app, there are 3 cells A1, B1 and C1 and A1+B1=C1 (the formula is simplied to illustrate the point. In reality, it is complicated formula that needs to execute in the server side). When I change A1 then B1, there are 2 requests send back to the server for calculation. Ideally, the response of the last request will tell me the final result of C1. But if the response of first request comes after the 2nd. The change of B1 will be lost! How to guarantee that?

(2) Error Handling
If the request is lost somehow, will XMLHttpRequest timeout? Besides, if each request use a new XMLHttpRequest, are there any thread contention when the responses come back that may modify the same field… (again, it sounds like ordering or synchronous issues…)

thanks
ray

There are quite a few frameworks popping up, aren’t there? We can’t hope to cover all of them, and more will inevitably appear between finalising the manuscript and the publication date. Looking at a lot of the frameworks from a design point of view, I see a lot of similarities in style, so there’s plenty to talk about without getting stuck on a particular implementation.