There seems to be a lot of stress/load testing tool that support AJAX.
I am wondering how well does these tool implemented.
Do they only record http request and replay it?
Is it the right way to test AJAX app?
How does google test their ajax apps?
Answer
Most of the load testing tools out there do AJAX load testing the same way: they execute the raw HTTP traffic that is seen during a "recording" phase (which can be page requests, image requests, or even AJAX requests). The main difference among them is how good their recorder/IDE tool is and how easily it helps you parameterize the HTTP requests such that they reflect real world traffic based on dynamic/realtime results.
Warning, blatant plug: The only real exception to this is my company, BrowserMob. Instead of simulating the traffic observed, it actually uses real web browsers to drive back load. As such, the AJAX stuff is handled by the browser.
Useful link: Separate from the blatant plug above (though I do hope you check it out - we're up front with the pricing and provide a free trial), I recently wrote an article for Ajaxian about AJAX load testing. It goes in to more detail about the technical implications of using real browser users (RBUs) vs. virtual users (VUs).