<p>I really love the way Ajax makes a web app perform more like a desktop app, but I'm worried about the hits on a high volume site.  I'm developing a database app right now that's intranet based, that no more then 2-4 people are going to be accessing at one time.  I'm Ajaxing the hell out of it, but it got me to wondering, how much Ajax is too much?</p>

<p>At what point does the volume of hits, outweigh the benefits seen by using Ajax?  It doesn't really seem like it would, versus a whole page refresh, since you are, in theory, only updating the parts that need updating.  </p>

<p>I'm curious if any of you have used Ajax on high volume sites and in what capacity did you use it?  Does it create scaling issues?</p>

On my current project, we do use Ajax and we have had scaling problems. Since my current project is a J2EE site that does timekeeping for the employees of a large urban city, we've found that it's best if the browser side can cache data that won't change for the duration of a user session.  Fortunately we're moving to a model where we have a single admin process the timekeeping for as many employees as possible. This would be akin to how an ERP application might work (or an email application). Consequently our business need is that the browser-side can hold a lot of data, but we don't expect the volume of hits to be a serious problem. So we've kept an XML data island on the browser-side. In addition, we load data only on an as-needed basis.
I highly recommend the book
Ajax Design Patterns
or their
site
.