There are many different ways to provide Ajax support from Rails. Some use Rails itself as a foundation, a provider of web services, while others more tightly bind the Rails application to the details of the client interface.
If a manager comes running down the hallway shouting that you should scrap everything you’ve built because he’s going to replace it with an Ajax application, you may, if you’ve built your application using Rails’ RESTful features, remain pretty calm. If you’ve built a consistently RESTful API, then Ajax applications issue their requests to it without you needing to tear it down and start over. Some refactoring to let the Ajax application grab smaller pieces might be a good idea, but overall you’ll have a solid foundation.
The RESTful foundation is solid enough, in fact, that anyone who
has access to your site could potentially build an Ajax application
XmlHTTPRequest calls to the same server
the script came from, anyone who wants to can build a mashup
combining your data with their interface if they’re willing to set
up some creative HTTP proxying.
That doesn’t mean the job is done, of course—it just means that most of what you’ve built will work. You can focus on the clearly Ajax bits, mostly in your views, rather than having to sort through a huge mess of code that used to generate one kind of HTML data structure and trying to figure out how to make it generate another ...