This post on Http GET and side effects - in light of Google's Web Accelerator - made me think about something. I only just posted on this, but now I've read a bit more and thought about it. Here's what BitWorking has to say:

Let's start talking about you. And by you I mean all the web application developers out there that have been mis-using HTTP. Now I've been telling you over and over how to use HTTP correctly, how to build you web applications in a RESTful manner, and the consequences you could face if you didn't. Well, you didn't listen to me, did you 37Signals? Did you? Now Google releases their Web Accelerator and your application is broken. Apparently you used GET to delete items from Backpack and when Googles Web Accelerator pre-fetches that URI in you web app, items get deleted. Let's make this clear, Google didn't break your application, your application was broken, Google was just the first person to point it out to you. Do you feel the pain? Do you feel it?

Well, let's not get too self righteous or too far ahead of ourselves, hmm? Let's talk about something that's been an off and on topic in the syndication world for awhile - RFC 3229. That spec defines how a web server can answer incremental changes from a client. In the context of a news aggregator, that means only items that are new/changed since the last such request. In order to implement that on the server side, you'll need a dynamic feed, using header information in a GET request.

Well. The GET request effects a state change in the server with respect to the client in question. This is arguably a good thing for any conforming clients, since it reduces the amount of traffic that needs to be served. It's also something that Google's accelerator could render completely useless. I emphasize could - it's unclear to me whether the request sent by Google would include the requisite header info.

The point though, is this - not all client requests that result in a server state change are bad things. Heck, if they are we better get rid of 304, right now. The purists shouting from the rooftops about state free GET requests haven't thought this one all the way through.