ideally the web should follow the REST principle and be completely stateless. Therefore a single URL should identify a single resource, without having to keep the navigation history of each user.
...and I read the Wikipedia page http://en.wikipedia.org/wiki/REST and it really sounds good, but I don't get how to actually implement it. I'm working in ASP .NET Webforms NOT MVC.
For example, in the application I am about to build - I need my user to Login before I allow them to do anything. There are a couple of hoops they have to jump through before they are allowed to do much useful - like Accept T's and C's and confirm their basic details are unchanged. Finally they are allowed to do somthing they really want like BuyAProduct!
It seems to me (I come from the HEAVILY stateful world of the Rich client) that I need state to record what they have done and infer from that what they are allowed to do. I don't see how I can support them (say) bookmarking the BuyAProduct URI. When they arrive at the bookmark how do I know if they have logged in and if they agreed to the T's and C's and if they dutifully checked their basic details?
I love the idea of the app being stateless, partly because it seems to completely solve the problem of "What the heck do I do when the user clicks on the Back and Forward buttons?" I don't see how I can still get it to work properly. I feel I am missing somthing really fundamental about this.
The advice isn't suggesting that the app should be stateless - it's suggesting that the resources in the app should be stateless. That is, a page called "www.mysite.com/resources/123" will always represent the same resource, regardless of which user is accessing it or whether they're logged in or not.
(The fact that you might deny a non-logged-in user access is a separate issue - the point is that the Uri itself doesn't rely on user-specific data to work.)
For example, the kind of sites that break this rule are those where you navigate to a product page, email the Uri to your friend, and on clicking it they see a message along the lines of "I'm sorry, your session has expired" or "This product does not exist" or similar. The reason this happens is because the Uri includes something specific to the user's session on the site, and if a different user tries to use the link (or the same user at a later time), it's no longer valid.
So, you will always still need some form of state for your application, but where that state is implemented is the important factor.
If you want to do Web forms, that's cool. If you want to do REST that's cool too. But please for the love of everything sacred, please don't attempt to adhere to the principles of REST using Web Forms.
Just to clarify this point further, I don't believe webforms is a wise choice for REST because the conceptual model that WebForms is based on is one where you abstract away the web. It was built to emulate the VB development model.
REST embraces HTTP and the distributed nature of web applications. The two approaches are not compatible.
A cookie would seem to be the answer to your question. You can use the the .net authentication provider which will set a cookie, that your application can check for and require the presence for if they're to buy anything.
The thing you want to try and avoid is keeping a state representation of them on the server, aka session cookie. That will make scaling more difficult.
WebForms uses POST for everything. You should only POST to a URL when you are creating a new item and don't know where it will live. If you know its url, then you should use PUT to create or replace. If you need intermediary steps for, say, a shopping cart, then you should create resource representations for those intermediary steps. Your browser and server communicate by passing full representations between each other. It's simple request/response message passing.
WebForms doesn't encourage this. You can build RESTful systems in WebForms, but the default model will push you away from it towards a RPC approach. I can think of two ways around this: Front Controller(in which case you should really consider MVC) or using .ashx files for almost everything. The Postback model pretty well obliterates any real hope of doing true REST with real WebForms/.aspx (i.e. PUT and DELETE are always POSTs and thus fail the REST model).