Probably 99.99% of HTML applications and websites are served over HTTP exclusively. (I'm referring here to HTTP as a transport protocol, not HTTP vs.HTTPS, for example, and I realize that HTTP is an application-layer protocol according to OSI; but developers generally treat it as an abstraction for "the network"). As anybody who has done web programming knows, HTTP is a stateless protocol; that is, it's based on a request-response model, and in general, one request has no knowledge of previous requests. This has posed some challenges for web developers over the years, and some brilliant abstractions of state on top of the statelessness have been devised.
The hard part now, though, isn't to deal with statelessness. It's dealing with the request-and-response model.
All network communication is inherently request-and-response. There are some applications that utilize full-duplex communications to get around that (think of chat software), but for the most part, that isn't really available behind the firewall. Web sockets are still yet to be standardized (and there are some questions about long-term compatibility with WebSocket-ignorant proxies). And typically, corporate firewalls say no to outbound connections except on ports 80 or 443. Some applications (think Meebo) have been able to get around this limitation by cleverly using long-timeout delays on AJAX requests. The client makes a request to the server, and the server either responds immediately (if an event is in queue) or holds the request for 30-90 seconds to see if an event comes in. I even did this once myself with good success, although I never took that app into production. (There was also some question about the total # of clients an ASP.NET server could sustain whilst holding threads in that way).
In many respects, Windows developers haven't had to deal with this. We could issue synchronous requests, and the UI would stand still for a second, and either it would work or it would fail. But usability concerns over this process, as well as issues with high network latency (imagine pressing the "Submit" button and having to wait 20 seconds while your app freezes - by then, I've force-closed the app) have seen platform providers decree that asynchrony is the only way to go.
HTML isn't the only application provider dealing with this limitation. Adobe Flash has long had an asynchronous-communication-only model, Microsoft Silverlight has also carried on this principle; of course, these two applications have lived predominantly in browsers, where a hanging UI probably means interfering with other apps as well as the one making the request. Interestingly, WinRT - the Windows 8 developer framework - is also going to mandate an asynchronous model, following in the Silverlight-based foodsteps blazed by Windows Phone 7.
So as we trek out into the world of asynchrony, well, we have a whole mess of questions to deal with now:
- If there's an error, does it show up in the calling method or in the callback method? Does it even show up?
- Does a network (transport-level) error surface differently than an application error? What if the server returned an HTTP 403 Forbidden response?
- What are all of the different kinds of errors that can crop up? Do I need to handle SocketException or is that going to be abstracted to something more meaningful to my application?
- What do I do if a network error comes up? Do I assume that I'm offline, panic, and quit? What if my application only makes sense "online"?
- Do I surface an error to the customer? Silently fail? I might generally fail silently if I'm a background process, but then again, what if it's an important one? What if the customer thought he was saving his draft while all along it was offline, and then the customer closes the browser?
- During the async operation, should I show the user a timeout spinner or something to that effect?
- How should I design my async operations? For example, consider a Save operation. Should I capture all of my state at once and send it off, and let the user immediately keep working? Should I make the user wait until saving completes? Should I even use Save, or automatically save whenever something changes?
- If I use auto-save, how do I handle undo? What if I want to undo between sessions? Is there a way to go back if the hosting application crashes? (Worst case scenario: the user accidentally hit Select All, Delete and then the browser crashed after the auto-save).
- Write a singleton object? This might be easier and afford strong member protection, but I can only have one widget, unless I somehow differentiate between them and multiplex, which can become hairy quickly.
- Should the monitoring function accept a callback, or should it be event-based, so that multiple subscribers can listen? (Maybe an event-based model offers some interesting ways to deal with the complexities of a singleton?)
- Should the widget manipulate the view directly, or should I write separate code that handles the view based on the state of the object (or objects)?
The list goes on.
We're moving faster and faster into an asychronous world. It is already happening, and we as developers need to be prepared to handle these difficulties. We also need to understand how to communicate these kinds of questions to our business analysts, our supervisors, and our customers. We need to be able to equip ourselves to ask the right questions of our customers, so that when it's time to make a decision, we have the information we need.