What's the specific problem? Why do you need the results to be ordered? If you really do need to order the results, just attach a sequential ID to each request, then hold a priority queue of results sorted by request ID. Pseudo-code follows:

  next_request_id = 0;
  next_response_id = 0;

  // the following code assumes there's a PriorityQueue class implementing a
  // priority queue with push(object, priority) and pop() operations
  response_queue = new PriorityQueue();

  function send_request(request) {
      request.request_id = next_request_id;
      next_request_id++;
      ajax_send(request);
  }

  function receive_response(response) {
      response_queue.push(response, response.request_id);

      while(!response_queue.empty()
              && response_queue.top().request_id == next_response_id) {
          handle_response(response_queue.pop());
          next_response_id++;
      }
  }

The above code isn't very robust as it doesn't deal with requests that die in transit and never return a response, etc.

Right - this is what I'm about to try to implement (and I think I'd trash the entire queue if a response isn't returned within some timeout limit and display a subsequent error message). I don't know if this is the best practice though and as this seems like a pretty generic AJAX problem, it feels like there should be a pretty generic solution...

I doubt most AJAX applications are that reliant on request/response ordering. However, a priority queue is the generic solution. :-)

One way to handle it is to design your system so that it doesn't matter if the requests come back out of order. Could you give an example of where order matters, so we can work on designing it so that order doesn't matter?

I'd like to implement a note-taking system, with an auto-save feature (say, set by a timer). If the requests that go out to the server are received out of order, the wrong state will be auto-saved.

Don't save again until the first save has been registered. If it times out, try again.

Right - so this would amount to creating queue? Nevermind, I see what you're saying... This is a very specific instance of something pretty general though. Lets say the user edits a note and then deletes it. Do I have the delete action wait for any saves that went out complete first? As the system gets more and more complex it would be something of a pain to try to work out dependencies every time I want to add a new feature.

Yes, queue up the deltas, then only send the full set of deltas every time the timer fires. This will also prevent you from hammering your server with a stream of requests backing up.

Now, lets say I want to stay away from auto-save... The app I'm working on will actually be a widget on the side of the main site and so I'm not confident the user will pay enough attention to know if his/her info has been saved before they navigate to another page. In this case, can I still avoid a queue? I'd want every action a user takes (adding a note, editing it, deleting it) to result in a server request and some spinny wheel notification that the request is being processed. By the way, thank you both for your help!

You don't need a queue.. Just issue the commands as the user makes them. Each command should be separate from every other command, so that order doesn't matter.

I think order does matter - lets say a user types in 'hello' and then saves the note, then quickly changes his/her mind and types in 'hello world' and saves the note. Now the backend will save the version of the note which it receives last, which could be 'hello' depending on network lag.

You should probably use some kind of versioning so that when you receive version 123 and the version in the database is 121, you won't commit 123 until you receive #122.

If the user deletes the note and the server receives the delete request before other modification deltas, simply ignore the deltas that arrive after the delete command.

In my opinion it's a lot more simple to solve this kind of problems on the server side of a web application.

In that instance, you'd abort the previous request, create a new XmlHTTPRequest object (important step), then re-issue the request.

How does one abort the previous request unless a queue is being kept track of? Also, there could have been a few other requests made between the first save and the second save. Its definitely possible I'm just not understanding how XMLHTTPRequest objects work - please do explain if you think that is the case.

Also - I'm not necessarily using XMLHTTPRequest, I'm leaning towards dynamic scripting as my site will need this widget to run across several different subdomains - dynamic scripting doesn't have a lot of the nice status functionality that XMLHTTPRequest does...

Okay, here's an alternate method.

 When a user needs to save their note
  If the note is already saving
   Set note.saveAgain = true
  else
   Start the process of saving the note
 
 When the note has saved
  If note.saveAgain == true
   Set note.saveAgain = false
   Start the process of saving the note again

That doesn't require a queue. It also only saves the most up to date version.

Cool.

Thanks again.

I can has karma now?

Edit: How dare I ask to be compensated for my time. I should be ashamed of myself.

Thanks!

If you have any more questions, feel free to ask.

No, you don't wait. You delete it immediately.. If the delete command arrives before the edit command, the edit command will simply fail, which is fine.