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,

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.