Multiple ajax call handling

Question

I am using jQuery to make some ajax calls and wonder how people handle this situation.

1. An ajax request is made to the server to retrieve some info.
2. Before the request returns another request is made. The first request is now invalid and out of date.

How do I tell that the initial request is now invalid and can be ignored when it returns. I only want to display the results of the second request and ignore (or cancel) the first.

Answer 1

I've never been in this situation before, but you could send over a key that you increment when making the request and have the key sent back with the response. When the response arrives, you could then check the key to see if it is what you expected.

var incrementor = 1;
var lastSent = 0;

jQuery(document).ready(function() {

    jQuery('a.submitter').click(function(event) {
     event.preventDefault();
     lastSent = incrementor;
     incrementor++;
     jQuery.post(
      'some-url.php',
      {
       'request-id': lastSent,
       'other-data': 'some-data'
      },
      function( data, textStatus ) {
       if( data.requestId == lastSent ) {
        // Do stuff
       }
      },
      'json'
     );
    });

});

Answer 2

"How do I tell that the initial request is now invalid"

You DON'T...! You queue up Ajax Requests at the client layer and don't let them fire before the previous ones have returned and finished up their manipulation of the DOM...

You can get some details about how to do this at my blog about "how to create an Ajax library" at; http://ra-ajax.org/how-to-create-an-ajax-library-part-7-the-ra-ajax-class.blog

Answer 3

Keep a record of a variable (eg:"request_id") that identify the request. Add 1 to the variable at each new request. Only process the request if the request_id returned by the server is equal to the variable you have on the client.

My 2 cents.

Answer 4

jQuery returns the XmlHttpRequest object from the call to ajax(), which I've used to achieve what you desire as follows:

var lastRequest;
function getSomeData() {
    if(lastRequest) {
        lastRequest.abort();
        lastRequest = null
    }
    lastRequest = $.ajax(...);
}

The net effect is that earlier requests' responses are ignored if a subsequent request is made.

Answer 5

I prefer not to involve the service call, you may not always have control over the service definintion. I would like to see something like this

$(function() {
    $('.ajaxButton').click(function() {
        $.currentCallId = (new Date()).getTime();

        $.ajax({
            type: "get",
            url: 'http://www.google.com/',
            beforeSend: function(xhr) {
                this.callId = $.currentCallId;
            },
            success: function(data) {
                if (this.callId === $.currentCallId) {
                    // process it
                } else {
                    // throw it out
                }
            }
        });
    });
});