jquery $.when and Simultaneous request issue

Question

I wrote my first $.when routine but is not working as expected:

     $(document).ready(function () 
     {
      var   UserUrl = 'API/StartGetUser.asp';
      var   StoreUrl = 'API/StartGetStore.asp';
      var   FeedbackUrl='API/StartGetFeed.asp';
      var   ItemsUrl='API/StartGetSellers.asp';
    $.when(
        $.get(UserDetailsUrl),
        $.get(StoreCategoryUrl),
        $.get(FeedbackUrl),
        $.get(ItemsUrl)
    ).then(function(user,store,feed,items) {
        //$.get(mailUrl)
        alert(user+' '+store+' '+feed+' '+items);
    }).fail(function(err) {
      alert(err);
    });

It works, but I was expecting that the 4 get would have fired simultaneously, while I found that are executed one after the previous has finished... since is

$.get

it is surely async..

what can be wrong? What shall I check?

btw, serverside is classic asp and request are on same machine as can see from url.

thanks for any hints

Joe

Answer 1

The four requests are initiated one after the other because Javascript executes code sequentially. There is no actual parallel execution in Javascript (it is single threaded - except for webWorkers which are not involved here).

What will happen here is that all four requests will be "in process" at the same time. When exactly the responses come back is determined by the responsiveness of the server. Some servers will only handle one request at a time and will cause the responses to come back sequential, even though they were all initiated at once. Other servers will process them in a time-sliced fashion (or even using multiple processors) such that all responses are finished at about the same time.

You can verify the timing of the sending of the requests and the receiving of the responses by opening the network tab in the Chrome debugger before your operation and then looking at the time trace that it shows.