Why don't these $.getJSON calls return in parallel?

Question

<script type="text/javascript">
    var sourceData = {sites:[{ name: "London" , Id: "0"}, { name: "Bath", Id: "1"}, { name: "Cambridge", Id: "2"}]};
    $(document).ready(function () {
    $(sourceData.sites).each(function (index, data) { var objName = "<div class='bold' id = place" + data.Id + ">" + data.name + "</div>"; $('#results').append(objName); });

    $(sourceData.sites).each(function (index, data) {
    $.getJSON("GenerateData.aspx?place=" + data.name, function (retData) {
            $('#place' + data.Id)
                .wrap("<a href=GenerateData.aspx?place=" + data.name + "></a>")
                .append(retData.Wait); });
        });
    });
</script>

So basically generateData just returns a json which looks like:

{"name": "(place parameter passed back)", "Wait": 5000}

Except 5000 is actually Rand(0,5000) and also corresponds to a thread.sleep that executes before the request is returned.

What I can't understand is that while I'm getting some parallelisation (the order in which results are returned is random), the order in which they return is not from lowest to highest, and also the total return time is equal to the total wait times.

So there's no actual parallelisation of the requests against GenerateData, even though the requests seem to be sent out in parallel.

I've seen Asynchronous Controller is blocking requests in ASP.NET MVC through jQuery but that seems specific to MVC which I'm not using (yet).

Am I doing anything wrong from the jquery side, or is this another case of ASP.net sessions messing parallelisation up?

look at the latencies in the javascript console

they are not send in parallel but sequentially. The $.getJSON is asynchronously so it will not block until it receives a reply but that's different from parallel and will affect the order they are handled in server side — Rune FS, Feb 12 '13 at 13:06

score 1 · Answer 1 · answered Feb 12 '13 at 13:07

1

It's not a coincidence that $.getJSON() is an asynchronous operation. The requests responses are not going to be ordered in the same way the requests are or at least very rarely they will. If you want your requests to return in the order that you sent them then you need to create an Ajax queue and then the requests would be sent upon completion of previous requests. You can achieve this using jQuery.queue().

answered Feb 12 '13 at 13:07

Konstantin Dinev

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Not really answering the question being asked: 'why do my requests appear sequential instead of parallel?'. – Mr47 Feb 12 '13 at 13:10
1

@Mr47 "Why don't these $.getJSON calls return in parallel?" the question states this. They are sent out in parallel because you are looping and sending requests one after another. – Konstantin Dinev Feb 12 '13 at 13:12
they are not send in parallel but sequentially (asynchronously) – Rune FS Feb 12 '13 at 13:20
I meant sequentially not in parallel. – Konstantin Dinev Feb 12 '13 at 13:31
OK, but even if they are sent sequentially in a non-blocking manner, the requests are being handled sequentially but in a blocking (on the server?) manner. This is really the problem I face. I'll try to add a screenshot of the issue I face. – Eterm Feb 12 '13 at 14:18

Why don't these $.getJSON calls return in parallel?

1 Answers1