Understanding JS event loop

Question

I am trying to understand the js event loop. This is my js code:

var start = new Date().getTime();
var url = "/WebForm1.aspx/Test1";
$.ajax({
    type: "POST",
    url: url,
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function (data) {
        console.log('Test1, elapsed: ' + (new Date().getTime() - start) + 'ms');
    },
});

url = "/WebForm1.aspx/Test2";
$.ajax({
    type: "POST",
    url: url,
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function (data) {
        console.log('Test2, elapsed: ' + (new Date().getTime() - start) + 'ms');
    },
});

url = "/WebForm1.aspx/Test3";
$.ajax({
    type: "POST",
    url: url,
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function (data) {
        console.log('Test3, elapsed: ' + (new Date().getTime() - start) + 'ms');
    },
});

This is my server side code:

[WebMethod]
public static void Test1()
{
    System.Threading.Thread.Sleep(1500);
}

[WebMethod]
public static void Test2()
{
    System.Threading.Thread.Sleep(2000);
}

[WebMethod]
public static void Test3()
{
    System.Threading.Thread.Sleep(3000);
}

Now the result is :

Test1, elapsed: 1542ms
Test3, elapsed: 4578ms
Test2, elapsed: 6636ms

Now what i dont understand is, why doesnt they all execute together? is it because the server side can process only one request at a time, or does it revolves with the js event loop?

what do you mean "looks like" :) is it single threaded or not — omriman12, Sep 08 '15 at 14:06
The ajax calls should fire off rapidly in sequence, but if the server is blocking and handling them serially, each has to wait its turn. It's server-side. — Dan Lowe, Sep 08 '15 at 14:06
http://stackoverflow.com/questions/4428413/why-would-multiple-simultaneous-ajax-calls-to-the-same-asp-net-mvc-action-cause — Stryner, Sep 08 '15 at 14:07
So until the server side finishes one request, all other requests are stuck? — omriman12, Sep 08 '15 at 14:08
Have you tried this multiple times? Is the response consistent? — MinusFour, Sep 08 '15 at 14:10
now you can see why I said "looks like", because ... well, it DOES appear that way — Jaromanda X, Sep 08 '15 at 14:11
yea, thats something new i am happy to have learned, ty guys — omriman12, Sep 08 '15 at 14:11

score 2 · Answer 1 · answered Sep 08 '15 at 14:09

2

What this experiment shows is that the server handles the requests one-by-one, instead of in parallel. Why this is is not immediately apparent.

It's possible that the server is single threaded. Another possibility is that the requests require a shared (locked) resource for instance the user session or a locked database table.

answered Sep 08 '15 at 14:09

Halcyon

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as i see from @Stryner link its because it uses the same session id – omriman12 Sep 08 '15 at 14:11
Possibly, I see nothing in your code that indicates sessions are used though. Is there some wrapper code that does? – Halcyon Sep 08 '15 at 14:13

Miguel Lattuada · Answer 2 · 2015-09-08T14:30:01.207

0

The two arguments are correct. Your server can handle just one request at a time (with the same origin), and javascript has to wait for the server. Javascript anyways will execute one response at time, because it's single threaded.

edited Sep 08 '15 at 14:30

answered Sep 08 '15 at 14:23

Miguel Lattuada

5,327
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_Your server can handle just one request at a time_ - **proof?** _Javascript anyways will execute one request at time_ - **no** (unless asynch=false is passed, which it is not) – Halcyon Sep 08 '15 at 14:25
Because is how asp.net works http://stackoverflow.com/questions/1735107/can-a-single-asp-net-user-make-more-than-one-request-at-a-time-if-the-session-is , and you are right with javascript, it could execute n requests, but responses will be handled one by one. It's more about server side. – Miguel Lattuada Sep 08 '15 at 14:29
This has nothing to do with asp.net, but with how sessions work; or locked resource in general. You see the same behaviour in PHP. If you close the session before you `sleep()`, the other requests wont need to wait. – Halcyon Sep 08 '15 at 14:31

Understanding JS event loop

2 Answers2