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On most pages on Stack Exchange sites, there is an Ajax request made to a URL like this one:

http://meta.stackexchange.com/questions/ticks?_=xxxxxxxxxxxxx

And the response is usually an alphabetic string like;

DkLMTD

What is this all about and what do the 'ticks' do?

If you visit any of the Stack Exchange sites in Firefox with Firebug enabled, you can see this as well.

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Just a guess, so this is only a comment, but perhaps that's a "heartbeat" - used to keep a session active and for showing things like when someone else adds an answer to the question you're viewing. –  Joel Coehoorn Jul 28 '09 at 14:18
    
Do they still ticks? –  YOU Apr 17 '11 at 13:14
    
Ian's answer seem correct and complete, why didn't you accept it? I doubt you'll get any better answer or "real" answer from the site developers. –  Shadow Wizard Apr 17 '11 at 14:13

1 Answer 1

up vote 8 down vote accepted
+50

Ticks are used by the WMD editor, seemingly it is a pseudo random generated string. It is appended to your post via a hidden input when you load a questions page. Using the tick they can determine when you really loaded the page, and whether or not they need to serve you with a captcha. Don't use this knowledge for evil!

$.get("/questions/ticks", function (b) 
{
   $("#post-editor-hidden").append("<input type='hidden' name='bullet' value='" + b + "' />")
});

Sessions don't need to be kept active because they never expire, and new post activity is designated by a 'heartbeat', which is a request sent to /posts/#ID/new-activity-heartbeat, which returns JSON that specifies whether or not new posts have been made and if so how many.

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1  
The source code changed a bit, but the logic is the same. It's now: $.get("/questions/ticks", function(h) { g.append("<input type='hidden' name='i1l' value='" + h + "' />"); }); –  Shadow Wizard Apr 17 '11 at 14:14

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