Question

AJAX response returns the same value only requesting current date time

Asked by: n2alexan

Hi,

I have a very simple webapp that has this index.htm page:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Untitled Page</title>

    <script type="text/javascript">
            
            function GetXmlHttp() {      
      var xmlhttp = false;
      if (window.XMLHttpRequest)
      {
            xmlhttp = new XMLHttpRequest();
  }
      else if (window.ActiveXObject)// code for IE
      {
            try
            {
                  xmlhttp = new ActiveXObject("Msxml2.XMLHTTP");
            }
            catch (e)
            {
                  try
                  {
                        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
                  } catch (E) {
                        xmlhttp=false;
                  }
            }
      }
      return xmlhttp;
}

            function ajaxFunction(url)
  {  
    var xmlHttp = GetXmlHttp();
 
    xmlHttp.onreadystatechange=function()
      {
      if(xmlHttp.readyState==4)
        {
        document.getElementById("ajaxin").value = document.getElementById("ajaxin").value + '|' + xmlHttp.responseText;
        }
      }
    xmlHttp.open("GET",url,true);
    xmlHttp.send(null);  
   }
   

    </script>
   
</head>
<body>
    <input id="Button1" onclick="ajaxFunction('ajax/tracker.aspx');" type="button" value="button" />
    <input id="ajaxin" style="width: 100%; height: 544px" type="text" />
</body>
</html>

======
and I use aspx page to write back the date and time

<% Response.Write(DateTime.Now) %>

but every time I press the button I get the same datetime and it's value is not changed, even after 1 minute.
The values of seconds is not updated.

Could someone explain what is happenning here.

Thank you very much,
N.

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Asked On
2007-02-22 at 09:26:16ID22406912
Tags

ajax

,

response

,

time

,

get

,

same

Topics

Programming for ASP.NET

,

JavaScript

,

Microsoft Visual Basic.Net

Participating Experts
1
Points
500
Comments
3

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Answers

 

by: mark-bPosted on 2007-02-22 at 10:26:40ID: 18590049

Maybe an always changing variable at the end of the request URL so no caching occurs? :

<input id="Button1" onclick="ajaxFunction('ajax/tracker.aspx?x='"+ new Date().getTime() + "');" type="button" value="button" />

-Mark

 

by: n2alexanPosted on 2007-02-23 at 02:13:07ID: 18594740

Hi Mark,

Thank you for your response. Your solution does work!

I have found the explanation to the problem by searching for 'ajax caching' terms in Google. I came across David's blog http://www.enja.org/david/?p=25 who has stated Microsoft Internet Explorer Cache issues which he found  on this http://en.wikipedia.org/wiki/XMLHTTP page.

"
Microsoft Internet Explorer cache issues
Internet Explorer implements caching for GET requests. Authors who are not familiar with HTTP caching expect GET requests not to be cached, or for the cache to be avoided as with the refresh button. In some situations, failing to circumvent caching is a bug. One solution to this is to use the POST request method, which is never cached; however, it is intended for non-idempotent operations.

Setting the "Expires" header to reference a date in the past will avoid caching of the response.
" (XMLHttpRequest - Wikipedia, the free encyclopedia, Wikipedia 2007).


 

by: mark-bPosted on 2007-02-23 at 04:43:28ID: 18595319

Glad to help.  And good luck!

Cheers,

-Mark

20120131-EE-VQP-002

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