4.5 How do I prevent a WML deck from being read from the cache?

4.5 How do I prevent a WML deck from being read from the cache?

You might want to check this section if you have problems controlling cache the way it's described below.

After a WML deck has been downloaded to the WAP device, it lives in the WAP device's memory for a certain period of time. Until this period has expired, the deck will not be downloaded from the server but instead from the WAP device memory. This is known as caching, and it obviously speeds things up considerably.

However, in some rare cases you might not want the deck to be read from the cache memory but from the server. A typical case is where you have a deck containing cards with information that gets updated frequently. By adding some cache information to the HTTP header you send out from the server, you can tell the WAP device that the following deck should never be stored in the cache.

Note that this requires that you are able to produce HTTP headers on the server side, either using PHP, ASP, Perl or some other type of server side scripting language. The lines cannot be included in the deck code as they are HTTP header information, not WML elements.

IMPORTANT! For static decks, or if you do not have access to a server side script language, many browsers support META tags that control caching on the browser. See the end of this section for examples on using META tags for controlling the cache.

See the W3C HTTP specifications for more details on HTTP.

With the following lines added to the HTTP header, the deck will expire immediately:

Expires:  05:00:00 GMT
Last-Modified: DD. month YYYY HH:MM:SS GMT
Cache-Control: no-cache, must-revalidate
Pragma: no-cache

The first line tells the micro browser that the deck should expire some time in the past. The second line tells the browser when the deck was last modified, and "DD" should be replaced by the current day-of-the-month (eg. "28"), "month" with the current month (eg. "January"), "YYYY" with the current year (eg. ""), "HH:MM:SS" with the current time (eg. (08:19:23"). The third and the fourth line have the same effect, i.e. to tell the browser that the deck should not be cached - line three is HTTP 1.1 and line four is HTTP 1.0.

The following PHP example produces the above header each time the deck is accessed.

<?
  header("Content-type: text/vnd.wap.wml");				// set the correct MIME type
  header("Expires: 05:00:00 GMT");		        // expires in the past
  header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT");     // Last modified, right now
  header("Cache-Control: no-cache, must-revalidate");	        // Prevent caching, HTTP/1.1
  header("Pragma: no-cache");		                        // Prevent caching, HTTP/1.0
?>

Here's an example using WebClasses in VB, using "-1" as a value for Response.Expires to prevent caching.

Private Sub WebClass_Start()
    'Set correct MIME type
    Response.ContentType = "text/vnd.wap.wml"
    'Make sure no caching
    Response.Expires = -1
    Response.AddHeader "Pragma", "no-cache"
    Response.AddHeader "Cache-Control", "no-cache, must-revalidate"
    'Use basicwml(my own) as template
    Set NextItem = basicwml
End Sub

And here's an example in ASP, again using "-1" as a value for Response.Expires to prevent caching.

<%
  Response.ContentType = "text/vnd.wap.wml"
  Response.Expires = -1
  Response.AddHeader "Pragma", "no-cache"
  Response.AddHeader "Cache-Control", "no-cache, must-revalidate"
%>

And finally a couple of examples on controlling the caching of static decks using META tags:

The following deck will always be loaded off the server. Note that you should never use this unless you know what you're doing since it eats bandwidth and "wastes" time, every time the deck is requested.

<?xml version="1.0"?>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://...">
<wml>
  <head>
    <meta forua="true" http-equiv="Cache-Control" content="max-age=0"/>
  </head>
  <card id="alwaysexpire">
    <p>This deck will never be stored in the cache</p>
  </card>
</wml>

The following deck will be loaded instantly from the browser's cache memory instead of from the server until 86400 seconds (24 hours) have passed.

<?xml version="1.0"?>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.1//EN" "http://...">
<wml>
  <head>
    <meta forua="true" http-equiv="Cache-Control" content="max-age=86400"/>
  </head>
  <card id="expire1day">
    <p>This card will live in the cache for a day</p>
  </card>
</wml>

Some browsers such as the UP.Browser will not reload a deck if it is reached by going "back" from another card. (The object of the cache is to save bandwidth and time). To force this behaviour, you must use the must-revalidate parameter to the META tag.

<meta forua="true" http-equiv="Cache-Control" content="must-revalidate"/>

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