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Preventing session expiration with AJAX

Lately I have noticed an increase in issues related to session expiration in web pages. There are two cases that come to mind:

  • Traditional forms that may take the users too long to fill and submit, perhaps because they need to seek data from several sources, such as curriculum info or references.
  • Rich Internet Applications, which may delay to communicate with the server long enough for a session to expire, even though the users act under the delusion of a constant communication and a consistent state of their data.

Increasing the session timeout is only a partial solution, as it affects the server load and doesn’t fully address the problem since users may still face an unexpected session timeout if their behavior diverts from the expected scenarios. A better way is to use a simple AJAX call to periodically renew your session, as long as your web page is still open in the browser. You can do that using the setInterval() JavaScript function or its equivalent in the framework of your choice. With jQuery, you could use something like this:

    $(document).ready( function() {
        var refreshTime = 600000; // in milliseconds, so 10 minutes
        window.setInterval( function() {
            var url = 'http://mysite.mydomain/refreshSessionURL';
            $.get( url );
        }, refreshTime );

The url variable should point to a page that does nothing but refreshing a session. If your application is in PHP, a simple session_start() will do.  If you are using an MVC framework, you could use a controller that renders nothing back and let the framework handle the session renewal.

One possible caveat using this approach is AJAX caching. If it is enabled (some browsers, including IE, enable it by default), it is possible that only your first call will be sent to the server. There are two approaches. The most simple one is to disable AJAX caching completely. In jQuery, this can easily be done using a $.ajaxSetup( {cache:false} ) call. Another possible way is by attaching some random parameter to your session-refreshing URL that will be ignored by the server.

Finally, you should be aware of the security implications of never letting a session to expire. Depending on the requirements of your application, you should consider using a user inactivity test to prevent session renewal or even cause a session to expire.


June 19, 2010 Posted by | Programming, web | , , , , , , | 6 Comments

Selecting language of multilingual web sites

Multilingual sites will usually offer a way to their users to switch between languages of the content, either through a link in their pages or through the configuration of user preferences. For first-time visitors, however, a site needs a way to determine their prefered language(s). The standard way to identify this is by inspecting the Accept-Language HTTP header sent to the site by the user’s browser.

According to the HTTP 1.1 Specification, the Accept-Language header can be used to assign a weight to each language, determining the users’ prefered order of natural languages of multilingual content. For example,

Accept-Language: el,en;q=0.5,fr;q=0.4

means that the user prefers Greek content, but if it is not available then English and French are also acceptable, with English having a higher priority.

You can parse the Accept-Language header to determine the appropriate language. Although a simple parsing can be used in most cases, addressing the gritty details of the specification can be a bit tricky. Here is an implementation of the parsing algorithm in PHP. It might be an overkill, but it gives you a pretty good idea:


function sort_descending_weights$a$b )
# Each array element is a (lang,weight) pair
if ( $a] != $b] )
        return ( 
$a] < $b] ) ? : -1;

    # If two languages have the same weight, then we might want to impose 
    # our own precedence. Put your own ordering code here. For simplicity, we
    # just assume that the default language takes priority.
if ( $a] == $default_lang ) return -1;
    else if ( 
$b] == $default_lang ) return 1;
    else return 

function is_language_available$lang )

function get_prefered_language( )
# If no Accept_Language header exists, use the site's default language
if ( !in_array'HTTP_ACCEPT_LANGUAGE'$_SERVER ) ) 

    # Parse the header. A * indicates any language not explicitly specified
$list explode","$h );
    if ( 
count$list ) == ) return $default_lang;
$prefs = array();
    foreach ( 
$list as $langs )
$tmp explode";q="$langs );
$lang $tmp];
$weight count$tmp ) == 1.0 $tmp];
array_push$prefs, array( $lang$weight ) );

    # The specification doesn't enforce weight to be in descending order.
    # Sort the parsed values.
usort$prefssort_descending_weights );    

    # Pick an available language. is_language_available() is a stub.
foreach ( $prefs as $pref )
$lang$weight ) = $pref;
        if ( 
is_language_available$lang ) ) return $lang;

echo get_prefered_language();

In Firefox 3.0, users may specify their prefered content languages in the Preferences / Content / Languages Menu:

prefs lang

For ease of use, exact weights don’t need to be specified. When users change the order of the languages in the list, the browser will calculate and send the appropriate weights in the HTTP request.

September 3, 2009 Posted by | Programming | , , , , , | 4 Comments