This commit will fix a common problem in CodeIgniter.

After $config['sess_time_to_update']  expires the session will be destroyed if concurrent AJAX calls are made to the framework.
Original discussion on this problem can be found on the following URL

http://stackoverflow.com/questions/7980193/codeigniter-session-bugging-out-with-ajax-calls

application/libraries/MY_Session.php

@@ -0,0 +1,19 @@
+<?php
+class MY_Session extends CI_Session {
+
+    /**
+     * Update an existing session
+     *
+     * @access    public
+     * @return    void
+    */
+    function sess_update() {
+       // skip the session update if this is an AJAX call! This is a bug in CI; see:
+       // https://github.com/EllisLab/CodeIgniter/issues/154
+       // http://codeigniter.com/forums/viewthread/102456/P15
+       if ( !($this->CI->input->is_ajax_request()) ) {
+           parent::sess_update();
+       }
+    }
+}
+