keeping custom session variable id - Joomla! Forum - community, help and support

i set random id in session variable per browser.
this random id not regenerate anyhow until browser closed.here code trying achieve -

code: select all

$session = jfactory::getsession();
$mymessage = $session->get('mymessage');
if($mymessage!= null)
{ 
   //some task id number stored in $mymessage
}
else
{
   //i set random id here in $rnd_id variable.code did not show here
   $session->set('mymessage',$rnd_id);
}

but happens when login user in local joomla site , logout random id generated code above because $mymessage becomes null after user logs out.
so every1 can understand problem here.i guess creating cookie can me cookie better session?