var XHRFactory = (function(){
 // static private member
 var stack = new Array();
 var poolSize = 10;
 
 var nullFunction = function() {}; // for nuking the onreadystatechange
 
 // private static methods
 
 function createXHR() {
  if (window.XMLHttpRequest) {
       return new XMLHttpRequest();
     } else if (window.ActiveXObject) {
       return new ActiveXObject('Microsoft.XMLHTTP')
     }
    }

 // cache a few for use
 for (var i = 0; i < poolSize; i++) {
  stack.push(createXHR());
 }
 
 // shared instance methods
 return ({
  release:function(xhr){
   xhr.onreadystatechange = nullFunction;
   stack.push(xhr);
  },
  getInstance:function(){
   if (stack.length < 1) {
    return createXHR();
   } else {
    return stack.pop();
   }
  },
  toString:function(){
   return "stack size = " + stack.length;
  }
 });
})();