Measuring visitor HTTP cache hit ratio for external CDN resources

Question

My site uses several common CDN hosted resources, like bootstrap.css, jquery.js and fontawesome.css. Is it possible to get information, probably with JavaScript, do my site visitors have warm caches for these resources in their web browsers?

Answer 1

While not an answer, some interesting insight I found when working on this problem with Chrome:

Fetching the resource from CDN is a deferred call, while fetching an item from the cache seems in fact to be a blocking call, despite being quite fast.
Firefox didn't seem to this exhibit behavior consistently (and I didn't even bother with IE).

To test this observation further, I built the following small fiddle, which works reliably for me on Chrome. Please leave a comment on your own test results if you have the time.

var testSource = function(href) {
  var timeLimit = 5;
  var l = document.createElement("link");
  l.rel = "stylesheet";
  l.type = "text/css";
  l.href = href;
  var s1 = document.createElement("script");
  s1.innerHTML = "window.d = new Date();";
  var s2 = document.createElement("script");
  s2.innerHTML = "window.d2 = new Date();";
  document.head.appendChild(s1);
  document.head.appendChild(l);
  document.head.appendChild(s2);
  window.setTimeout(function() {
      var p = document.createElement("p");
      if (typeof(d2) === "undefined" || d2 - d > timeLimit) {
        p.innerHTML = "guess cache";
      } else {
        p.innerHTML = "guess load";
      }
      p.innerHTML += " (" + href + ")";
      document.body.appendChild(p);
    },
    timeLimit * 10);
}

btn.onclick = function() {
  testSource(inp.value);
}

<input type="text" id="inp" value="//maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css" />
<input type="button" id="btn" value="test url" />