Question

Let’s imagine a web page page needing to load a javascript file (i.e. my.js). Is it possible to organize the following fail-over loading sequence?

1. If server A is up, load my.js from server A.
2. Else, if server B is up, load my.js from server B.
3. Else, if server C is up, load my.js from server C.
4. …

If yes, how to proceed? Thanks.

P.S.: I have just found yepnopejs. Does anyone recommend it?

Answer 1

I have seen this technique to allow a fallback if a CDN is down.
If your js file has some testable property like a global variable (I’ve called it marker), you can attempt to load the file from server A, test for the marker and if it is not found script another attempt.

<script type="text/javascript" src="http://server_A.tld/my.js"></script>
<script type="text/javascript">
if( !window.marker ) {
    document.write(
        '<script type="text\/javascript" src="http:\/\/server_B.tld\/my.js"><\/script>'
    );
}
</script>

Update
There is no danger that all the scripts will run using this technique. John Resig explains this in a blog post.. Scripts can download in parallel and in any order but they must execute in order.

Here is a fiddle that demonstrates