Where I can find examples of Drupal bootstrap levels? I recently came through some Drupal apps that I needed to send some silent Ajax requests that performs a database update, and nothing else. Every such requests bootstraps Drupal to its full level which, as far as I can see, is a waste of resources. 

Where I can find examples that someone would bootstrap Drupal, but not to its full level?

One example is the statistics module in core. ./modules/statistics/statistics.php. Do you know any others?

Please note this question is not about "how to bootstrap". Rather, in trying to see some examples of applications of it. 

Thanks, and hats off everyone in advance.
The JS module is specifically for this. It allows you to create ajax callback that only bootstrap to a level you specify via a hook_js().  

So you could either use this module or take a look at its code to see how it works.  

Good blog post about it HERE