I know there are monitoring services that simply visit a site from time to time, and send a notification when the site isn't available, or if the page can't be found. What I'm looking for though is a monitoring service that'll notify me e.g. in case of a mysql problem (but where the server still returns a "page found"). The most basic approach would be a notification if the page is smaller than a certain size, e.g. if the page size suddenly drops way below 10k then it's obvious that there's some problem (like the page only displaying a "couldn't connect to database" error). Of course, a more sophisticated version could also scan the page content for signs of an error.

Does anyone know if a service or script like that exists? Thanks!