Bash script, that guards the availability or functionality of more cascaded proxy servers in your network and switches trough them to minimize drop-out times. The result is squid.conf, re-applied immediately after change.
Use cron to execute this script regularly (each 3-10 minutes, depends on how quickly you want your clients happy).
Two modes are available: automatic and manual - automatics act according to custom settings and switches the cascaded proxy server by priorities of your servers, if the server above in the list doesn't work. Manual settings blocks the automatics and sets an uplink of your choice.
Furthermore, to test the functionality of the proxy, the script uses links (text web browser), or availability of the site by pinging. Each has its pros and cons: checking functionality - finds out practical real functionality, but is dependant on a website; availability - lesser data rate, but server can be available without working proxy service.