How does the database actually work?
XROXY.COM is powered by C++, PHP, mySQL, and the occasional human touch. The actual list of proxies is stored in a MySQL database, which keeps track of each proxy's IP address, port number, status (open, closed, unknown, or blacklisted), anonymity, and latency. A suite of C++ tools and PHP scripts keep up with the maintenance of the database and the website itself.
One of the scripts runs automatically every few hours, probing all current open proxies to see whether or not they're still open. Records in the database are updated accordingly. If a previously open proxy becomes unreachable or the connection times out, that proxy is marked and will be tested once more later.
Another script reads in the list of visitor-submitted proxies, adds open ones to the database, and discards hosts which don't respond or are running private proxies. Several other checks are made during this part of the process, the most significant of which is that IPs which resolve to .gov, .mil, or .int hostnames are skipped without being tested.