So I'm the monkey in this ugly ugly code wrangling project. My particular problem, at the moment, is that I need to write a serializing component to ensure that n users can be browsing the site, purchasing (potentially) the same items, maybe not buy everything they say they want, and still not worry about inventory being misupdated. The server process is single-threaded, although potentially running on multiple processors. (or machines, network translucent toolkit)

The server's being modeled in Twisted, and the "DBMS" is an in-house XML sparse file system of Sisyphan proportions. Any recommendations as to open-source preexisting components which may provide algorithm ideas are highly welcome, even sought.