Conflict Scheduling of Transactions on XML Documents

In the last few years an interest in native XML databases has surfaced. With other authors we argue that such databases need their own provisions for concurrency control since traditional methods are inadequate to capture the complicated update-behavior that is possible for XML documents. Ideally, updates should not be limited to entire document trees, but should involve subtrees and even individual elements. Providing a suitable scheduling algorithm for semistructured data can significantly improve collaborative systems that store their data — e.g. word processing documents or vector graphics — as XML documents. In this paper we improve upon earlier work which presented two equivalent concurrency control schemes based on Path Locks, and a commit scheduler for these schemes. In contrast to the earlier work, we now introduce a conflict scheduler for XML databases which uses the same path lock conflict rules and the same basic query and update languages. This new scheduler has significantly different properties than the commit scheduler. We also give a comprehensive proof of serializability of schedules accepted by the new scheduler.