A Transaction Model for XML Databases

Abstract

The hierarchical and semistructured nature of XML data may cause complicated update behavior. Updates should not be limited to entire document trees, but should ideally involve subtrees and even individual elements. Providing a suitable scheduling algorithm for semistructured data can significantly improve collaboration systems that store their data—e.g., word processing documents or vector graphics—as XML documents. In this paper we show that concurrency control mechanisms in CVS, relational, and object-oriented database systems are inadequate for collaborative systems based on semistructured data. We therefore propose two new locking schemes based on path locks which are tightly coupled to the document instance. We also introduce two scheduling algorithms that can both be used with any of the two proposed path lock schemes. We prove that both schedulers guarantee serializability, and show that the conflict rules are necessary.