Semistructured data models provide an important tool for studying methods for representing, searching, and processing heterogeneous data in distributed systems with autonomous components (especially, on the WWW). However, the problem of data consistency remains little investigated. In this paper, various approaches to this problem are discussed, and the usability of various mechanisms suggested in the context of databases, including real-time databases, is analyzed. An approach based on the concept of multilevel transactions is presented in detail. In the framework of this approach, commutativity predicates for basic operations on semistructured data are constructed.