We present a new formal model of query and computation on the Web. We focus
on two important aspects that distinguish the access to Web data from the
access to a standard database system: the navigational nature of the access
and the lack of concurrency control. We show that these two issues have si
gnificant effects on the computability of queries. To illustrate the ideas
and how they can be used in practice for designing appropriate Web query la
nguages, we consider a particular query language, the Web calculus, an abst
raction and extension of the practical Web query language WebSQL. (C)1998 P
ublished by Elsevier Science Ltd. All rights reserved.