Formal Models of Web Queries \Lambda Extended abstract

The paper presents a new 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 signiicant eeect on the computabil-ity of queries. To illustrate the ideas and how they can be used in practice for designing appropriate Web query languages , we consider a particular query language, the Web calculus, an abstraction and extension of the practical Web query language WebSQL. 1 Introduction Tools and techniques for retrieving information from the World Wide Web are rapidly being developed 14, 15, 16, 5]. Most of these works are based on the metaphor of the Web as a database, in order to carry over and adapt familiar query languages such as SQL or Datalog. However, the Web is not a database, and querying it diiers signiicantly from querying a conventional database. Two essential aspects that make a query on the Web diierent from a query on a standard database are the lack of concurrency control and the limited data access capabilities. First, consider the impact of the lack of concurrency control on which queries can and cannot be eeectively evaluated. Concurrency control on the Web is very crude: the owner of a document can lock it to prevent others from ac-cessing it while it is being modiied, but no other user can do this, and there is no transaction mechanism. This makes many queries in principle impossible to compute in a nite time. For example, nding the documents reachable directly or indirectly from a given starting point may require following a chain in which documents are being added to the end of the chain faster than we can discover them, so the computation may never terminate.