We propose the use of the universal relation as a user interface to
provide transparent access to a network of distributed, heterogeneous,
and autonomous information sources. We implement this interface in
two layers. The lower layer consists of access scripts, which
encapsulate knowledge about information sources and are capable of
answering basic queries. The upper layer uses combinations of these
scripts to answer user queries phrased in terms of a universal relation.
Access scripts know how to obtain information either directly from
sources or from service providers (mediators, traders, and the like).
They present this information in relational form, but with an inherent
direction, in the sense that whenever values for a fixed subset of
attributes of the relation are given, the access script will deliver
values for the rest of the attributes in the relation. In this paper,
we address the problem of defining the semantics of a user query posed
against the universal relation and of finding a sequence of access
script invocations that gathers the information requested in the