A Data Model and Query Language for Distributed Service Discovery

In a large heterogeneous distributed system spanning administrative domains such as a DataGrid, it is often desirable to maintain and query dynamic and timely information about active participants such as services, resources and user communities. This paper develops a suitable database and query model as well as a generic and dynamic data model for such database systems. Unlike in the relational model the elements of a tuple in our data model can hold structured or semi-structured data in the form of any arbitrary well-formed XML document or fragment. An individual tuple element may, but need not, have a schema (XML Schema), in which case the content must be valid according to the schema. The elements of all tuples may, but need not, share a common schema. The concepts of (logical) query and (physical) query scope are cleanly separated rather than interwoven. A query is formulated against a global database view and is insensitive to link topology and deployment model. The query scope, on the other hand, is used to navigate and prune the link topology and filter on attributes of the deployment model. Example service discovery queries are given. Three query types are identified, namely simple, medium and complex. An appropriate query language (XQuery) is suggested. The suitability of the query language is demonstrated by formulating the example prose queries in the language. Detailed requirements for a query language supporting service and resource discovery are given. The capabilities of various query languages are compared.