One of the first problems concerning network information systems is that of locating services. Today many organizations and institutes are making efforts to bring their own information on-line, but actually none of them has an obligation to provide an overview of all the astronomical on-line facilities that are currently available. The need for an overview has been recognized by Heinz Andernach, Robert J. Hanisch and Fionn Murtagh, and is addressed in their paper Network Resources for Astronomers. Their paper is also available in an on-line hypertext version, which allows easy access to the facilities and services mentioned in the paper. An even more ambitious programme to provide an overview, or directory service of on-line astronomical information and facilities, is the AstroWeb project. This initiative is supported by a `consortium' of seven institutions. It seems the AstroWeb project is well established now, and that the members of the `consortium' have de facto assumed the responsibility to provide this service for a longer period of time.
Currently, the AstroWeb service may be regarded as a directory or `yellow pages' service, and the various participating institutes are providing several, different ways to access and search the master database. The master database contains records in an agreed `data interchange format', which accommodates key words, and short and long descriptions of an entry. This should allow for sophisticated indexing and, in combination with e.g. a WAIS search engine, free search capabilities. We understand that work on such facilities is in progress. They are needed, to make the electronic AII as transparent and accessible for the End-User as the paper AII, with its long standing tradition of indexing and abstracting through libraries, library catalogues, and printed volumes of abstracts.
Quite another matter is that of the integrated functionality of services. The End-User is very interested in efficient ways of feeding the information, which he retrieved from one service, into another, so he may, in successive steps, obtain the information he is interested in. A researcher could, for example, start his investigations concerning a particular astronomical object with the SIMBAD database. This provides him with references to literature and data sets. The researcher will then want to retrieve some of those data sets and the software to analyse them. Today he needs to consult various sources separately, to find out if, and where, these data and software are available at a site which may be accessed through the Internet. The subsequent retrieval and installation of data and software again require several actions from the researcher or his system manager. Effectively, the researcher needs to be familiar with things like `ftp', `archie', WWW, operating systems, etc. to be able to use the various on-line facilities to his benefit. Obviously, one would like the required actions to be simplified and automated, allowing the researcher to concentrate on the science, rather than on computer and network trivia.
In fact, there are two systems that provide precisely this kind of automated and integrated functionality: the Astrophysics Data System, ADS, operated by NASA, and the European Space Information System, ESIS, operated by ESA. However, at this moment in time the continuation of both systems is rather uncertain (see section 1.2). The scope of the integrated functionality is, for both the ADS and ESIS, limited to the tasks that have been built into each system. Since no single effort will ever be able to cover all of astronomy, ways will have to be sought which allow integration of functionality across different information systems. This may be achieved by defining or adopting a data exchange and query format, similar to the FITS standard for filing astronomical data, and SQL, the Structured Query Language.
Apart from this data exchange format, one would require a flexible or modular client-server system, with a client that is able to send data, that were retrieved from one server, to another, either directly or after some modification. The concepts of such a modular client-server system are discussed in section 4.