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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
CDS GLU, a Tool for Managing Heterogeneous
Distributed Web Services
P. Fernique, F. Ochsenbein and M. Wenger
Centre de Donn’ees astronomiques de Strasbourg, 11 rue de l'universit’e,
67000 STRASBOURG ­ FRANCE, Email: question@simbad.u­strasbg.fr
Abstract. The Web development has been a very important milestone
in unifying the access to remote databases : a unique user interface, a
unique network protocol, and a hypertext link mechanism (via URLs ­
Uniform Resource Links). But if creating URLs is easy, maintaining them
is the real challenge for database managers.
In this context, the CDS (Centre de Donn’ees astronomiques de Stras­
bourg) has developed the GLU system (G’en’erateur de Liens Uniformes).
This tool allows all managers participating in the system to describe and
easily distribute the URL definitions required to access their databases.
The GLU system ensures that all GLU members receive these definitions
and will receive all future modifications. So, the GLU system is particu­
larly adapted to design cooperative Web services, allowing one to generate
links to other services which remains always up­to­date.
1. Introduction
The World Wide Web has allowed significant progress towards astronomical
service interoperability : the Web is understood through all the Internet, it is
very easy to interface it with databases, and hypertext is a powerful concept to
obtain good interoperability between databases.
However for this last functionality, a keystone, the ability to define a unique
access key to a given information location, the URN (Uniform Resource Name),
is still missing. A IETF Working Group has been working on it for a few years,
but there is presently no implementation of such a system (Berners­Lee 1994)
Therefore everybody is still using URLs (Uniform Resource Locators) instead
of URNs, with their well known drawbacks: at any time, any component of
an URL (the hostname, the directory, the resource name and/or the parameter
syntax) can be modified without any possibility of informing potential users.
The real challenge is thus to maintain all these URLs. For this, the Cen­
tre de Donn’ees astronomiques de Strasbourg (CDS 1 ) has developed the GLU
(G’en’erateur de Liens Uniformes ­ Uniform Link Generator) which allows one to
use symbolic names instead of hard­coded URLs in data.
1 http://cdsweb.u­strasbg.fr
466

Managing Heterogeneous Distributed Web Services 467
...keywords : galaxies: individual: http://simbad.u­strasbg.fr/simbad?id=M31>
M31 ...
The result sent to the user
... keywords : galaxies: individual: <&simbad M31>...
In the data base
%ActionName
%Description
%Url
simbad
Simbad query by identifier
http://simbad.u­strasbg.fr/simbad?id=$1
In the GLU dictionary
Figure 1. Usage of GLU tag (symbolic name: simbad, parameter: M31)
2. The GLU System
To fulfill this aim, the GLU implements two concepts:
1. the GLU dictionary which is a compilation of symbolic names with their
corresponding URLs maintained up­to­date.
2. the GLU resolver which replaces symbolic names by the relevant URLs on
the fly.
Using the GLU system, data managers can forget the URLs and just use
their symbolic names ­ using GLU tags ­ in all their Web documents. Every
time a server sends a database result, the GLU replaces, on the fly, the GLU
tags by the corresponding URLs and substitutes the parameters at the proper
locations (see the example in Figure 1).
The GLU system di#ers from other similar tools (such as the PURL system
­ developed by OCLC O#ce of Research) in that the GLU allows each of its
members to locally resolve the URLs using its own view of the GLU dictionary.
This is an important choice to ensure the resolution speed and the security of
the system. So, for this replacement mechanism, only a simple modification
to the Web server configuration is required: specifying that the data streams
have to be filtered by the GLU resolver during output. For the advanced Web
programmer, the GLU has two libraries, one in Perl, the other in C to resolve
GLU tags directly in CGI (Common Gateway Interface) programs.
An important piece of the GLU system is the mechanism maintaining each
view of GLU dictionary. This task is performed by a daemon, called glud, which
has the responsibility to send and to receive the GLU records (the entries in the
dictionary).
The GLU protocol used by these daemons has the following characteristics:

468 Fernique, Ochsenbein and Wenger
. It is based on distribution domains. Managers choose the GLU domains
in which they want to distribute their own GLU records. In the same
way, they choose the GLU domains from which they want to receive GLU
records.
. It uses a hierarchical name space for the GLU record identifiers (this en­
sures their uniqueness through the whole system).
. It is pretty fault tolerant and secure: independent views of the GLU dic­
tionary, mechanism authenticating the update sender, ...
Several other functionalities are also addressed by the GLU system, the
most important being:
. The management of clones : in the GLU dictionary, it is possible to specify
several URLs for a symbolic name. The GLU daemon will test each of
them regularly and will label the best for the future resolutions (based on
connection speed and proximity).
. The capability to use the GLU system as a general macro system : in­
stead of a simple URL, it is possible to specify in the GLU record a full
HTML text. In this way, homogeneous HTML pages (for headers, menus,
forms,...) are easy to maintain (see the CDS pages).
3. GLU Tools
To take the full benefit of all GLU facilities, three GLU tools have been devel­
oped:
1. The Dictionary Browser 2 . It is used by the data managers to check the
GLU dictionary content (see Figure 2).
2. The Test Sequencer. Its purpose is to check all the URLs of the GLU
dictionary to inform the managers of a disfunctionality.
3. The Service Browser (called AstroGLU). An ``intelligent'' service direc­
tory using the capability of the GLU to give access to the participating
Web services by commonly known data types (see Egret 1997).
4. Conclusion
Presently, two centers use the GLU system : the CDS, for its own Web services,
and the CNES, the French Space Agency for the Space Physics database project
(CDPP).
The GLU package 3 has been designed for a large cooperation and is opened
to other astronomy partners.
2 http://simbad.u­strasbg.fr/demo/cgi­bin/glu dic.pl
3 http://simbad.u­strasbg.fr/demo/glu.html

Managing Heterogeneous Distributed Web Services 469
Figure 2. GLU Browser tool
References
Egret, D., et al., 1998, this volume
Wenger, M, 1996 et al., Bull. American Astron. Soc., 189, 602
Berners­Lee, T., 1994, Request For Comments 1630