ADASS 2002 Conference Proceedings

In this article we describe how the GLU system (Uniform Link Generator) allows the Aladin browsing tool to integrate several image and data servers through a single interface. We describe how it works, how it is updated and how it is implemented in a Java context. We also present the future evolution we foresee for the GLU system in order to allow interaction with the emerging Web Services.

1. Interoperability Needs

Aladin is now widely known as a tool to display and cross-match heterogeneous data and images anticipating future VO portals. It offers transparent access to Simbad, VizieR, CDS image servers (DSS, MAMA, 2MASS), NED, SkyView and SuperCosmos databases, NVSS and FIRST archives, as well as mission logs such as CFHT, Chandra, HST, HUT, ISO, IUE and Merlin.

2. GLU Functionalities

The GLU system is a distributed repository of Web server descriptions. It was designed and written in 1996 by the Centre de Données astronomiques de Strasbourg (CDS). Presently, the GLU manages thirty replicated sites all over the world and about five hundred Web resource descriptions.

A GLU resource description, also called a GLU record, typically contains a unique identifier for the resource, a short description, a URL template to query the resource, and additional information describing the data type of the query parameters. The latter may include celestial coordinates, astronomical objects designations, bibliographic codes and so forth. The format of the results, VOTable, FITS image, HTML pages etc must also be included.

Figure 1: Example GLU record for the SuperCosmos resource.


   <RESOURCE ID="CDS/aladin/SSS.img">
      <DESCRIPTION>SuperCOSMOS image server- Edinburgh (UK)</DESCRIPTION>
      <QUERY>
         <URL>http://www-wfau.roe.ac.uk/~sss/cgi-bin/sss_aladin_pix.cgi?
	         ra=$1&dec=$2&mime-type=image/x-gfits&
	         x=$3&y=$4&waveband=$5</URL>
         <VAR NAME="1">
            <DESCRIPTION>Right Ascension (J2000)</DESCRIPTION>
            <TYPE REF="Coo(J2000,RA)"/>
         </VAR>
         <VAR NAME="2">
            <DESCRIPTION>Declination (J2000)</DESCRIPTION>
            <TYPE REF="Coo(J2000,DE)"/>
         </VAR>
         <VAR NAME="3">
            <DESCRIPTION>Width (arcmin)</DESCRIPTION>
            <TYPE REF="Field(RAm)"/>
            <VALUE DEFAULT="true">10</VALUE>
         </VAR>
         <VAR NAME="4">
            <DESCRIPTION>Height (arcmin)</DESCRIPTION>
            <TYPE REF="Field(DEm)"/>
            <VALUE DEFAULT="true">10</VALUE>
         </VAR>
         <VAR NAME="5">
            <DESCRIPTION>Waveband</DESCRIPTION>
            <VALUE DEFAULT="true">1 - UKST Blue (Bj)</VALUE>
            <VALUE>2 - UKST Red (R)</VALUE>
            <VALUE>3 - UKST Infrared (I)</VALUE>
            <VALUE>4 - ESO red (R)</VALUE>
            <VALUE>5 - POSS-I red (R)</VALUE>
         </VAR>
      </QUERY>
      <RESULT><CONTENT-TYPE>image/gfits</CONTENT-TYPE></RESULT>
      <DOC ROLE="user" HREF="http://www-wfau.roe.ac.uk/sss/"/>
   </RESOURCE>

The GLU is distributed with a library toolkit in C and Perl to access the repository (gludic tool) and to generate the URL corresponding to a Web resource (glufilter tool). In this latter task, the GLU not only maps the query parameters passed by the user into the correct fields of the query URL template, but the GLU is also able to make the appropriate transformations to adapt the query parameters to the format and parameter types required by the remote server. For example, astronomical object identifiers are converted into their celestial coordinates, or the coordinates can be precessed and/or edited according to the server's requirements.

3. Aladin/GLU Algorithm

To understand how Aladin and GLU interact, we describe step by step the Aladin/GLU algorithm:

**Figure 2:** GLU/Aladin interactions
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4. GLU and Web Services

With SOAP, WSDL and UDDI, the new Web Services will offer an alternative to describe and access Web resources. With these new standards we anticipate that servers will provide one or more SOAP options within the next two years in addition to the classical HTTP access.

In this context, we have planned to extend the next GLU release in three directions:

References

Fernique, P. Ochsenbein, F. Wenger, M. 1998, in ASP Conf. Ser., Vol. 145, Astronomical Data Analysis Software and Systems VII, ed. R. Albrecht, R. N. Hook, & H. A. Bushouse (San Francisco: ASP), 466

A Bit of GLUe for the VO: Aladin Experience

Abstract:

1. Interoperability Needs

2. GLU Functionalities

3. Aladin/GLU Algorithm

4. GLU and Web Services

References

Footnotes