VODataService: a VOResource Schema Extension for Describing Collections and Services
Version 1.01

Status of this document

A list of current IVOA Recommendations and other technical documents can be found at http://www.ivoa.net/Documents/.

Acknowledgements

This document has been developed with support from the National Science Foundation's Information Technology Research Program under Cooperative Agreement AST0122449 with The Johns Hopkins University, from the UK Particle Physics and Astronomy Research Council (PPARC), and from the Eurpean Commission's Sixth Framework Program via the Optical Infrared Coordination Network (OPTICON).

The Virtual Observatory (VO) is general term for a collection of federated resources that can be used to conduct astronomical research, education, and outreach. The International Virtual Observatory Alliance (IVOA) is a global collaboration of separately funded projects to develop standards and infrastructure that enable VO applications.

XML document validation is a software process that checks that an XML document is not only well-formed XML but also conforms to the syntax rules defined by the applicable schema. Typically, when the schema is defined by one or more XML Schema [schema] documents (see next section), validation refers to checking for conformance to the syntax described in those Schema documents. This document describes additional syntax constraints that cannot be enforced solely by the rules of XML Schema; thus, in this document, use of the term validation includes the extra checks that goes beyond common Schema-aware parsers which ensure conformance with this document.

The eXtensible Markup Language, or XML, is document syntax for marking textual information with named tags and is defined by the World Wide Web Consortium (W3C) Recommendation, XML 1.0 [XML]. The set of XML tag names and the syntax rules for their use is referred to as the document schema. One way to formally define a schema for XML documents is using the W3C standard known as XML Schema [schema].

This document defines the VOResource schema using XML Schema. The full Schema document is listed in Appendix A. Parts of the schema appear within the main sections of this document; however, documentation nodes have been left out for the sake of brevity.

Reference to specific elements and types defined in the VOResource schema include the namespaces prefix, vr, as in vr:Resource (a type defined in the VOResource schema). Reference to specific elements and types defined in the VODataService extension schema include the namespaces prefix, vs, as in vs:DataCollection (a type defined in the VODataService schema). Use of the vs prefix in compliant instance documents is strongly recommended, particularly in the applications that involve IVOA Registries (see [RI], section 3.1.2). Elsewhere, the use is not required.

Contents

• Abstract
• Status of this document
• Acknowledgments
• Conformance-related definitions
• Syntax Notation Using XML Schema
• 1. Introduction
• 2. The VOResource Data Model
  • 2.1. The Schema Namespace and Location
  • 2.2. Summary of Metadata Concepts
• 3. The VODataService Metadata
  • 3.1. Resource Type Extensions
  • 3.2. Coverage
  • 3.3. Tabular Data
  • 3.4. Interface Type Extension: ParamHTTP
• Appendix A: the VODataService XML Schema
• Appendix B: Compatibility Issues with VODataService 1.0
• Appendix C: Change History
• References

1. Introduction

The VOResource standard [VOR] provides a means of encoding IVOA Resource Metadata [RM] in XML. VOResource uses XML Schema [schema] define most of the XML syntax rules (while a few of the syntax rules are outside the scope of Schema). VOResource also describes mechanisms for creating extensions to the core VOResource metadata. This allows for the standardization of new metadata for describing specialized kinds of resources in piecemeal way without deprecating the core schema or other extensions. This document defines one such extension referred to as VODataService.

The purpose of this extension is to define common XML Schema types--particularly new resource types--that are useful for describing data collections and services that access data. In particular, it allows one to describe the data's coverage: the parts of the sky the data is associated with and the time and frequency ranges that were observed or modeled to create the data. It also allows one to describe tables in detail. In particular, one can describe each of the columns of a table--providing, for example, its name, type, UCD, and textual description. When this metadata is part of a resource description in a registry [VOR], it becomes possible to discover tables that contains particular kinds of data.

It is intended that VODataService will be central to describing services that support standard IVOA data access layer protocols such as Simple Image Access [SIA] and Simple Cone Search [SCS]. While other VOResource extensions would define the protocol-specific metadata (encapsulated as a standard capability [VOR]), the general service resource description would share the common data concepts such as coverage and tabular data. Note, however, that a service described using the VODataService schema need not support any standard protocols. With the VODataService extension schema plus the core VOResource schema, it is possible to describe a custom service interface that accesses data.

As a legal extension of VOResource [VOR], the use of VODataService is subject to the rules and recommendations for XML [xml], XML Schema [schema], and VOResource itself.

The VODataService extension in general enables the description of two types of resources: data collections and services that access data. Here's an example of a VOResource document (abbreviated for the purposes of illustration) that describes a service from the NASA Extragalactic Database (NED) that provides measured redshifts for a given object.

2 



1
1
1

3
3
3
3
3
3

4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4

4
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
4

6
6
6
6
6
6
6
6
6

6
6
6

7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7

<?xml version="1.0" encoding="UTF-8"?>
<ri:Resource xmlns="" 
             xsi:type="vs:CatalogService" status="active" 
             updated="2008-04-29T14:51:54Z" created="2005-10-14T01:46:00Z"
             xmlns:ri="http://www.ivoa.net/xml/RegistryInterface/v1.0" 
             xmlns:vr="http://www.ivoa.net/xml/VOResource/v1.0" 
             xmlns:vs="http://www.ivoa.net/xml/VODataService/v1.1" 
             xmlns:stc="http://www.ivoa.net/xml/STC/stc-v1.30.xsd" 
             xmlns:xlink="http://www.w3.org/1999/xlink" 
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://www.ivoa.net/xml/VOResource/v1.0
                                 http://www.ivoa.net/xml/VOResource/v1.0
                                 http://www.ivoa.net/xml/VODataService/v1.1
                                 http://www.ivoa.net/xml/VODataService/v1.1
                                 http://www.ivoa.net/xml/STC/stc-v1.30.xsd
                                 http://www.ivoa.net/xml/STC/stc-v1.30.xsd">

  <title>The NASA/IPAC Extragalactic Database</title>
  <shortName>NED_redshift</shortName>
  <identifier>ivo://ned.ipac/Redshift_By_Object_Name</identifier>
  <curation>
    <publisher>The NASA/IPAC Extragalactic Database</publisher>
    <contact>
      <name>Olga Pevunova</name>
      <email>contact@datacenter.edu</email>
    </contact>
  </curation>
  <content>
    <subject>redshift</subject>
    <subject>galaxies</subject>
    <description>
      NED is built around a master list of extragalactic objects for
      which cross-identifications of names have been established,
      accurate positions and redshifts entered to the extent possible,
      and some basic data collected. This service will return recorded 
      redshifts for a given object.
    </description>
    <referenceURL>http://nedwww.ipac.caltech.edu/help/data_help.html#zdat</referenceURL>
    <type>BasicData</type>
    <contentLevel>Research</contentLevel>
  </content>

  <capability>
    <interface xsi:type="vs:ParamHTTP">
      <accessURL use="base">
         http://nedwww.ipac.caltech.edu/cgi-bin/nph-datasearch?search_type=Redshifts&
      </accessURL>
      <queryType>GET</queryType>
      <resultType>application/xml+votable</resultType>
      <param use="required">
        <name>objname</name>
        <description>Name of object</description>
        <dataType>string</dataType>
      </param>
      <param use="required">
        <name>of</name>
        <description>Output format parameter, must be "xml_main" for VOTable output.</description>
        <dataType>string</dataType>
      </param>
    </interface>
  </capability>

  <coverage>
    <stc:STCResourceProfile>
       <stc:AstroCoordSystem xlink:type="simple"
                             xlink:href="ivo://STClib/CoordSys#UTC-FK5-TOPO" 
                             id="UTC-FK5-TOPO"/> 
       <stc:AstroCoordArea coord_system_id="UTC-FK5-TOPO">
          <stc:AllSky/>
       </stc:AstroCoordArea>
    </stc:STCResourceProfile>
    
    <waveband>Radio</waveband>
    <waveband>Optical</waveband>
  </coverage>

  <tableset>
    <name>default</name>
    <table role="out">
      <qualifiedName>default</qualifiedName>
      <column>
        <name>No.</name>
        <description>
           A sequential data-point number applicable to this list only.
        </description>
        <ucd>meta.number</ucd>
        <dataType>int</dataType>
      </column>
      <column>
        <name>Name in Publication</name>
        <description>
           The object's name in NED's standard format, of the object to which the data apply.
        </description>
        <ucd>meta.id;name</ucd>
        <dataType>string</dataType>
      </column>
      <column>
        <name>Published Velocity</name>
        <description>
          The radial velocity , derived from derived from the shift of some spectral feature, in km/sec
        </description>
        <unit>km/sec</unit>
        <ucd>src.spect.dopplerVeloc</ucd>
        <dataType>int</dataType>
      </column>
    </table>
  </tableset>
</ri:Resource>

1. the extra namespaces associated with VODataService metadata; if STC coverage information is not included, then only the VODataService namespaced is needed.
2. the specific type of resource indicated by the value of the xsi:type attribute; in this case vs:CatalogService indicates that this is describing a service that accesses tabular data.
3. the location of the VOResource-related schema documents used by this description,
4. the core VOResource metadata,
5. an interface described by the VODataService interface type, vs:ParamHTTP; this type can indicate input arguments it supports.
6. a description of the coverage, including an STC description plus waveband keywords.
7. a description of the table that is returned by the service.

2.1. The Schema Namespace and Location

The namespace associated with VODataService extensions is "http://www.ivoa.net/xml/VODataService/v1.1". Just like the namespace URI for the VOResource schema, the VODataService namespace URI can be interpreted as a URL. Resolving it will return the XML Schema document (given in Appendix A) that defines the VODataService schema.

Authors of VOResource instance documents may choose to provide a location for the VOResource XML Schema document and its extensions using the xsi:schemaLocation attribute. While the choice of the location value is the choice of the author, this specification recommends using the VODataService namespace URI as its location URL (as illustrated in the example above), as in,

xsi:schemaLocation="http://www.ivoa.net/xml/VODataService/v1.1
                    http://www.ivoa.net/xml/VODataService/v1.1"

Note: The IVOA Registry Interface standard [RI] actually requires that the VOResource records it shares with other registries provide location URLs via xsi:schemaLocation for the VOResource schema and all legal extension schemas that are used in the records. This rule would apply to the VODataService schema.

The prefix, vs, is used by convention as the prefix defined for the VODataService schema; however, instance documents may use any prefix of the author's choosing. In applications where common use of prefixes is recommended (such as with the Registry Interface specification [RI]), use of the vs prefix is recommended. Note also that in this document, the vr prefix is used to label, as shorthand, a type or element name that is defined in the VOResource schema, as in vr:Resource.

Note: One reason one may not be able to use vs to represent the VODataService schema, version 1.1, is because it is already in defined to represent VODataService v1.0 and cannot be overridden. At this writing, there are no IVOA applications in which this is the case. Consult Appendix B for more details on compatibility issues.

As recommend by the VOResource standard [VOR], the VODataService schema sets elementFormDefault="unqualified". This means that it is not necessary to qualify element names defined in this schema with a namespace prefix (as there are no global elements defined). The only place it is usually needed is as a qualifier to a VODataService type name given as the value of an xsi:type attribute.

References

[RFC 2119]

Bradner, S. 1997. Key words for use in RFCs to Indicate Requirement Levels, IETF RFC 2119, http://www.ietf.org/rfc/rfc2119.txt

[RM]

Hanisch, Robert (ed.) 2004. Resource Metadata for the Virtual Observatory, Version 1.01, IVOA Recommendation, http://www.ivoa.net/Documents/REC/ResMetadata/RM-20040426.htm

[xml]

Bray, Tim, Paoli, Jean, Sperberg-McQueen, C. M., Maler, Eve, Yergeau, Francois (editors) 2004, Extensible Markup Language (XML) 1.0 (Third Edition), W3C Recommendation 04 February 2004, http://www.w3.org/TR/REC-xml

[schema]

Fallside, David C., Walmsley, Priscilla (editors) 2004, XML Schema Part 0: Primer Second Edition, W3C Recommendation 28 October 2004, http://www.w3.org/TR/xmlschema-0/

[VOR]

Plante, Raymond, Benson, Kevin, Graham, Matthew, Greene, Gretchen, Harrison, Paul, Lemson, Gerard, Linde, Tony, Rixon, Guy, Stébé, Aurélien 2008, VOResource: an XML Encoding Schema for Resource Metadata, v1.03, IVOA Recommendation, http://www.ivoa.net/Documents/PR/ReR/VOResource-20080222.html

[SIA]

Tody, Doug and Plante, Ray 1997, Simple Image Access Specification Version 1.0, IVOA Working Draft, http://www.ivoa.net/Documents/WD/SIA/sia-20040524.html.

[RI]

Benson, Kevin, Plante, Ray, Auden, Elizabeth, Graham, Matthew, Greene, Gretchen, Hill, Martin, Linde, Tony, Morris, Dave, O'Mullane, Wil, Rixon, Guy, Andrews, Kona 2008, IVOA Registry Interfaces, v1.02, IVOA Recommendation, http://www.ivoa.net/Documents/latest/ResourceInterface.html

[STC]

Rots, Arnold, Space-Time Coordinate Metadata for the Virtual Observatory, v1.33, IVOA Recommendation, href="http://www.ivoa.net/Documents/REC/STC/STC-20071030.html">