VOResource: an XML Encoding Schema for Resource Metadata
Version 1.01

Status of this document

This is an IVOA Working Draft for review by IVOA members and other interested parties. It is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use IVOA Working Drafts as reference materials or to cite them as other than "work in progress."

Comments on this document are due 10 June 2006 for consideration in the next version of this document. They should be sent to registry@ivoa.net, a mailing list with a public archive or on the VOResource twiki discussion page. General discussion of related technology is also welcome on the Rwp03 wiki site.

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.

UTC refers to Universal Coordinated Time as defined by....

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). Use of the vr prefix in compliant instance documents is not required (see section 2.1); its use in this document is simply to indicate that it is an entity defined in the VOResource schema.

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. The Core Structural and Semantic Model
  • 2.3. Extending the VOResource Schema
• 3. The VOResource Metadata
  • 3.1. The Base Resource Type
  • 3.2. Resource Type Extensions: Organisation and Service
• Appendix A: the VOResource XML Schema
• References

1. Introduction

The IVOA Standard, Resource Metadata for the Virtual Observatory [Hanisch et al. 2004] (hereafter referred to as RM) defines metadata terms for describing resources. The RM defines a resource as:

... VO element that can be described in terms of who curates or maintains it and which can be given a name and a unique identifier. Just about anything can be a resource: it can be an abstract idea, such as sky coverage or an instrumental setup, or it can be fairly concrete, like an organization or a data collection. This definition is consistent with its use in the general Web community as "anything that has an identity" (Berners-Lee 1998, IETF RFC2396). We expand on this definition by saying that it is also describable.

The resource metadata are, then, the terms and concepts that describe a resource in general. The RM defines the terms as well as describes reasonable or allowed values; it does not, however, describe how the terms and values should be encoded. This is because resource metadata may be encoded in several different formats, depending on the context. This document specifically describes an encoding called VOResource.

The primary intended use of VOResource is to provide an XML interchange format for use with resource registries. A registry is a repository of resource descriptions [RM] and is employed by users and applications to discover resources. VOResource can be used to pass descriptions from publishers to registries and then from registries to applications. Another inended use is as a language for services to describe themselves directly. VOResource may be used in other ways, in whole or in part, using the standard XML mechanisms (e.g., import, include).

The VOResource schema provides XML encoding for so-called core metadata from the RM that (with a few exceptions) can apply to all resources; however, it is recognized that a full and useful description of a specific resource will require additional metadata that is relevant only to a resource of its type. Thus, the VOResource schema has been especially designed to be extended. The model for doing this is described in section XX.

Note: The name "VOResource" has in practice had two meanings within IVOA discussions. The first refers specifically to the core XML schema defined by this document. The second refers more broadly to the core schema plus the set of legal extensions. In this document, use of the name "VOResource" corresponds to the first meaning. Reference to the broader set of schemas will be indicated explicitly with the annotating phrase, "and its legal extensions."

2. The VOResource Data Model

The primary use for VOResource, of course, is to describe a resource using the metadata concepts defined in the RM. Here's an example of a VOResource document describing an organisation, the Radio Astronomy Imaging Group at the National Center for Supercomputing Applications.

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<?xml version="1.0" encoding="UTF-8"?>
<resource xsi:type="Organisation"
  xmlns:vr="http://www.ivoa.net/xml/VOResource/v1.0"
  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">

    <title>NCSA Radio Astronomy Imaging</title>
    <shortName>NCSA-RAI</shortName>
    <identifier>ivo://rai.ncsa/RAI</identifier>

    <curation>
      <publisher ivo-id="ivo://ncsa.uiuc/NCSA">
         National Center for Supercomputing Applications
      </publisher>
      <creator>
        <name>   Dr. Richard Crutcher    </name>
        <logo>
           http://rai.ncsa.uiuc.edu/rai.jpg
        </logo>
      </creator>
      <date>1993-01-01</date>
      <contact>
        <name>Dr. Raymond Plante</name>
	<email>rplante@ncsa.uiuc.edu</email>
      </contact>
    </curation>

    <content>
      <subject>radio astronomy</subject>
      <subject>data repositories</subject>
      <subject>digital libraries</subject>
      <subject>grid-based processing</subject>
      <description>
        The Radio Astronomy Imaging Group at the National Center for
        Supercomputing Applications is focused on applying
        high-performance computing to astronomical research.  Our
        projects include the NCSA Astronomy Digital Image Library, 
        the BIMA Data Archive, the BIMA Image Pipeline, and the
        National Virtual Observatory. 
      </description>
      <referenceURL>http://rai.ncsa.uiuc.edu/</referenceURL>
      <type>Organisation</type>
      <contentLevel>Research</contentLevel>
    </content>

    <facility>Berkeley-Illinois-Maryland Array (BIMA)</facility>
    <facility>
       Combined Array for Research in Millimeter Astronomy (CARMA)
    </facility>

</resource>

This example illustrates some important components of a VOResource record:

1. the VOResource namespace,
2. the specific type of resource indicated by the value of the xsi:type attribute,
3. the location of the schema documents used by this description,
4. values for the three main types core metadata: identity, curation, and content,
5. a reference to another resource is made by providing that resource's IVOA identifier,
6. string values can be padded with spaces for easier readability,
7. extension metadata specific to the type of resource.

2.1. The Schema Namespace and Location

The VOResource schema namespace is "http://www.ivoa.net/xml/VOResource/v1.0". The namespace URI has been chosen to allow it to be resolved as a URL to the XML Schema document (given in Appendix A) that defines the VOResource schema. Applications may assume that the namespace URI is so resolvable.

Authors of instance documents that use the VOResource schema may choose to provide a location for VOResource XML Schema document using the xsi:schemaLocation attribute; the choice of the location value is the choice of the author. In general, the use of xsi:schemaLocation is recommended by this specification with a the namespace URI given as the location as illustrated in the example above, unless the application prefers otherwise.

xsi:schemaLocation="http://www.ivoa.net/xml/VOResource/v1.0
                    http://www.ivoa.net/xml/VOResource/v1.0"

Whenever instance validation is needed, use of the VOResource schema and its legal extensions must be declared using the standard namespace declaration attribute, xmlns:prefix (where prefix is an arbitrary prefix). The prefix, vr, is used by convention as the prefix defined for the VOResource schema; however, instance documents may use any prefix of the author's choosing. 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.

Because the VOResource XML schema sets elementFormDefault="unqualified", documents that use the VOResource schema should not use the namespace declaration attribute, xmlns (used to set the default namespace), anywhere in the document where the VOResource schema is in effect. (This is a restriction set by the rules of XML Schema.) Furthermore, in accordance with the Schema rules for unqualified elements, the VOResource namespace prefix must not used to qualify VOResource elements. In general, namespace prefixes are only used to qualify type names given as values to the xsi:type attribute (see next section). Legal extensions of the VOResource schema SHOULD also set elementFormDefault="unqualified" for consistancy.

The VOResource schema only defines global types; it does not define any global elements (often refered to as root elements). It is the responsibility of the application to define the root element of the VOResource-employing documents it operates on. Typically, the root element is defined in a separate application-specific schema. The type of an application document's root element is not assumed to be any particular type defined in the VOResource schema (nor any of its legal extensions). In fact, it need not be of a type from the VOResource at all; rather, VOResource types may appear anywhere in the document.

Note: The IVOA Registry Interface standard, for example, includes a small schema that defines a single global element, <VOResources>, that can contain a series of <resource> child elements. The child element is defined to be of the type Resource from the VOResource schema.

Note: In the example instance document at the beginning of section 2, the root element, <resource> is not defined in any schema. Nevertheless, this document is still legal and verifiable XML. This is because the element's type is explicitly specified with the xsi:type attribute.

VOResource uses the following conventions for names of elements and types:

• all global types it defines have names that are capitalized. (This practice would extend to global elements, if they existed in the VOResource.)
• Locally defined elements begin with a lower-case character.
• For all types and element names that are made up of multiple words, such as shortName, upper-case letters are used demarcate the start of appended word (the "camel" format).
• Names that include abbreviations, such as IdentifierURI, all letters in the abbreviation are capitalized.

Applications describe a single resource using an element of the type vr:Resource or a legal derivation of it. The content of the vr:Resource type is referred to as the core VOResource metadata, and they fall into four categories (corresponding to the sections 3.1, 3.2, 3.3, and 4 of the RM):

• identity metadata: the <title>, <shortName>, and <identifier> elements;
• curation metadata: the contents of the <curation> element;
• general content metadata: the contents of the <content> element;
• metadata quality flags: the <validationLevel> element.

These elements are defined in more detail in section 3.

Many of the elements in VOResource that are meant to have string or URI values are defined as being of the type xs:token. This allows authors of VOResource instance documents to pad string and URI values with spaces and include carriage returns to improve readability. The definition of these types will cause an XML Schema-compliant parser to replace tab, line feed, and carraige return characters with simple spaces, then replace multiple sequential occurrences of spaces with a single space, and then remove all leading and trailing spaces.

All VOResource elements and attributes that contain dates must be interpreted as UTC dates and must be encoded in compliance with ISO8601 standard Date and Time Format [ISO8601]. The vr:UTCTimestamp type provides a special restriction of the format that requires includsion of date and time, but disallows the timezone format. This enforces a restricted form of this format which allows encoding of the date and time, but disallows the timezone format:

<xs:simpleType name="UTCTimestamp">
   <xs:restriction base="xs:dateTime">
      <xs:pattern value="\d{4}-\d\d-\d\dT\d\d:\d\d:\d\d(\.\d+)?"/>
   </xs:restriction>
</xs:simpleType>

<xs:simpleType name="UTCDateTime">
   <xs:union memberTypes="xs:date vr:UTCTimestamp"/>
</xs:simpleType>

The vr:UTCDateTime type allows the date to appear either as a simple date--i.e. conforming to xs:date--or as a date and time as restricted by vr:UTCTimestamp. When either of these VOResource types is used, authors must provide an applicable UTC date and time, and applications must interpret the time and date as UTC.

All VOResource types and elements have an associated semantic meaning which is given in the first <xsd:documentation> node within the type or element's definition in the schema. The meaning associated with a type is generic, indicating the kind of information the type provides. The semantics that are delivered by a VOResource instance document, however, are those associated with VOResource elements. The meaning of a VOResource element can be thought of as having two parts: the generic meaning of the set of information it contains as defined by its type, and a specific meaning describing the context in which that information applies. Because all VOResource elements are locally defined (in the XML Schema sense), they do not have an absolute meaning, but rather have a meaning tied to the thing being described by that element as represented by the enclosing type.

1. The vr:Curation type describes the curation of a resource. The <curation> element describes curation of the specific resource described by the enclosing vr:Resource type and identified by its identifier element.

2. The vr:ResourceName type is a generic reference to another resource. The <publisher> element gives a reference to the publisher of the specific resource being described which may itself be a registered resource described elsewhere.

3. The <title> element gives the title of the resource being described the enclosing vr:Resource type and identified by its <identifier> element. The <title> element's type, xs:token (a restriction on xsd:string), has no inherent meaning associated with it.

Additional semantics are transmitted through the use of derived types using the xsi:type attribute. In the sample instance document above, the use of xsi:type="Organisation" means that the resource being described is specifically an organisation as defined by the vr:Organisation type. This type provides additional metadata that are not part of the core resource metadata. The semantics associated with the use of xsi:type is described further in the next subsection.

When a resource is described with an element explicitly of the type vr:Resource, it is being described in the most generic sense. The metadata presented in this type, including both free text values and controlled vocabulary, can give some sense of what type of resource is being described and what it might be used for. However, the most useful descriptions of resources will not explicitly use the vr:Resource type; rather, they will use types that are derived from vr:Resource.

Defining derived vr:Resource types accomplishes two things:

1. it sharpens the semantic meaning of the resource description by indicating what specific type of resource it is, and
2. it may allow additional metadata not part of the core but specific to the that type of resource.

The VOResource schema defines two types derived from vr:Resource: vr:Organisation and vr:Service. The vr:Organisation adds metadata describing the astronomical facilities such as telescopes that are associated with the organisation it describes. The vr:Service type adds an element called capability which describes the service's interface as well as information regarding its behavior.

Extensions of the vr:Resource type is a key way derivation is used in VOResource to provide refined resource descriptions. Two other important parent types in the schema are vr:Capability and vr:Interface; these are extended to provide more refined descriptions of services (see section 2.2.2). The motivation for extending these types are the same as for vr:Resource: to provide more specific semantic meaning through the derived type's name, and to provide additional, specialized metadata that is not part of the parent type. Note, however, that in general, a derived type need not alter the content model of its base type. This allows derived types to add more specific meaning with out adding any additional metadata.

As described in section 2.2, it is intended that derived vr:Resource, vr:Capability and vr:Interface types be invoked in instance documents using the xsi:type attribute (as illustrated in the sample document above). This method illustrates a polymorphism for resource metadata in that any place where an element of parent type is expected, the derived type may be inserted. The use of xsi:type illustrates both what specific type is being inserted in as well as what it is being inserted in for. That is, as in our example, the resource being described is an organisation.

The other mechanism for polymorphism provided by XML Schema [Schema] is substitution groups. Invoking derived vr:Resource types via elements in a substitution group is discouraged because it is less obvious from looking at the instance document that a substitution is being made.

The vr:Service type extends the core vr:Resource metadata data by adding the capability element (see section 3.2.2). This element is used to describe a major functionality of the service, usually accessible through a single service endpoint URL. In particular, it is used to describe support for an IVOA service standard (e.g. Simple Image Access Protocol). A service resource record may have multiple child capability elements, each describing a different major functionality; however, these capabilities should be related in an obvious, logical way by virtue of sharing same core VOResource metadata.

Note: Whether multiple related capabilities are grouped together in a single Service record or are described in separate Service records is expected to be the choice of the VOResource record author. However, it is also expected that resource registry providers will provide some guidance to authors on best practices. This guidance could in part come in the form of a GUI that naturally encourage or contrains to aggregation of capabilities in a single record.

The capability element, through its type vr:Capability, describes the behavior of service capability and how to access it. The latter is described by a child interface element. As for the behavior, the base vr:Capability type only provides a description element that can contain human-readable text on what this capability provides. More structured behavioral information must be provided through specialized vr:Capability extensions. In particular, it is expected that a service standard (e.g. Simple Image Access Protocol) would define an extension of vr:Capability that adds additional metadata that can describe the service's behavior in relation to the standard; for example, the added metadata can describe limitations of the service implementation or indicate support for optional features. The specific vr:Capability type is invoked using the xsi:type mechanism described in section 2.2.1.

<capability xsi:type="sia:SimpleImageAccess"
            standardID="ivo://ivoa.net/std/SIA">
  ...
</capability>

As the example above suggests, a common way for locating or otherwise identifying support for a standard service capability is by looking for the appropriate value in a capability's xsi:type attribute.

If the service capability being described does not conform to any standard or if the standard does not require any specialized capability metadata for describing an implementation's behavior, then no vr:Capability extension is required. In this case, the base capability element is simply used without any xsi:type attribute provided. It is expected that the metadata provided outside of the capability element as well as (optionally) it's description element is sufficient for describing what the service does.

Because each vr:Capability extension features a different set of behavioral metadata, introducing a new vr:Capability extension can impose a non-trivial cost on applications that process VOResource records. Thus, an alternative way to indicate support for a service standard is provided by the standardID attribute which is useful when the standard does not require any specialized behavioral metadata to be provided. The value is set to a URI which represents the service standard. Some service standards that do extend vr:Capability may force the value of this attribute to be set to the appropriate value (see section 2.3.2); this allows one to use, when appropriate, the standardID as a way to locate support for a standard regardless of whether an extension type has been defined or not.

Each capability element can contain one or more child interface elements, each describing how the capability can be accessed. The interface element's type, vr:Interface, is abstract; thus, the interface element must be accompanied by an xsi:type attribute that indicates an vr:Interface extension type. The VOResource schema defines two vr:Interface extension types: vr:WebBrowser, for describing an interface access via web browser, and vr:WebService, for accessing a service described by a Web Service Description Language (WSDL) document (see section XX for details).

<interface xsi:type="vr:WebService">
   <accessURL> http://archive.org/service/query </accessURL>
</capability>

When a capability contains more than one interface, each interface should be interpreted as an alternative interface for accessing essentially the same underlying capability. The interfaces can differ in their overall type (e.g. vr:WebBrowser, vr:WebService) or in the supported input parameters or output products.

When a standard capability is being described (i.e. either the vr:Capability sub-type is defined by a standard or the standardID is provided), then at least one of the interface elements should describe an interface required by the standard. The role attribute is used to mark the standard interfaces (typically with the value "std"; see section XX for details). All other interfaces are considered non-standard alternatives.

Another important way interfaces inside the same capability element can be different is in the version of the service standard the interface supports. Whenever, an interface supports a version other than "1.0", the interface element must include a version attribute set to the version being supported. Valid values for version are defined by the standard.

<capability xsi:type="sn:SkyNode"
            standardID="ivo://ivoa.net/std/SkyNode">

  <!--  version 1.0 of the standard SkyNode interface --> 
  <interface xsi:type="vr:WebService" role="std" version="1.0">
     <accessURL> http://archive.org/service/skynode </accessURL>
  </interface>

  <!--  version 1.1 of the standard SkyNode interface --> 
  <interface xsi:type="vr:WebService" role="std" version="1.1">
     <accessURL> http://archive.org/service/skynode1.1 </accessURL>
  </interface>

  <!--  a interactive alternative interface, assesible via a browser  --> 
  <interface xsi:type="vr:WebBrowser">
     <accessURL> http://archive.org/skynode.html </accessURL>
  </interface>
  ...
</capability>

A schema made up only of global type definitions provides great flexibility for extension. Any global type defined in the VOResource schema may be used as the base of a derived type defined in another schema. The schema containing the derived types must declare its own namespace URI or default to the null namespace; it must not adopt the VOResource namespace URI. The application must then define what schemas, identified by their namespace URIs, are supported and/or required.

A VOResource extension is an XML Schema document whose primary purpose is to define new types derived from those defined in the VOResource schema for use in resource descriptions. It is recommended that VOResource extensions follow the definition styles used by the core VOResource. In particular:

• Provide semantic definitions of all types and elements within the first <xsd:documentation> element inside the type or element definition. Subsequent <xsd:documentation> elements may provide additional comments or discussion.

• Avoid the use of xsd:choice elements. VOResource does not use the choice structure because it does not map readily into any object-oriented software language structure. Choices are handled instead as multiple derived types that can be inserted in place of a parent type.

• Avoid the use of substitution groups. VOResource prefers instead the use of xsi:type which are (with a few exceptions) functionally equivalent to substitution groups in terms of structure; however, xsi:type serves as an obvious flag in the instance document that a substitution has been made.

• Choose semantically meaningful names for derived types. When the derived type appears in the pattern <elname xsi:type="derivedType">, choose a derivedType name such that the sentence, "a derivedType is a kind of elname" makes natural and obvious sense. For example, "an Organisation is a kind of resource."

• Follow the VOResource naming conventions.

There are two types of derivation that are particularly important to the VOResource data model: derivation of the vr:Resource type, used to define specific types of resources, and the derivation of service metadata elements.

Derivation of vr:Resource to define new kinds of resources should be done by extension (i.e. using <xsd:extension>) rather than restriction. It is not required that the derived type change the content model from that of the vr:Resource base type; in this case, the purpose of the derivation is only to sharpen the semantic meaning of the resource description.

As described in section 2.2.3, a service standard will often define a new vr:Capability extension type to allow implementations to describe how they support the standard. This definition of the vr:Capability extension should be done in a schema document with a namespace identifier that is dedicated to that standard (hereafter referred to as the standard's extension schema). The extension type should include elements representing the applicable Capability metadata described in section 5.2 of the RM (e.g. Service.MaxReturnRecords, Service.MaxReturnSize) but can also include other concepts that are specific to that standard.

The standard's extension schema may create a derived vr:Capability type that forces the value of the standardID attribute to be set to a given URI. This should be done by first deriving from vr:Capability by restriction (i.e. using <xsd:restriction>), keeping all of the parent's content model except adding to the standardID's attribute definition use="required" along with the fixed modifier set to the desired URI. Since this restricted type is not intended for direct use in an instance document, it should be marked as abstract. The restricted type should then be extended to add the specialized capability metadata required by the standard. (See the example below.)

It is not recommended that standard's extension schema attempt to force the inclusion of a required interface type.

An extension schema can define new interface types, though not necessarily in the context of any specific standard service capability. The basic vr:Interface type provides only accessURL and securityMethod as child elements. A derived vr:Interface type must indicate in the documentation how the <accessURL> should be interpreted and used. The derived type may also include other added metadata describing how to use the service (e.g., a description of the input arguments). If the interface extension type is expected to be referenced by a standard service capability, then it is recommended that the additional metadata be optional unless the metadata is absolutely required by clients in order to invoke the service.

Note: It is intended that a set of common generic interface types would be defined in a separate VOResource extension schema. At the time of this writing, this schema is called VODataService. It currently defines an interface type for describing traditional GET and POST services. More specific interfaces, particularly those associated with standard IVOA services (like a Registry Service) would derive its specific interface descriptions from one of the common types as appropriate.

Note: The Simple Image Access Protocol [SIA] is an example of a standard service that defines capability metadata. These include "maxRecords" that list the maximum number of records the SIA implementation can return at a time, and "MaxImageSize" gives the maximum image size that the service can return.

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" 
           xmlns:vr="http://www.ivoa.net/xml/VOResource/v1.0"
           xmlns:vs="http://www.ivoa.net/xml/VODataService/v1.0"
           xmlns:sia="http://www.ivoa.net/xml/SIA/v1.0"
           targetNamespace="http://www.ivoa.net/xml/SIA/v1.0"
           elementFormDefault="unqualified" attributeFormDefault="unqualified"
           version="1.0">

   <xs:annotation>
      <xs:documentation>
        An XML Schema for describing a Simple Image Access Service
        implementation.   
      </xs:documentation>
   </xs:annotation>

   <xs:import namespace="http://www.ivoa.net/xml/VOResource/v1.0"/>

   <xs:complexType name="SIACapRestriction" abstract="true">
      <xs:annotation>
         <xs:documentation>
            an abstract capability that fixes the standardID to the
            URI for the SIA standard.
         </xs:documentation>
      </xs:annotation>
      <xs:complexContent>
         <xs:restriction base="vr:Capability">
            <xs:sequence>
               <xs:element name="validationLevel" type="vr:Validation"
                           minOccurs="0" maxOccurs="unbounded"/>
               <xs:element name="description" type="vr:token" 
                           minOccurs="0"/>
               <xs:element name="interface" type="vr:Interface" 
                           minOccurs="0" maxOccurs="unbounded"/>
            </xs:sequence>
            <xs:attribute name="standardID" type="vr:IdentifierURI"
                          use="required" fixed="ivo://ivoa.net/std/SIA"/>
         </xs:restriction>
      </xs:complexContent>
   </xs:complexType>

   <xs:complexType name="SimpleImageAccess">
      <xs:annotation>
         <xs:documentation>
            The behavior and limitations of an SIA implementation.  
         </xs:documentation>
      </xs:annotation>

      <xs:complexContent>
         <xs:extension base="sia:SIACapRestriction">
            <xs:sequence>

               <xs:element name="maxRecords" type="xs:int">
                  <xs:annotation>
                     <xs:documentation>
                        The largest number of records that the Image Query web
                        method will return. 
                     </xs:documentation>
                  </xs:annotation>
               </xs:element>

               <!-- other capability metadata -->
            </xs:sequence>
         </xs:extension>
      </xs:complexContent>
   </xs:complexType>

</xs:schema>

<resource xsi:type="vr:Service"
  xmlns:vr="http://www.ivoa.net/xml/VOResource/v1.0"
  xmlns:sia="http://www.ivoa.net/xml/SIA/v1.0"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

    <!-- the core VOResource metadata -->

    <capability xsi:type="sia:SimpleImageAccess">
       <interface xsi:type="...">
         <!-- interface description -->
         <accessURL> http://archive.org/services/sia <accessURL>
       </interface>

       <maxRecords>5000</maxRecords>
       <!-- other capability metadata -->
    </sia:capability>

</resource>

This section enumerates the types and elements defined in the VOResource schema and describes their meaning in terms of the RM.

A resource, as defined by the RM, is any entity or component of a VO application that is describable and identifiable by a IVOA Identifier. A resource is described using VOResource by an element of the type vr:Resource or one of its legal extensions. The schema definition (below) includes elements that encode the identity, curation, and general content metadata for a resource (see sections 3.1 thru 3.3 of the RM). The RM states that certain metadata are required in a minimally compliant resource description; this requirement is enforced by the VOResource schema definition.

<xs:complexType name="Resource">
   <xs:sequence>

      <xs:element name="validationLevel" type="vr:Validation"
                  minOccurs="0" maxOccurs="unbounded">
      <xs:element name="title" type="vr:token"/>
      <xs:element name="shortName" type="vr:ShortName" minOccurs="0"/>
      <xs:element name="identifier" type="vr:IdentifierURI"/>
      <xs:element name="curation" type="vr:Curation"/>
      <xs:element name="content" type="vr:Content">

   </xs:sequence>

   <xs:attribute name="created" type="vr:UTCDateTime"/>
   <xs:attribute name="updated" type="vr:UTCDateTime"/>
   <xs:attribute name="status" default="active"/>
      <xs:simpleType>
         <xs:restriction base="xs:string">
            <xs:enumeration value="active"/>
            <xs:enumeration value="inactive"/>
            <xs:enumeration value="deleted"/>
         </xs:restriction>
      </xs:simpleType>
   </xs:attribute>
</xs:complexType>

The child elements for vr:Resource are described in subsequent sections.

The vr:Resource attributes represent a special class of metadata: they describe the resource metadata description contained within the vr:Resource itself as opposed to the resource being described. Their meaning are as follows:

The following sections define the elements that encode the specific metadata from the RM. In all cases, the semantic meaning of an element is defined by the RM metadatum it corresponds to (labeled "RM Name" below). All rules and advice given in the "Comments" portions in the RM definition apply. Any textual differences in the semantic definitions given below from those given in the RM are intended only for clarification for the XML encoding context.

The identity metadata described in the RM (section 3.1) are represented as top-level children of the vr:Resource type.

Two special types, vr:ShortName and vr:identifierURI are defined to support identity metadata. The vr:ShortName definition enforces the 16-character limit on shortNames.

<xs:simpleType name="ShortName">
  <xs:restriction base="xs:string">
     <xs:maxLength value="16"/>
  </xs:restriction>
</xs:simpleType>

The vr:IdentifierURI enforces the URI syntax for IVOA Identifiers as defined by the IVOA Identifier standard [ID].

<xs:simpleType name="IdentifierURI">
  <xs:restriction base="xs:anyURI">
    <xs:pattern value="ivo://[\w\d][\w\d\-_\.!~\*'\(\)\+=]{2,}
(/[\w\d\-_\.!~\*'\(\)\+=]+(/[\w\d\-_\.!~\*'\(\)\+=]+)*)?"/>
  </xs:restriction>
</xs:simpleType>

Two additional types which are not used within the vr:Resource type but are available to support the two components of an IVOA Identifier [ID]: vr:AuthorityID and vr:ResourceKey.

<xs:simpleType name="AuthorityID">
  <xs:restriction base="xs:string">
    <xs:pattern value="[\w\d][\w\d\-_\.!~\*'\(\)\+=]{2,}"/>
  </xs:restriction>
</xs:simpleType>

<xs:simpleType name="ResourceKey">
  <xs:restriction base="xs:string">
    <xs:pattern value="[\w\d\-_\.!~\*'\(\)\+=]+(/[\w\d\-_\.!~\*'\(\)\+=]+)*"/>
  </xs:restriction>
</xs:simpleType>

The curation metadata described in the RM (section 3.2) are bundled together into the vr:Resource child element, <curation>. Its content is defined by the vr:Curation complex type.

<xs:complexType name="Curation">
  <xs:sequence>

    <xs:element name="publisher" type="vr:ResourceName"/>
    <xs:element name="creator" type="vr:Creator"
                minOccurs="0" maxOccurs="unbounded"/>
    <xs:element name="contributor" type="vr:ResourceName"
                minOccurs="0" maxOccurs="unbounded"/>
    <xs:element name="date" type="vr:Date" 
                minOccurs="0" maxOccurs="unbounded"/>
    <xs:element name="version" type="xs:token" minOccurs="0"/>
    <xs:element name="contact" type="vr:Contact"/>

  </xs:sequence>
</xs:complexType>

vr:Curation Metadata Elements
Element	Definition
publisher	RM Name: Publisher, PublisherID (see type definition below) Value type: string with optional ID attribute: vr:ResourceName Semantic Meaning: Entity (e.g. person or organisation) responsible for making the resource available Occurrences: required Comments: The PublisherID is encoded as an optional attribute
creator	RM Name: Creator, Creator.Logo (see type definition below) Value type: composite: vr:Creator Semantic Meaning: The entity (e.g. person or organisation) primarily responsible for creating the content or constitution of the resource. Occurrences: optional; multiple occurrences allowed Comments: If <creator> element appears, it must contain at least a name. If multiple creator elements appear, but only one logo is provided (see type definition below), applications may assume that the one logo applies to all creators.
contributor	RM Name: Contributor Value type: string with optional ID attribute: vr:ResourceName Semantic Meaning: Entity responsible for contributions to the content of the resource. Occurrences: optional; multiple occurrences allowed
date	RM Name: Date Value type: UTC date (without time) with optional role attribute: vr:Date Semantic Meaning: Date associated with an event in the life cycle of the resource. Occurrences: optional; multiple occurrences allowed
version	RM Name: Version Value type: string: xs:token Semantic Meaning: Label associated with creation or availablilty of a version of a resource. Occurrences: optional
contact	RM Name: Contact, Contact.Name, Contact.Email Value type: composite: vr:Contact Semantic Meaning: Information that can be used for contacting someone with regard to this resource. Occurrences: required; multiple occurrences allowed

Several of the curation elements (most importantly, <publisher>) make use of the vr:ResourceName type. This type is provides a means of refering to another resource both by name and by its IVOA identifier. Not all resources refered to using this type will necessarily be registered and, therefore, will have an identifier; thus, the identifier (which is encoded as an attribute) is optional.

<xs:complexType name="ResourceName">
  <xs:simpleContent>
    <xs:extension base="xs:token"> 
      <xs:attribute name="ivo-id" type="vr:IdentifierURI"/>
    </xs:extension>       
  </xs:simpleContent>
</xs:complexType>   

The <creator> element is defined by the vr:Creator complex type which bundles together the RM metadata Creator and Creator.Logo.

<xs:complexType name="Creator">
   <xs:sequence>
     <xs:element name="name" type="vr:ResourceName"/>
     <xs:element name="logo" type="xs:anyURI" minOccurs="0"/>
   </xs:sequence>
</xs:complexType>

vr:Creator Metadata Elements
Element	Definition
name	RM Name: Creator Value type: string with optional ID attribute: vr:ResourceName Semantic Meaning: The name and (optionally) the ID of the entity (e.g. person or organisation) primarily responsible for creating the content or constitution of the resource. Occurrences: required
logo	RM Name: Creator.Logo Value type: URL: vr:paddedURI Semantic Meaning: URL pointing to a graphical logo, which may be used to help identify the information source Occurrences: optional

The <Date> element's type, vr:Date, is an extension of the UTCDateTime type that adds an optional attribute called role.

<xs:complexType name="Date">
   <xs:simpleContent>
      <xs:extension base="xs:date">
        <xs:attribute name="role" type="xs:string" default="representative"/>
      </xs:extension>
   </xs:simpleContent>
</xs:complexType>

The purpose of the role attribute is to indicate what aspect of the resource the date describes. This allows several <date> elements to be provided, each with a different role. The possible values for this attribute are not controlled; however, several values are recommended below and may be recognized by applications. Other roles may be given. If a role is not given, applications should interpret the date as representative as defined below.

It is important to note that the <date> element describes the resource itself, not the resource record that describes it. Dates describing the resource record are covered by vr:Resource attributes created and updated

The <contact> element is defined by the vr:Contact type which bundles together several component pieces of information, including the RM metadata Contact.Name and Contact.Email.

<xs:complexType name="Contact">
  <xs:sequence>

     <xs:element name="name" type="vr:ResourceName"/>
     <xs:element name="address" type="xs:token" minOccurs="0"/>
     <xs:element name="email" type="xs:token" minOccurs="0"/>
     <xs:element name="telephone" type="xs:token" minOccurs="0"/>

   </xs:sequence>
</xs:complexType>

vr:Contact Metadata Elements
Element	Definition
name	RM Name: Contact.Name Value type: string with optional ID attribute: vr:ResourceName Semantic Meaning: The name and (optionally) the ID of the entity (e.g. person or organisation) that can be contacted regarding the resource. Occurrences: required
address	RM Name: n. a. Value type: string: xs:token Semantic Meaning: All components of the mailing address of the contact person or organisation. Occurrences: optional
email	RM Name: Contact.Email Value type: string: xs:token Semantic Meaning: The email address of the contact person or organisation. Occurrences: optional
telephone	RM Name: n. a. Value type: string: xs:token Semantic Meaning: The telephone number of the contact person or organisation. Occurrences: optional Comments: Complete international dialing codes should be provided.

The general content metadata described in the RM (section 3.3) are bundled together into the vr:Resource child element, <content>. Its content is defined by the vr:Content complex type.

<xs:complexType name="Content">
  <xs:sequence>

    <xs:element name="subject" type="xs:token"
                minOccurs="0" maxOccurs="unbounded"/>
    <xs:element name="description" type="xs:token"/>
    <xs:element name="source" type="vr:Source" minOccurs="0"/>
    <xs:element name="referenceURL" type="vr:PaddedURI"/>
    <xs:element name="type" type="vr:Type" minOccurs="0"/>
    <xs:element name="contentLevel" type="vr:ContentLevel" 
                minOccurs="0" maxOccurs="unbounded"/>
    <xs:element name="relationship" type="vr:Relationship" 
                minOccurs="0" maxOccurs="unbounded"/>

</xs:sequence>
</xs:complexType>

vr:Content Metadata Elements
Element	Definition
subject	RM Name: Subject Value type: string: xs:token Semantic Meaning: a topic, object type, or other descriptive keywords about the resource. Occurrences: required; multiple occurrences allowed
description	RM Name: Description Value type: string: xs:token Semantic Meaning: An account of the nature of the resource. Occurrences: required Comments: The description may include but is not limited to an abstract, table of contents, reference to a graphical representation of content or a free-text account of the content.
source	RM Name: Source Value type: a formatted reference (e.g. ADS Bibcode): vr:Source Semantic Meaning: a bibliographic reference from which the present resource is derived or extracted. Occurrences: optional
referenceURL	RM Name: ReferenceURL Value type: URL: vr:PaddedURI Semantic Meaning: URL pointing to a human-readable document describing the resource. Occurrences: required
type	RM Name: Type Value type: string with controlled vocabulary: vr:Type Semantic Meaning: Nature or genre of the content of the resource Occurrences: optional; multiple occurrences allowed Allowed Values: Other resource that does not fall into any of the category names currently defined. Archive Collection of pointed observations Bibliography Collection of bibliographic reference, abstracts, and publications Catalog Collection of derived data, primarily in tabular form Journal Collection of scholarly publications under common editorial policy Library Collection of published materials (journals, books, etc.) Simulation Theoretical simulation or model Survey Collection of observations covering substantial and contiguous areas of the sky Transformation A service that transforms data Education Collection of materials appropriate for educational use, such as teaching resources, curricula, etc. Outreach Collection of materials appropriate for public outreach, such as press releases and photo galleries EPOResource Collection of materials that may be suitable for EPO products but which are not in final product form, as in Type Outreach or Type Education. EPOResource would apply, e.g., to archives with easily accessed preview images or to surveys with easy-to-use images. Animation Animation clips of astronomical phenomena Artwork Artists' renderings of astronomical phenomena or objects Background Background information on astronomical phenomena or objects BasicData Compilations of basic astronomical facts about objects, such as approximate distance or membership in constellation Historical Historical information about astronomical objects Photographic Publication-quality photographs of astronomical objects Press Press releases about astronomical objects Organisation An organization that is a publisher or curator of other resources Project A project that is a publisher or curator of other resources Registry a query service for which the response is a structured description of resources
contentLevel	RM Name: ContentLevel Value type: string with controlled vocabulary: vr:ContentLevel Semantic Meaning: Description of the content level or intended audience Occurrences: optional; multiple occurrences allowed Allowed Values: General Resource provides information appropriate for all users. Elementary Education Resource provides information appropriate for use in elementary education (e.g. approximate ages 6-11). Middle School Education Resource provides information appropriate for use in middle school education (e.g. approximate ages 11-14). Secondary Education Resource provides information appropriate for use in elementary education (e.g. approximate ages 14-18). Community College Resource provides information appropriate for use in community/junior college or early university education. University Resource provides information appropriate for use in university education. Research Resource provides information appropriate for supporting scientific research. Amateur Resource provides information of interest to amateur astronomers. Informal Education Resource provides information appropriate for education at museums, planetariums, and other centers of informal learning.
relationship	RM Name: Relationship, RelationshipID Value type: composite: vr:Relationship Semantic Meaning: a description of a relationship to another resource Occurrences: optional; multiple occurrences allowed

The <source> element's type, vr:Source, is an extension of the xs:token type that adds an optional attribute called format.

<xs:complexType name="Source">
   <xs:simpleContent>
      <xs:extension base="xs:token">
        <xs:attribute name="format" type="xs:string"/>
      </xs:extension>
   </xs:simpleContent>
</xs:complexType>

The format indicates the syntactic format of the value of the <source> element. The possible values for this attribute are not controlled; however, applications should recognize a value equal to bibcode as referring to the standard astronomical bibcode format [Bibcode].

The <relationship> is defined by the vr:Relationship complex type which bundles together the RM metadata Relationship and RelationshipID.

<xs:complexType name="Relationship">
   <xs:sequence>
      <xs:element name="relationshipType" type="xs:token"/>
      <xs:element name="relatedResource" type="vr:ResourceName" 
                  minOccurs="1" maxOccurs="unbounded"/>
   </xs:sequence>
</xs:complexType>

vr:Relationship Metadata Elements
Element	Definition
relatedResource	RM Name: RelationshipID Value type: string with optional ID attribute: vr:ResourceName Semantic Meaning: the name of resource that this resource is related to. Occurrences: required, multiple values allowed Comments: The attribute value provides the RM metadatum RelationshipID.
relationshipType	RM Name: Relationship Value type: string with controlled vocabulary Semantic Meaning: the named type of relationship Occurrences: required Allowed Values: mirror-of The current resource mirrors another resource. service-for The current resource is a service that provides access to a data collection. served-by The current resource can be accessed via the identified service. derived-from The current resource is derived from another resource. related-to The current resource is related to another resource in an unspecified way. Comments: The allowed values are not enforced by the schema itself. The "other" resource refered to in the above definitions refers to the resource given by the <relatedResource> element.

It is important to note that the <relationshipType> controlled vocabulary is not enforced by the schema itself. This allows a future version of this specification to define additional terms without requiring a change in the XML Schema document.

The RM's ResourceValidationLevel is encoded in the VOResource schema with the <validationLevel> element, which can appear in multiple places within a vr:Resource type or sub-type. It may appear one or more times as the first children of a vr:Resource type and/or, if the resource is a vr:Service type or sub-type, one or more times as the first children of any <capability> element.

The validationLevel element requires an attribute called validatedBy which takes an IVOA ID as a value. This ID must refer to a registered organisation or registry that assigned the numerical value. This element can appear multiple times, each with a different validatedBy value, to reflect the code assigned by different organisations.

<xs:complexType name="Validation">
  <xs:simpleContent>
    <xs:extension base="vr:ValidationLevel">
      <xs:attribute name="validatedBy" type="vr:IdentifierURI"
                    use="required"/>
    </xs:extension>
  </xs:simpleContent>
</xs:complexType>

<xs:simpleType name="ValidationLevel">
  <xs:restriction base="xs:integer">
     <xs:whiteSpace value="collapse"/>
     <xs:enumeration value="0"/>
     <xs:enumeration value="1"/>
     <xs:enumeration value="2"/>
     <xs:enumeration value="3"/>
     <xs:enumeration value="4"/>
  </xs:restriction>
</xs:simpleType>

The VOResource schema defines two extensions of the base vr:Resource type for describing two specific types of resources: vr:Organisation and vr:Service. In addition to providing more refined semantic meaning over vr:Resource, they add additional metadata for the describing the resource which don't necessarily apply in the generic case.

The Organisation resource type is used to describe an organisation in the sense defined by the RM:

An organisation is a specific type of resource that brings people together to pursue participation in VO applications. Organisations can be hierarchical and range greatly in size and scope. At a high level, an organisation could be a university, observatory, or government agency. At a finer level, it could be a specific scientific project space mission, or individual researcher.

The Organisation type extends the Resource type by adding two additional elements to the core set of metadata, <facility> and <instrument>:

<xs:complexType name="Organisation">
   <xs:complexContent>
      <xs:extension base="vr:Resource">
         <xs:sequence>

            <xs:element name="facility" type="vr:ResourceName"
                        minOccurs="0" maxOccurs="unbounded"/>
            <xs:element name="instrument" type="vr:ResourceName"
                        minOccurs="0" maxOccurs="unbounded"/>

         </xs:sequence>
      </xs:extension>
   </xs:complexContent>
</xs:complexType>

vr:Organisation Extension Metadata Elements
Element	Definition
facility	RM Name: Facility Value type: string with optional ID attribute: vr:ResourceName Semantic Meaning: the observatory or facility used to collect the data contained or managed by this resource. Occurrences: optional; multiple occurrences allowed
instrument	RM Name: Instrument Value type: string with optional ID attribute: vr:ResourceName Semantic Meaning: the instrument used to collect the data contained or managed by this resource. Occurrences: optional; multiple occurrences allowed

The main role of an organisation in the VO (that is, the main reason for describing organisations in a registery) is as a provider or publishier of other resources. In particular, an organisation description in a registry declares the association of an IVOA identifier [ID] with the organisation. The organisation can then be referred to in other resource descriptions. For example, an organisation identifier will appear as the publisher identifier of service resource (as illustrated in our example from section 2).

The Service resource type is used to describe a service--a resource that actually does something--in the sense defined by the RM:

A service is any VO resource that can be invoked by the user to perform some action on their behalf.

The general data model is described in section 2.2.2. The Service type extends the Resource type by adding two elements: <rights> which indicates who may access it, and <capability> which describes how the service behaves and how it is invoked.

<xs:complexType name="Service">
   <xs:complexContent>
      <xs:extension base="vr:Resource">
         <xs:sequence>
            <xs:element name="rights" type="vr:Rights" 
                        minOccurs="0" maxOccurs="unbounded"/>
            <xs:element name="capability" type="vr:Capability" 
                        minOccurs="0" maxOccurs="unbounded"/>
         </xs:sequence>
      </xs:extension>
   </xs:complexContent>
</xs:complexType>

As described in section 2.2.2, multiple capability elements may appear, each describing a different capability of the service.

   <xs:complexType name="Capability">
      <xs:sequence>
         <xs:element name="validationLevel" type="vr:Validation"
                     minOccurs="0" maxOccurs="unbounded"/>
         <xs:element name="description" type="xs:token" minOccurs="0"/>
         <xs:element name="interface" type="vr:Interface" 
                     minOccurs="0" maxOccurs="unbounded"/>
      </xs:sequence>
      <xs:attribute name="standardID" type="xs:anyURI"/>
   </xs:complexType>

vr:Capability Metadata Elements
Element	Definition
validationLevel	RM Name: ResourceValidationLevel Value type: integer with required validatedBy attribute taking an ID: vr:Validation (see definition for listing of allowed values.) Semantic Meaning: An evaluation via a numeric grade that indicates the quality of the capability description and whether its implementation is functionally with applicable standards. Occurrences: optional; multiple occurrences allowed
description	Value type: string: xs:token Semantic Meaning: A human-readable description of what this capability provides as part of the over-all service Occurrences: optional
interface	Value type: composite: vr:Interface Semantic Meaning: a description of how to call the service to access this capability Occurrences: optional

The capability element is sufficient for describing a custom service capability--i.e., a service that is particular to one provider and does not conform to a specific standard (be it recognized by the IVOA or some other sub-community). However, service standards will often create a vr:Capability extension that adds additional metadata that are specific that that are specific to the behavior of that particular type of service.

Note: The RM defines three metadata that may be important for several standard query services: Service.MaxSearchRadius, Service.MaxReturnRecords, and Service.MaxReturnSize. These are examples of service-specific metadata that should be encoded as child elements in a type derived from vr:Capability.

vr:Capability Attribute
Attribute	Definition
standardID	RM Name: Service.StandardURI Value type: a URI: xs:anyURI Semantic Meaning: A unique identifier for the standard that the service complies to. Comments: Normally this attribute is optional; however, a vr:Capability extension may force this attribute to be required and set to a fixed URI representing the service standard.

<xs:complexType name="Interface" abstract="true">
   <xs:sequence>
      <xs:element name="accessURL" type="vr:AccessURL"
                  maxOccurs="unbounded"/>
      <xs:element name="securityMethod" type="vr:SecurityMethod"
                  minOccurs="0" maxOccurs="unbounded"/>
   </xs:se