Registry Service Metadata Concepts

This wiki document provides further information to support section 4, "Service Metadata Concepts", in Bob Hanisch's Resource Registry Metadata: http://www.ivoa.net/internal/IVOA/IvoaResReg/ResourceServiceMetadataV7.pdf.

Interface Metadata

• ServiceInterfaceURL (URL):
  • Bob Hanisch's resource metadata definition: A URL pointing to a document that presents or describes the service interface.
  • Elizabeth's definition: This is a document that can be accessed on the internet that tells you how to access the service interface. As Bob comments in the resource metadata document, the interface description depends on how the service interface is implemented. If the service interface is a web service, then the ServiceInterfaceURL will give the web address of the WSDL describing the web service. If the service interface is a form that can be filled out and submitted using an internet browser, then the ServiceInterfaceURL is the web address of the form. Similarly, if the service interface is described by GLU, a system which uses symbolic service names instead of hard-coded URLs, then the ServiceInterfaceURL will be a GLU record that can be looked up in a GLU dictionary or directory.
• ServiceBaseURL (URL):
  • Bob Hanisch's resource metadata definition: The base protion of a URL used to invoke a service with the expectation that an additional string must be appended for the service to execute properly. The syntax of the appended string is defined by the specific service.
  • Elizabeth's definition: The ServiceBaseURL is the basic web address you use to submit a request to a service before any data is actually submitted. Depending on how the service is implemented, the submitted request may use a web address identical to the ServiceBaseURL; ie, no extra strings or characters are appended to the URL. If a service uses the GET method common in HTML forms, however, the contents of the form query are appended to the end of the URL. The ServiceBaseURL is the portion of the web address that has does not include appended strings or characters.
• ServiceHTTPResults (MIME type):
  • Bob Hanisch's resource metadata definition: The MIME type that is returned by a service.
  • Elizabeth's definition: The ServiceHTTPResults tells you how the service results will be formatted when the results are returned to you. MIME stands for Multi-purpose Internet Mail Extensions, and MIME types describe standard classifications of files on the internet. For a complete over view of MIME types available, see http://www.iana.org/assignments/media-types/. Some of the more common MIME types to be returned from a service are text/html, text/xml, text/plain, image/jpeg, image/gif, audio/mpeg, and video/quicktime.

Capabilities Metadata

• ServiceStandardURI:
  • Bob Hanisch's resource metadata definition: A URI identifying a standard service.
  • Elizabeth's definition: A name or web address that uniquely identifies a service specification standard for a registered service. A service specification standard provides all the information needed to invoke a service, such as input, output, and procedure calls. WSDL is an example of a service specification standard for a web service; the WSDL includes information on request structure, input and output data types, method names, and XML schema elements. The ServiceStandardURI can be a web address or a unique identifying name for the standard, such as the W3C specification standards.
• ServiceStandardURL:
  • Bob Hanisch's resource metadata definition: A URL that points to a human readable document that describes the standard upon which a service is based.
  • Elizabeth's definition: The ServiceStandardURL provides a web address for a document that clearly explains the service requirements set out by the service specifiction standard listed at the ServiceStandardURI. Although these two elements have a subtle difference in name, the distinction between their roles is clear; if a person wants to find a document located on the internet that explains how the service works, the document can be found at the ServiceStandardURL. For example: as mentioned above, a service's WSDL description clearly delineates the requirements of the service. However, while WSDL is a formatted text file, it's not exactly human readable. An application server can generate a WSDL service specification standard, but a real person (or a graduate student) needs to provide clear documentation that describes the service to other real people.
• ServiceMSR (float, decimal degrees):
  • Bob Hanisch's resource metadata definition: Service providers may choose to restrict the scope of searches done against their services, lest they be swamped with requests for millions or billions of results record. Service MSR restricts searches to some maximum radius (in decimal degrees) about a celestial coordinate.
  • Elizabeth's definition: If the service in questions allows a user to search on coordinates, the ServiceMSR metadata element will inform users of the maximum allowable radius that a single query can handle, given in decimal degrees. This element will not be relevant to services that search datasets by metadata other than coordinates, as well as for services that do not search datasets at all (such as astronomical calculation services, data storage services, etc.).

Terms - URI vs URL vs URN:

A Uniform Resource Identifer can be a name (URN) or a location (URL) that identifies a resource. A Uniform Resource Locator is a type of URI that tells you where to find a resource on the internet and how to access it. URIs refer to names and addresses of a resource, whereas URLs refer specifically to the address and access protocol of a resource on the internet. . For example, a familiar web address format is "http://www.google.com". This URL is composed of a protocol for access - "http:" - and a path that gives the resource location - "//www.google.com". A resource can be a web page, an image, a data file, a web service, etc. Other common access protocols seen in URLs are "ftp:" (ie: "ftp://ftp.javasoft.com/docs/tutorial.zip") and "mailto:" (ie: "mailto:registry@ivoa.net").

A good explanation of Uniform Resource Names is given by R. Moats in "URN Syntax" [3]:

" Uniform Resource Names (URNs) are intended to serve as persistent, location-independent, resource identifiers and are designed to make it easy to map other namespaces (which share the properties of URNs) into URN-space. Therefore, the URN syntax provides a means to encode character data in a form that can be sent in existing protocols, transcribed on most keyboards, etc."

Please see these three resources (both URIs and URLs are given) for more information:

1. L. Masinter, "Guidelines for New URL Schemes", Network Working Group, The Internet Society, 1999. URL: ://www.ietf.org/rfc/rfc2718.txt
2. T. Berners-Lee, " Uniform Resource Identifiers (URI): Generic Syntax", Network Working Group, The Internet Society, 1998. URL: http://www.ietf.org/rfc/rfc2396.txt
3. R. Moats, "URN Syntax", Network Working Group, The Internet Society, 1997. URL: http://www.ietf.org/rfc/rfc2141.txt

-- ElizabethAuden - 11 Jun 2003