NGDF Metadata Gateway
Architecture V1.1
4. Distributed Directories
4.1 Z39.50
Z39.50 is an international standard protocol (ISO 23950) that was developed by
the library community to discover bibliographic records simultaneously from
different but similar structured databases. It provides for a client and
server that establishes a temporary connection enabling a user to post a
structured query, receive a summary result and request details from specific
records that meet the search criteria.
On the host (server) computer, Z39.50 server software interacts with a search
engine, which may be in the form of a database or alternative indexing
software, to process the query and return the results. The Z39.50 protocol
(through use of a single, standards-based protocol) provides a method to access
existing databases, without having to redesign the system. Z39.50 provides a
protocol that is not prescriptive on hardware, operating systems or
applications, as long as participants agree on a common mapping to one or more
Z39.50 profiles.
It is possible to register specific sets of attributes and rules of
implementation as a Z39.50 Application Profile. The FGDC has developed a
profile for geospatial metadata ("GEO") which provides guidance on
how to implement FGDC metadata elements within a Z39.50 service
<
http://www.blueangeltech.com/Standards/GeoProfile/geo22.htm
>. There is
potential for interoperability between the existing profiles and in fact most
profiles have been developed from other existing profiles.
For more details on Z39.50 and links to associated references see
http://lcweb.loc.gov/z3950/agency
.
4.2 Distributed Metadata Indexes
Any organisation can provide access to its metadata through the Gateway.
Metadata, once collected, should be stored and indexed on a server networked to
one or more distributed search engines (see 4.3). The data can be stored in a
database or text files and indexed to assist search requests from the NGDF
Gateway. Typically data is stored in XML format to make use of the Z39.50
technology and provides efficient searching of the metadata. Some Z39.50
search engines are now utilising relational database technologies. The
metadata index software is typically included as a component of the search
engine software. Organisations may, however, decide to develop an interface
between their the underlying metadata management system (e.g. database) and a
Z39.50 search engine.
To make use of the Z39.50 services, the distributed metadata indexes will
typically hold metadata in three formats
a) XML for indexing, searching
and exchange of the metadata
b) HTML for presenting the
required metadata via a web browser following a successful search
c) Text as an alternative
presentation format to HTML (for non web interfaces)
4.3 Distributed Search Engines
Any organisation can also implement a distributed search engine that responds
to Z39.50 search request, searches the local metadata indexes and provides a
response. Access to a search engine can be established from a local server,
from a web-site or as a distributed directory service linked to one or more
existing gateways (e.g. NGDF). By linking to a Gateway service organisations
are able to increase the awareness of and accessibility to the directory index.
A number of systems provide general Z39.50 support of which there are currently
three options (two commercial options) that comply with the GEO profile
requirements (see Appendix B). It is important for organisations to assess
their technical requirements before deciding on the appropriate search engine
to use. As part of this technical assessment, the search engine will need to
support Z39.50 profiles (e.g. GEO, Dublin Core) being utilised by the relevant
Gateways.
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