The NGDF Metadata Gateway is based on a distributed architecture that will require a collaborative venture between the various government and private sector bodies participating in NGDF. The implementation is designed to provide direct access to a number of directory systems distributed throughout UK. These individual directory systems or "nodes" can be implemented by any organisation (public or private) willing to provide on line access to their directory of GI data holdings. Individual directories may be for the exclusive use of an organisation, be a collective resource for an industry sector or be provided as a commercial service for users to advertise their data holdings.
The Metadata Gateway Service is built on an architecture consisting of a number of components that provide a non prescriptive approach for organisations to participate in the initiative. The Gateway architecture provides participants with the opportunity to participate at any of the components and, in fact, encourages broad participation to ensure the project realises its ambition and can be sustainable. Because of the distributed nature of the architecture, there is a critical co-ordination function required to ensure that participants (users and providers) are able to effectively link to the gateway.
The architecture can be viewed as a number of distinct components, though this will be dependent on the actual implementation at each node. A typical implementation may consist of the following components:
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Web Browser (User)
use of the Internet Web browsers is seen as a standard mechanism for the distribution of information over the Internet and will enable users to search the distributed directories. Typically these are the later versions of the Netscape and Internet Explorer browsers.
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Distributed Search Interface (Gateway)
search interface provides a web form to enable the user to define their search criteria, communicates the query to the distributed metadata search engines and displays results back to the users web browser. The NGDF Gateway architecture permits the establishment of alternate, or duplicate, gateways. This duplication of gateways serves to provide redundancy of the service and to permit alternative thematic or regional gateways (e.g. English, Scottish, environmental or international).
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Distributed Search Engine
distributed server that performs the required search queries on its local metadata indexes and provides responses to the search interface. The search engine provides the interface between the generic gateway request and the local metadata index (e.g. database) query. Each search engine can be 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.
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Distributed Metadata Index
collection of metadata records indexed to assist the discovery of data through the gateway. Each metadata collection may consist of metadata for an individual organisation or metadata from a collection of organisations. To be searched from the gateway, each metadata index must be linked with a search engine.
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Metadata Management Systems (Provider)
Organisations will collect and manage metadata using data management systems compliant with their organisational requirements. The manner of data storage is not important for the delivery of the Gateway, however, it is important for the NGDF project that these data management systems provide metadata that can be indexed and searched from the respective metadata indexes.
The following diagram (figure 1) illustrates the general structure architecture of the proposed Gateway service.
Figure 1