A System for Managing Oceanographic Metadata Using XML and XQuery

摘要

Metadata, by definition, is information associated with data, covering where, how, when and by whom the data were acquired. The chance to take part in European projects such as the EU-SeaDataNet (the Pan-European infrastructure for ocean and marine data management) made it necessary to use XML (Extensible Markup Language) as a standard file format for sharing data and metadata. At present, the Italian National Oceanographic Data Centre (OGS/NODC) has all its data and metadata contained in an Oracle relational database, and the metadata is managed using XML formats and schema (XSD;XML Schema Definition), giving common vocabularies for parameters, instruments, ships, etc. in agreement with European project standards. One problem with XML is the dynamic change in metadata schemas (XSD), necessitating development of a system which is flexible and capable of managing the changes. This paper describes a system for managing oceanographic metadata using XML files and the functionalities provided by the Oracle database. To better manage the XML format, we chose to load into the OGS/NODC database the whole XML file, using a dedicated field. The database Oracle gives us the flexibility to manage the XML format locally, within the OGS/NODC information system, using the XML DB (XML DataBase) Oracle features. This, through the use of XMLType, allows the inclusion of the XML into the database. Furthermore, through the use of the XQuery functions it is possible to create a set of views through which information contained in the XML can be viewed more immediately in a relational form. Using the XML and the XQuery functions, it is possible to store, extract and manage different kinds of information that might be exchanged at the European level. Moreover, with a RESTful (Representational State Transfer) Web Service, we have a simple and standard interface for rapidly and easily creating, modifying and deleting records containing XML files inside the database. Finally, through the use of a RESTful Web Service, it is possible to decouple the application from the database, so that through the use of software that manages HTTP URLs, such as the Mikado (SeaDataNet project), the XML file can be inserted, updated and deleted inside the database without the need for a direct connection to it.