Aggregation and Push-Based Distribution of THREDDS Catalogs in Operational Information Systems

We present a solution for the implementation of a catalog service in the context of an European infrastructure; this has the strategic objective of providing an integrated service for global and regional ocean resources to intermediate users and policy makers. The current European information systems (e.g. MERSEA - Marine Environment and Security for the European Area, MOON - Mediterranean Operational Oceanography Network and SeaDataNet - PanEuropean infrastructure for Ocean and marine data management) are characterized by a three-level hierarchy. At the bottom level we find the regional and local data sources, providing observation data and model outputs. At the intermediate level, the operational projects have Thematic Portals (TEP's) aggregating the information on logical basis, e.g. Observation data portals (In situ and Satellite), Forcing (meteorological portal), Modelling/forecasting portals. At the top level, the Information Management Portal provides query services and links to TEP's. Currently, several architectural and technological general solutions are available for implementing catalog services, for typical use-cases in the context of geospatial information systems. However, if we consider Operational Oceanography a particular case, characterised by tighter non-functional characteristics -such as seamless integration with other services, responsiveness, etc.- we must conceive a tailored architectural solution. In the particular cases of operational projects, the hierarchical structure of the information system can be considered near-static and the needed scalability in terms of data sources is small; thus, a simpler solution based on datasets aggregation can be suitable and profitable. In such a schema, upper level catalogs are built merging lower-level catalogs. On the other hand, to adopt this approach, it is necessary that any update is notified to the upper-level catalogs, whenever a lower level catalog is modified. Hence, our solution implemented a push-based approach for updates notification, using Web technologies on the top of THREDDS/OPenDAP technology. Using Web interfaces (Web forms and services) users can subscribe to a notification service for receiving updates of one or more catalogs. Whenever a source THREDDS catalog changes, this variation is propagated to the upper levels and subscribers are notified. The implementation resulted in a simple and effective solution; this may be valuable for similar hierarchical information systems which are also common in other Earth Science application fields.