Incremental Update in Web Service Environment – CASE: Use of the OGC’s GeoSynchronization Service to Integrate Hydrographic Datasets
ISBN 978-85-88783-11-9
Authors
1Latvala, P.; 2Hietanen, E.
1FINNISH GEOSPATIAL RESEARCH INSTITUTE (FGI) Email: pekka.latvala@nls.fi
2FINNISH GEOSPATIAL RESEARCH INSTITUTE (FGI) Email: eero.hietanen@nls.fi
Abstract
The current trend in Europe where different governmental organizations are opening up their geospatial data sets has led to the increased use of spatial data. Different companies, organizations and individuals have downloaded the original data sets from the data providers and created their own products and services on top of them. The copied data sets must be updated regularly from the master databases in order to maintain good data quality in the services. The execution of a full update by replacing all data is often an unfeasible solution because of the large amount of data or because the data might have been changed or new data might have been added in the local service databases. An alternative to the full update is to perform the updates incrementally, feature-by-feature. The Open Geospatial Consortium (OGC) has created one solution to the incremental data update problem with GeoSynchronization Service (GSS) specification [1] that is currently still in development. The GSS is a web service that uses different ATOM feeds to deliver information on the changed features. Its workflow is based on three user roles: publisher, reviewer and subscriber. The publisher can suggest changes to the original database. These change proposals are then collected into an ATOM-based change feed. The change feed is inspected by the reviewer who decides whether the changes are accepted or rejected. The reviewer’s decisions are collected into another ATOM-based feed, called resolution feed that is sent back to the publisher. The accepted changes are executed in the database. The third ATOM-based feed is called a replication feed and it is used for collecting the information about all changes that have been carried out in the database. The subscribers can use the replication feed to execute the exact same changes in their local databases. The studied example case is related to the handling of updates of hydrographic data in a situation where the data is collected and maintained by the National Mapping and Cadastral Agency and used by another governmental organization that has copied the original data and created a network model from the hydrographic features. There is a strong need for keeping the network model up-to-date by updating the data from the original source. A prototype GSS implementation was created in the project for handling the incremental data update procedure. The prototype contains an OpenLayers-based client application that the publishers can use for editing the data. The created changes are packaged as WFS-T queries and sent to the GSS module that has been implemented as a Java Servlet. The GSS module parses the change information and stores it into the PostgreSQL/PostGIS database. The GSS module is used also for creating the replication feed. The replication feed can be queried with spatial, temporal or id-based filters that allow the subscribers to fetch only those changes that they are interested. The use of WFS-T queries provides the subscribers an easy way to carry out the selected changes in their target services. The change proposal reviewing stage was bypassed in the prototype implementation because all changes are seen as authoritative changes that have already been accepted. Overall, GSS is seen as a suitable solution for performing incremental data updates in a web service environment. References [1] Vretanos, P. A. (ed.), OWS 7 Engineering Report – Geosynchronization service, 2011, available at: https://portal.opengeospatial.org/files/?artifact_id=39476 (accessed 14.11.2014)