Schema Evolution in a Federated Spatial Database
A Federated Spatial Database System (FSDBS) is the integration of multiple spatial data sources which enables effective spatial data sharing. FSDBS environments are becoming increasingly popular as more and more spatial and non-spatial datasets are integrated, especially those across a number of independent organisations. However, in an FSDBS environment, database schemas are subject to changes due to the ever-changing nature of the real world represented by spatial data models and the management of these changes is complex and inefficient. This is because schema changes in one local database will affect or invalidate not only applications built against the local schema, but also applications built against the federated schema. The traditional approach of manually modifying invalid applications to adapt to the new evolved schema is expensive and time consuming.
In this research, an Automatic Schema Evolution (ASE) Framework has been developed in order to overcome the limitation of manual modifications of applications. This is applied research which aims to solve real life problems and the object-relational data model is the focus of interest due to its support of spatial data management and its popularity in contemporary database management systems (DBMSs). Therefore, methodologies and algorithms developed in the research are based on the object-relational data model.
The main components involved in the ASE include: Schema Element Dependency (SED), Schema Mapping, Metadata Repository and Query/View Rewriting. Based on the SED metamodel developed, SED is to generate and update schema element dependency metadata across the whole system which are then used to identify affected schema elements when a database schema change occurs. Schema Mapping is responsible for (1) generating new schema mapping according to the Schema Change Template (SCT) specified and (2) updating invalid schema mapping by schema mapping adaptation after database schema changes. The set of SCTs define corresponding schema mapping rules and have been developed based on the schema change taxonomy identified in a spatial database environment. Metadata generated and updated are then stored in the Metadata Repository. With the metadata and query rewriting algorithms developed, invalid views and queries (both spatial and non-spatial) can be identified and rewritten against the new schema by Query/View Rewriting. These aspects combined enable the management of schema evolution in an FSDBS in an automatic and transparent manner.
Based on the methodologies and algorithms developed as well as processes designed, the ASE prototype has been designed and developed in order to test the feasibility of the ASE. The working environment for the prototype system is Microsoft® SQL Server® 2008 and Microsoft® Visual Studio® C#. The ASE prototype contains the Metadata Repository, SED tool, Schema Mapping tool and Query Rewriting tool. The ASE prototype is then tested on a sample FSDBS and the results indicate that the ASE is effective for automatically managing schema evolution in an FSDBS.