Distributed Databases
In the area of distributed databases, the group has been involved in research into advanced techniques for database schema integration. In collaboration with a group at Birkbeck College, a framework for the definition of schema transformation rules has been developed, which may be applied between database schemas which are both physically and semantically heterogeneous in nature. This has the properties that it is readily adaptable to be used with any particular data modelling language, such as the ER model, UML or relational models, and that is may also be used to translate between those modelling languages. A simple prototype has been developed.
Current work includes the development in the AutoMed project of a fully functional tool for the specification and analysis of database schema transformation rules, and the investigation of the use of the those rules to automate the process of building mediators between heterogeneous database systems.
Current Projects
Current projects in this field include:
Key Publications
Peter McBrien and Alex Poulovassilis
Advanced Information Systems Engineering, 11th International Conference CAiSE'99
- Springer Verlag LNCS 1626, 333-348, 1999
Abstract: Whilst it is a common task in systems integration to have to transform between different semantic data models, such inter-model transformations are often specified in an ad hoc manner. Further, they are usually based on transforming all data into one common data model, which may not contain suitable data constructs to model directly all aspects of the data models being integrated. Our approach is to define each of these data models in terms of a lower-level hypergraph-based data model. We show how such definitions can be used to automatically derive schema transformation operators for the higher-level data models. We also show how these higher-level transformations can be used to perform inter-model transformations, and to define inter-model links.
Automatic migration and wrapping of database applications - a schema transformation approach
Peter McBrien and Alex Poulovassilis
Proceedings of ER99
- Springer Verlag LNCS 1728, 96-113, 1999
Abstract: Integration of heterogeneous databases requires that semantic differences between schemas are resolved by a process of schema transformation. Previously, we have developed a general framework to support the schema transformation process, consisting of a hypergraph-based common data model and a set of primitive schema transformations defined for this model. Higher-level common data models and primitive schema transformations for them can be defined in terms of this lower-level model. In this paper, we show that a key feature of our framework is that both primitive and composite schema transformations are automatically reversible. We show how these transformations can be used to automatically migrate or wrap data, queries and updates between semantically equivalent schemas. We also show how to handle transformations between non-equivalent but overlapping schemas. We describe a prototype schema integration tool that supports this functionality. Finally, we briefly discuss how our approach can be extended to more sophisticated application logic such as constraints, deductive rules, and active rules.
A Formalisation of Semantic Schema Integration
Peter McBrien and Alex Poulovassilis
Information Systems 23(5) 307-334, 1998
Abstract: Several methodologies for the semantic integration of databases have been proposed in the literature. These often use a variant of the Entity-Relationship (ER) model as the common data model. To aid the schema conforming, merging and restructuring phases of the semantic integration process, various transformations have been defined that map between ER representations which are in some sense equivalent. Our work aims to formalise the notion of schema equivalence and to provide a formal underpinning for the schema integration process.
- We show how transformational, mapping and behavioural schema equivalence are all variants of a more general definition of schema equivalence. We propose a semantically sound set of primitive transformations and show how they can be used to express the transformations commonly used during the schema integration process and to define new transformations. We differentiate between transformations which apply to any instance of a schema and those which require knowledge-based reasoning since they apply only for certain instances; this distinction could serve to enhance the performance of transformation tools since it identifies which transformations must be verified by inspection of the schema extension; it also serves to identify when intelligent reasoning is required during the schema integration process.
A General Formal Framework for Schema Transformation
Alex Poulovassilis and Peter McBrien
Data and Knowledge Engineering 28(1) 47-71, 1998.
Abstract: Several methodologies for integrating database schemas have been proposed in the literature, using various common data models (CDMs). As part of these methodologies transformations have been defined that map between schemas which are in some sense equivalent. This paper describes a general framework for formally underpinning the schema transformation process. Our formalism clearly identifies which transformations apply for any instance of the schema and which only for certain instances. We illustrate the applicability of the framework by showing how to define a set of primitive transformations for an extended ER model and by defining some of the common schema transformations as sequences of these primitive transformations. The same approach could be used to formally define transformations on other CDMs.