Object-Relational Impedance Mismatch: A Framework Based Approach

Ireland, Christopher Jon (2012). Object-Relational Impedance Mismatch: A Framework Based Approach. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f1dd


The term impedance mismatch was first used in 1984 to label problems that arise when a program uses a relational database for storage. For example, when transferring data from a relational database into a program any relational data structure is lost because a program operates at the row level. Consequently that data structure must somehow be reproduced when data is returned to a database. There are many such mismatches that cost time and effort to address. As new programming and database languages are introduced other kinds of impedance mismatch are anticipated.

Traditional approaches are concerned with pragmatic solutions to specific problems of implementation. They do not address the underlying cause and offer little rationale for the claim to a "solution". The motivation for this dissertation is to understand the cause of these mismatches so it is then possible to address each of them in an appropriate way.

Problem themes are introduced as a way to make sense of impedance mismatch. Such problems are not independent. Relationships between problem themes demonstrate the complex nature of impedance mismatch and they are used to identify three problems of particular significance. A structure to existing characterisations of impedance mismatch is identified and developed in order to organise the characterisations in a meaningful and useful way. This structure, based on four levels of abstraction, forms the foundation for a new framework.

The framework recognises a separation of concerns between a program and a database across levels of abstraction. At each level is observed a particular kind of impedance mismatch. Through a dialogue about a correspondence at each level it is possible to understand and address each kind of mismatch in a structured and consistent way. A technique based on equivalence is introduced in support of a dialogue.

The validity of the framework is demonstrated by identifying the cause of some significant mismatches. Across all the levels of the framework are explored both the cause of each mismatch and the effect of a solution. A four-stage process is described in support of an exploration and to inform others in the use of the framework. An option for change is linked to a conceptual problem not one of implementation and the fidelity and integrity of an existing solution is improved in a way that can be generalised for other solutions. New insights are also provided into the consequences of one solution.

Understanding cause and effect in this level of detail is not available using an alternative framework described in the literature. However despite the improved understanding of an impedance mismatch and the consequences of a solution there is a limit to what can be achieved using the framework.

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