Object Relational Model

Introduction

As introduced in the Web’s tiered architecture, objects are an integral component of today’s multi-tier Web accessible architectures and serve as the basis for emergent Service Oriented Architectures (SOA) and Web Services (Steiert, 2007; W3C, 2004).  Object oriented design’s (OOD) three guiding principles are data and method encapsulation, inheritance and polymorphism (Weisfield, 2009).  While detailed analysis of these OOD concepts is beyond the scope of this module, it is understood that a system based on objects provides a secure, flexible and scalable framework.  While these OOD advantages are particularly relevant to today’s networked and heterogeneous IS, there has been limited development of pure object-oriented databases due to the additional complexity they incur (Garcia-Molina, Ullman & Widom, 2009).  To accommodate OOD functionality, many RDBMS vendors have introduced object oriented features to their products and these products herald the emergence of the object-relational model (Garcia-Molina, Ullman & Widom, 2009).

The transition to an object-relational model must account for several changes to the relational model.  These changes include: (a) domain types may be aggregate objects rather than atomic attributes, (b) objects encapsulate methods that provide functionality beyond the SQL framework, (c) the aggregate nature of objects may necessitate the creation of identifiers for objects and their components, and (d) the RDBMS system may need to add support for object references (Codd, 1990; Garcia-Molina, Ullman & Widom, 2009).  It should be noted at the outset that object-relational models do not conform to Codd’s RDBMS theoretical constructs and Codd questions the inclusion of object oriented components in the relational model (Codd, 1990).   Recall that the relational model’s foundation is based on predicate logic however the move to non-atomic attributes renders predicate logic invalid and represents a serious flaw in the relational model (Codd, 1990).

The first conceptual change to accommodate the transition to the object relational model is that domain types may consist of objects and therefore may no longer be atomic.  Objects encapsulate both data and methods therefore the object-relational model must also account for method functionality listed above as the second requisite change.  As introduced above, domains that support aggregate data types cannot support predicate logic and thus do not support relational recursion.  In contrast to these vagaries, the move towards method encapsulation is consistent with typical RDBMS implementations since RDMBS are closely integrated with and accessed through a host programming language in any event (Codd, 1990; Garcia-Molina, Ullman & Widom, 2009).  Objects may encapsulate business logic in accord with an organization’s policies and procedures therefore an object’s methods can provide additional functionality and even serve as an additional constraint mechanism (Erl, 2005).

Since attributes may be non-atomic objects, the system may need mechanisms to identify and reference objects (Garcia-Molina, Ullman & Widom, 2009).  The inclusion of identifiers may be necessary to uniquely identify objects and references provide the object-relational model with a method to link objects and their components and eliminate redundancy (Garcia-Molina, Ullman & Widom, 2009).   Object oriented systems rely on references to link objects and components however the pure relational model is devoid of references since computation is based on comparison and evaluation of attribute values (Codd, 1990).   The pure OOD model hides references as exposing them represents a security risk however the object relational model may necessarily expose references for comparison and future reference (Malik, 2006; Weisfield, 2009; Wilander & Kamkar, 2003).

Summary

Despite its incongruence with the pure predicate logic based relational model, the move towards the object-relational model provides enhanced functionality to contemporary RDBMS.  As Codd (1990) asserts, the move towards an integrated relational and object oriented model will require that predicate logic evolve to support aggregate data types.

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