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Connecting High-Level Business Rules with Object-Oriented Applications: An approach using Aspect-Oriented Programming and Model-Driven Engineering

vrijdag, 22 juni, 2007 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
Faculteit: Science and Bio-engineering Sciences
María Agustina Cibrán

This dissertation addresses the problem of connecting high-level, executable business
rules with existing object-oriented applications.

State-of-the-art research on developing object-oriented software applications with rulebased
knowledge advocates making rules explicit and separate from the object-oriented
core functionality. Although many approaches target this goal and are a considerable improvement
on the solution that embeds rules as conditional statements in object-oriented
applications, these approaches still suffer from three major inherent problems.

First, even when business rules are successfully decoupled, the rule connection code is
still tangled with and scattered in the implementation of the core application functionality.
Therefore, when existing business rules need to be integrated differently, or when new
business rules need to be connected at unanticipated events, the source code of the core
application must be adapted manually at different places. Consequently, it becomes difficult
to localize, add, change or remove rule connections.

Secondly, regardless of the approach taken to decouple the business rules, executable
rules are ultimately low-level. This makes rules not understandable to domain experts who
are not adept at programming. A third, and closely related problem is a tight coupling
between the business rules and the existing implementation of the core application. This
causes rules to be fragile and not reusable, and prohibits the non-invasive realization of
rules in terms of unanticipated implementation elements. As a consequence, business rules
cannot be deployed by the domain experts without the intervention of a developer.

This dissertation presents a comprehensive solution to these problems, enabling existing
applications to integrate business rules at the domain level. The first problem is addressed
by encapsulating the rule connection in a separate module, decoupled from both the core
application functionality and the rules. This decoupling is not straightforward because
rule connections crosscut the core application functionality. Aspect-Oriented Programming
(AOP) provides new modularization mechanisms, i.e. aspects, for the encapsulation of
crosscutting code while ensuring dependency inversion between the core application and
the aspects. These properties make AOP suitable for encapsulating crosscutting rule connections.
This dissertation identifies commonalities and variabilities in the implementation
of rule connection aspects and proposes abstracting these recurrent issues as elements of
aspect patterns.

The second and third problems are addressed by building a layer of abstraction, a domain
model, which allows for the expression of business rules in terms of domain concepts. This
domain layer is able to represent domain concepts explicitly. A dedicated high-level business
rule language is provided which enables the expression of high-level rules in terms of the
domain concepts. Consequently, the coupling between the existing implementation of the
core application and the rules is loosened. Moreover, the domain model is evolvable, which
allows for the realization of unanticipated domain concepts and business rules that appear
as a result of domain evolution.

This dissertation observes that, although aspects are a suitable solution to the problem
of decoupling crosscutting rule connections, they exclude domain experts, as these aspects
reside at the implementation level. Moreover, rule connection aspects need to take into
account several recurrent issues, which renders the task of implementing these aspects difficult
for developers. This dissertation supports the expression of rule connections at the
domain level. A second dedicated high-level language is provided for this purpose.

The solution presented in this dissertation incorporates ideas from Model-Driven Engineering
(MDE) in order to achieve the automatic generation of executable implementations
for the high-level rules and rule connections. High-level rules and rule connections are
automatically transformed to rule objects and rule connection aspects respectively.

The approach presented in this dissertation is evaluated in the domain of Service-
Oriented Architecture (SOA). Service-oriented applications are very volatile: new services
appear, services become unavailable, non-functional properties of services vary (even at
run-time), and applications need to cope with all these changes. Moreover, clients also
change their requirements with respect to the selection and integration of services. This
dissertation shows how high-level business rules can automate the customization of serviceoriented applications. A Web services management layer, the WSML, is used as case study.
Two scenarios are presented: an evolution scenario, which shows that it is possible to add
new rules to the existing management framework, and a refactoring scenario, which shows
that existing rules in the core WSML implementation can be refactored and externalized
as high-level business rules.