A methodology to assess the impact of design patterns on software quality

Abstract

Context

Software quality is considered to be one of the most important concerns of software production teams. Additionally, design patterns are documented solutions to common design problems that are expected to enhance software quality. Until now, the results on the effect of design patterns on software quality are controversial.

Aims

This study aims to propose a methodology for comparing design patterns to alternative designs with an analytical method. Additionally, the study illustrates the methodology by comparing three design patterns with two alternative solutions, with respect to several quality attributes.

Method

The paper introduces a theoretical/analytical methodology to compare sets of “canonical” solutions to design problems. The study is theoretical in the sense that the solutions are disconnected from real systems, even though they stem from concrete problems. The study is analytical in the sense that the solutions are compared based on their possible numbers of classes and on equations representing the values of the various structural quality attributes in function of these numbers of classes. The exploratory designs have been produced by studying the literature, by investigating open-source projects and by using design patterns. In addition to that, we have created a tool that helps practitioners in choosing the optimal design solution, according to their special needs.

Results

The results of our research suggest that the decision of applying a design pattern is usually a trade-off, because patterns are not universally good or bad. Patterns typically improve certain aspects of software quality, while they might weaken some other.

Conclusions

Concluding the proposed methodology is applicable for comparing patterns and alternative designs, and highlights existing threshold that when surpassed the design pattern is getting more or less beneficial than the alternative design. More specifically, the identification of such thresholds can become very useful for decision making during system design and refactoring.

Introduction

Object oriented design patterns have been introduced in mid 1990s as a catalog of common solutions to common design problems, and are considered as standard of “good” software designs [31]. The notion of patterns was firstly introduced by Alexander et al. [2] in the field of architecture. Later the notion of patterns has been transformed in order to fit software design by Gamma, Helm, Johnson and Vlissides (GoF) [31]. The authors cataloged 23 design patterns, classified according to two criteria. The first, i.e. purpose, represents the motivation of the pattern. Under this scope patterns are divided into creational, structural and behavioral patterns. The second criterion, i.e. scope, defines whether the pattern is applied on object or class level. In [31], the authors suggest that using specific software design solutions, i.e. design patterns, provide easier maintainability and reusability, more understandable implementation and more flexible design. At this point it is necessary to clarify GoF are not the first or the only design patterns in software literature. Some other well known patterns are architectural patterns, computational patterns, game design patterns, etc. In recent years, many researchers have attempted to evaluate the effect of GoF design patterns on software quality. Reviewing the literature on the effects of design pattern application on software quality provides controversial results. Until now, researchers attempted to investigate the outcome of design patterns with respect to software quality through empirical methods, i.e. case studies, surveys and experiments, but safe conclusions cannot be drawn since the results lead to different directions. As mentioned in [37], [39], [53], [59], [69], design patterns propose elaborate design solutions to common design problems that can be implemented with simpler solutions as well.

In this paper we propose a methodology for comparing pattern designs. The proposed method is analytical in the sense that system comparison is not performed on specific system instances, but on design motifs that can describe every possible instance of a system during its extension. The methodology is generic so that it can be applied for comparing design alternatives that describe equivalent functionality and have specified axes of change. The proposed method attempts to formulate several quality attributes as functions of functionality addition on multiple, equivalent solutions to a design problem. Then the functions are compared so as to identify cut-off points during system maintenance when one solution gets better or worse than the other. In this study we illustrate the applicability of the methodology by employing it for comparing GoF design patterns to equivalent adhoc solutions. In the next section, we present background information on design patterns. In Section 3, the methodology is presented and in Section 4, three exploratory cases are demonstrated. A discussion on the methodology is presented in Section 5. Conclusions, threats to validity and future work are presented by the end of the paper.