The need of measuring software quality


People spend a lot of time debating about software quality but, very often, they speak in abstract terms. They don’t measure anything. Even worst, they make early design decisions in name of non-functional requirements (especially concerning perfomance issues). Without any measurement to support decisions, our intuition can be misleading most of the time. UML models let us to visualize large-scale structures better than the mere code, but again they are not always sufficient to fully describe the overall complexity of the system, nor they suffice to compare design alternatives. Consider, for example, the design illustrated in Figure 1 (real names are replaced by letters; in any case, what is important here is the model structure, not the specific domain discussed).

The structure of model A

Figure 1 - The first option to model a portion of a domain in UML

The model illustrates two aggregates, G and M, a hierarchy H and some infrastructure classes all around the aggregates (repositories, factories, and proxies). Looking at this design, the designer experimented a little bit in order to improve it. Pheraps the hierarchy merges two different dimensions, an abstraction and an implementation (Hj and Hjk, where j stands for the abstraction concern and k for the implementation concern, respectively). Thus, one possible refactoring strategy is to separate such dimensions, applying the Bridge pattern (the original hierarchy rooted at H now has been divided into two structures rooted at H1 and H2, respectively). Another improvement can be to merge the classes X and Y in order to provide a single, convenient access point to the hierarchy for the aggregates M and G (another Proxy interface?). Finally, some small modifications affecting the dependencies between the infrastructure classes and their aggregates, and we obtain the design described in model Figure 2.

Figure 2 - An alternative design for the original model

In his refactoring, the designer applied two well-known design patterns. This sounds good, but does he really improve the design? Looking at the structures of the two models, which one is better is not so evident. In general, there is no single answer. We need to understand which non-functional requirement is more important, in order to choose a suitable metric to measure it. Considering maintainability and testability, better means “simpler” in terms of complexity and coupling. We can calculate the CCD/NCCD metrics for both design alternatives in order to evaluate the dependecy structure of the two models. Despite of the fact that the second model applies some design patterns, the overall maintainability is not improved. The crude numbers are showed in Table 1:

 

Comparison between the two designs

Table 1 - Comparison between the two designs

The second design shows an increase in complexity of 9,334% according to NCCD metric despite of a reduction of the system size (-13,793%). This example illustrates several points:

  • The simple application of design patterns does not always guarantee an improvement in quality. A pattern represents a well documented solution to a recurring problem, but it comes with costs and consequences. If we choose a pattern which do not represents a good trade-off with respect to the specific non-functional requirement we want to optimize, the result will not be optimal.
  • Changing the non-functional requirement to prioritize, the numbers may tell a completely different story. In our example, if we take into account scalability, probably the alternative model of Figure 2 is better, at least considering the maintainance of the evolving hierarchy structure. After all, this is exactly the reason why the Bridge pattern was introduced.
  • Supported by a mathematical model such as that provided by the CCD/NCCD metric, we can measure one dimension of software quality objectively and systematically.
  • Without an objective evaluation, making strategic decisions (e.g. optimizations, design speculations, architectural choices, and so on) on the basis of the sole designer’s intuition can produce unexpected (unpleasant) results.
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6 thoughts on “The need of measuring software quality

    • Hi Alan! The CCD/NCCD metrics are explained in a previous post (How testable is a software architecture?), even if the better resource is the book “Large-Scale C++ Software Design”, written by John Lakos. I have found very few online resources that treat the topic (indeed it is the main reason that suggested me to write some articles about it in the first place). However, you can find an introduction to both physical design and its metrics in the Peter Grogono’s lecture on Software Quality Control.

      • > The CCD/NCDD metrics are explained in a previous post
        Ah, ok. Good to know, thanks.
        But,in that case, a “search” facility in your wordPress site would be appreciated: http://codex.wordpress.org/Creating_a_Search_Page

        The only alternative way to find that post (the best I could do) is to search “CCD/NCCD” with (for example) Google with ‘site’ attribute, and navigate through all given results (that starts with “The need of measuring software quality” part -sometimes with a old version-) and than one has all “How testable is a software architecture?” related links . Yes, achievable, but a bit tricky, don’t you think so?

        I really hope you will add “search” facility..

      • You are right. Unfortunately, the Search widget that I have added some time ago didn’t work properly. I will probably change the theme in order to overcome this and other limitations (especially concerning the site layout). Check it in the next weeks. Thanks a lot for your feedback.

        Andrea

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