# The Rectangle/Square Controversy

A software design problem that has often been discussed in the literature is whether a square should inherit from a rectangle or whether a rectangle should inherit from a square? Two opposing arguments are made by Meyer [1] and Martin, et.al. [2], both of which seem to have some merit. How is a software developer to decide which is the correct design? In this post I will discuss why I believe both approaches are wrong and I will provide the solution I prefer.

Meyer [1] proposes the design where square inherits from rectangle with square imposing the invariant that the width must equal the height. The conceptual design is shown in Figure 1. This design makes sense since a square “is-a” rectangle. Remember, that “is-a” is the litmus test for inheritance [3]. To state this more precisely: inheritance defines a subset relation between a child and a parent class. Thus, the Square class represents the set of squares which is a subset of the set of rectangles which is represented by the Rectangle class. This is shown in Figure 2. For a detailed discussion on the mathematical semantics of inheritance, see Meyer [1]. The implementation is shown in Figure 3.

Figure 1: Square extends Rectangle

Figure 2: The set of squares is a subset of the set of rectangles

Figure 3: The implementation of a Square extending a Rectangle

Martin’s criticism against the design of Figure 1 is that it violates the Liskov substitution principle, which states that subtypes must be substitutable for their base types [2]. Clients of instances of the Square class cannot set the width and height of a square as if they are using a rectangle. In particular they need to ascertain that the width and height of instances of square are equal. The main problem is that a square does not require both a width and a height and it therefore needs to ignore either of the attributes.

An alternative design that Martin discusses is where the setWidth (respectively setHeight) method is changed to set the height equal to the width (respectively to set the width equal to the height). However, the problem is when setWidth (respectively setHeight) is called it will overwrite the value of the height (respectively width) (see Figure 4). Hence, a client that first sets the width to 5 and then the height to 3 will expect the perimeter to be 16, but instead it will be 12. This again violates the Liskov substitution principle.

Figure 4: Forcing the width and height to be equal

The solution that Martin [2] proposes is that the Rectangle class must inherit from the Square class (see Figure 5). The objection that Meyer has against this design is that it violates the “is-a” relation in that a rectangle is not a square. Now you may reason that a rectangle is indeed a square when its width and height are equal, but the crucial point is that a rectangle is not always a square. Remember, inheritance enforces a subset relation between a child and a parent. That is, all instances of the child are necessarily instances of the parent, but not all instances of the parent are necessarily instances of the child.

The flaw in the solution that Martin proposes is that he uses inheritance for the sole purpose of reuse without there being an “is-a” relation. Meyer calls this “convenience inheritance” which should be avoided.

Figure 5: Rectangle extends Square

From a conceptual perspective a way around these objections is to introduce a class (i.e. Quadrilateral) from which both the Rectangle and Square classes can inherit. The related UML class diagram is shown in Figure 6. The {incomplete, overlapping} annotation indicates that other quadrilaterals may exist and that rectangles may occasionally be squares (when width=height).

Figure 6: Conceptual solution for Square and Rectangle problem

The implementation of this conceptual solution can be done where Quadrilateral is either an interface or an abstract class. Figure 7 shows the implementation where Quadrilateral is defined as an interface.

Figure 7: A correct implementation for the Square and Rectangle classes

Naturally I am not the first to point this solution out. [5] gives a general discussion of the problem and [4] provides a solution similar to mine.

### Bibliography

[1] B. Meyer, Object-oriented software construction, Prentice Hall, 1997.

[2] R. Martin and M. Micah, Agile principles, patterns, and practices in C#, Prentice Hall, 2006.

[3] G. Booch, R. A. Maksimchuk, M. W. Engel, B. J. Young, J. Conallen and K. A. Houston, Object-oriented analysis and design with applications, Addison-Wesley Professional, 2007.

[4] R. Carr, “Is a Square a Rectangle?,” [Online]. Available: http://www.blackwasp.co.uk/SquareRectangle.aspx.

[5] H. Makabee, “When a square is not a rectangle,” [Online]. Available: http://effectivesoftwaredesign.com/2010/09/20/when-a-square-is-not-a-rectangle/.

# Software Entropy: The Case for Organizational Change

In general enterprise software consists of large numbers of systems with complex cross-dependencies and with a high level of heterogeneity and redundancies. The ability of these software systems to address the needs of the enterprise changes over time for the worse. In order to understand the forces at work I will first consider entropy at the system level and then at the enterprise level.

## System Entropy

Initially after a software system is delivered, making changes to the system is usually relatively easy. The longer the system is in maintenance and the more changes are incorporated into the code base; the more the ability to make changes to the software system deteriorates [1] [2]. There are various reasons for the deterioration of software systems. Some of the reasons are due to technology, but mostly the reasons are organizational of nature [1].

From a technology perspective various best practices exist to counter entropy of software systems. For legacy technologies roadmaps exist to incorporate the latest systems design thinking to address short comings of earlier technologies. As an example IBM provides various strategies to improve the maintainability of RPG programs [3] [4].

At a system level the single most significant contributor to the decay of the code base is the lack of routine refactoring to ensure the long term maintainability of the system [1]. These refactoring exercises can for instance be geared towards ensuring the soundness of the design in the face of changing business needs or taking advantage of technology improvements. The lack of timely refactoring may be due to factors such as lack of skilled staff or time-to-market demands.

As such technology is not entirely to blame for the entropy of software systems. Rather, organizational level challenges are the most important contributors to the decay of software systems. Therefore, in order to address software system entropy, change firstly needs to be effected at the organizational level and subsequently at the technology level if appropriate. For example, replacing the existing RPG programs with the latest technology will inevitably in time propagate entropy in the new technology as well if the organizational challenges are not addressed.

Further factors that contribute to the decay of software systems originate at an enterprise level.

## Enterprise Software Entropy

The level of entropy found at the enterprise level is significantly higher than that of a single isolated software system. Typically the enterprise architecture is an accidental architecture. That is the enterprise architecture came about organically due to the independent evolution of the various systems and their cross dependencies rather than being planned. Since the various systems have been designed and developed in isolation, redundancies between systems are rife. Many enterprise architectures has grown over many years which results in a plethora of programming languages – often based on disparate programming paradigms – used across systems. With such a lengthy history these systems are frequently hosted on different kinds of servers running a variety of operating systems.

A characteristic of an accidental architecture is the occurrence of stovepipes (or silos). Stovepipes are systems that have been developed to fulfill a particular task without regard for the context of the task within the larger enterprise business process [2]. Since these systems have not been designed to interoperate [5], this approach sabotages efficiency at an enterprise level [6]. The most prevalent approach to alleviating the effect of silos in the enterprise is through different manifestations of point-to-point integrations. This results in the intertwined enterprise architecture as seen in Figure 1.

Figure 1: Typical Intertwined Enterprise Architecture

The effects of stovepipes are usually acutely noticeable between business units while the intra business unit level effects are often largely hidden from the enterprise. Understandably this causes business and management to have a vague idea of the true complexity of integration projects. This contributes to integration projects being invariable hugely under estimated in terms of cost and timelines [7]. Given this background it is no wonder that an estimated 70% of Enterprise Application Integration (EAI) projects fail [8]. The problems around integration are further exacerbated when e-Business and its associated need for business to business communications is considered.

If we want to address the problems of enterprise software effectively, we have to understand the forces contributing to entropy. These factors can be categorized into day-to-day operations and specific events [1].

The daily maintenance processes are geared towards the continuous delivery of economic value in the face of ever changing business needs. Enterprise systems generate economic value through value networks. These value networks frequently span multiple applications [9] since most applications or off-the-shelf solutions are not solving cross-functional or inter-enterprise needs [10] [11] [12]. In order to support the value network these applications need to be integrated [12] [11]. Day-to-day operations are typically project and short term focussed instead of being enterprise and long term focussed. This explains the proliferation of point-to-point integrations found in many of today’s enterprises.

Besides day-to-day operations specific events contribute to the decay of enterprise systems [1]. These events include but are not limited to the following:

• The maturation of organizations causes new specialized groups to be included in the Software Development Life Cycle (SDLC) [5].
• End-of-life events of a supported product [1] may cause skills to be available only at a premium.
• An enterprise may expand into new territories as a result of off shoring or globalization [5].
• Mergers or acquisitions can introduce new systems and technologies into the organization. This is likely to result in redundancies at various levels [1] [5] [9].
• New lines of business may be introduced to take advantage of new business opportunities [9]. Often business units have their own IT departments with opposing agendas causing infighting between business units [5].
• Obsoleteness or limitations of legacy technologies may drive technology modernization exercises [9]. Legacy applications further provide challenges in managing a retiring workforce and ever decreasing skills base [5].
• Government legislation may change the way business is conducted [9].

It is significant to again note that the role technology play in driving entropy at the enterprise level is negligible. Thus in order to find solutions to enterprise software entropy we cannot consider technical change alone – change must be effected at the organizational level as well.

## Conclusion

The traditional solution to the decay of software is to start from a clean slate and rewrite the application using a new technology. Often technology is blamed for the ills of the software industry, but technology is merely a tool at the disposal of the enterprise. The enterprise chose to use the technology because it has a business value. When a newer technology comes along it brings new opportunities for increased efficiencies – it does not nullify the value of the older technology. Therefore a rewrite should not be considered merely on technical grounds (for example the availability of a new technology), but should have a clear business case.

Continuous refactoring can be used to modify an application to take advantage of the latest design principles and stay relevant in the face of changing business needs. However, the prevalence of short term planning in organizations causes the continuous refactoring step often to be skipped altogether. Omitting the refactoring step in the long term leads to code that becomes too costly to refactor, hence the need for a rewrite.

# Bibliography

 [1] D. Krafzig, K. Banke and D. Slama, “Enterprise SOA: Service-Oriented Architecture Best Practices,” Amazon.com, 2004. [Online]. Available: http://www.amazon.com/Enterprise-SOA-Service-Oriented-Architecture-Practices/dp/0131465759/. [2] M. Juric, P. Sarang and B. Mathew, “Business Process Execution Language for Web Services BPEL and BPEL4WS 2nd Edition,” Amazon.com, 2006. [Online]. [3] L. Patterson, H. Araki, S. Bramley, G. Cobb, J. Eikenhorst, S. Milligan, J. Simons and M. Tregear, “Modernizing and Improving the Maintainability of RPG Applications Using X-Analysis Version 5.6,” http://www.redbooks.ibm.com/, 2006. [Online]. Available: http://www.redbooks.ibm.com/redpapers/pdfs/redp4046.pdf. [4] D. Cruikshank, “Modernizing Database Access: The Madness Behind the Methods,” IBM, 2006. [Online]. Available: http://www-03.ibm.com/systems/resources/systems_i_software_db2_pdf_Performance_DDS_SQL.pdf. [5] M. Matsumura, B. Brauel and J. Shah, “SOA Adoption for Dummies,” Software AG, 2009. [Online]. Available: http://zy.xjgame.com/SOBPAO/SOA%20Adoption%20for%20Dummies.pdf. [6] J. W. Ross, P. Weill and D. C. Robertson, “Enterprise Architecture As Strategy: Creating a Foundation for Business Execution,” Amazon.com, 2006. [Online]. Available: http://www.amazon.com/Enterprise-Architecture-Strategy-Foundation-Execution/dp/1591398398. [7] R. Schmelzer, “Understanding the Real Costs of Integration,” zapThink, 2002. [Online]. Available: http://www.zapthink.com/2002/10/23/understanding-the-real-costs-of-integration/. [8] G. Trotta, “Business Process Management (BPM) Best Practices: Dancing Around EAI ‘Bear Traps’,” ebizQ: The Insider’s Guide to Next-Generation BPM, 2003. [Online]. Available: http://www.ebizq.net/topics/int_sbp/features/3463.html. [9] F. A. Cummins, “Building the Agile Enterprise: With SOA, BPM and MBM,” Amazon.com, 2008. [Online]. Available: http://www.amazon.com/Building-Agile-Enterprise-SOA-Press/dp/0123744458. [10] F. Kuglin and R. Hood, “Using Technology to Transform the Value Chain,” Amazon.com, 2008. [Online]. Available: http://www.amazon.com/Using-Technology-Transform-Value-Chain/dp/1420047590. [11] M. Weske, “Business Process Management: Concepts, Languages, Architectures,” Amazon.com, 2012. [Online]. Available: http://www.amazon.com/Business-Process-Management-Languages-Architectures/dp/3642286151. [12] C. C. Poirier, L. Ferrara, F. Hayden and D. Neal , “The Networked Supply Chain: Applying Breakthrough BPM Technology to Meet Relentless Customer Demands,” Amazon.com, 2003. [Online]. Available: http://www.amazon.com/The-Networked-Supply-Chain-Breakthrough/dp/1932159088.

# Frameworks for Bespoke Software in the Context of Enterprise Architecture

In my previous post I spoke about the cost and complexities of creating a framework for bespoke software. In this post I want to look at how one can evaluate the appropriateness of a framework in the context of enterprise architecture. Such an evaluation needs to take into consideration the operating model of the business as well as the current stage of architecture maturity of the enterprise architecture of the organization.
Detailed information regarding operating models and architecture maturity can be found in [1] (with some information available at [2]). Here I will just provide sufficient information on these to support this discussion.

## Operating Model

An operating model defines the necessary level of business process standardization and data integration required to run the business effectively.  Business processes are standardized when they are the same across organizational units. The level of data integration required is typically driven by the proportion of customers shared across organizational units. Based on the required levels of business process standardization and data integration, different general types of operating models can be defined:

• Diversification – Different organizational units have different business processes and different customers.
• Coordination – Different organizational units share a large number of customers.
• Replication – Different organizational units have a set of shared business processes.
• Unification – A set of business processes and customers are shared across organizational units.

Organizational units in this context can refer to any unit within an organization, i.e. business unit, department, branch etc. Different operating models can be defined for different levels of organizational units.

## Architecture Maturity

Even though the business may be characterized by a specific operating model, this operating model may not be well expressed in the current enterprise architecture of the organization.  The different stages of architectural maturity give an indication of how well the operating model is realized within the software systems of the organization. The stages of architectural maturity are defined as follows:

1. Business Silos – The key business driver at this stage is to encourage innovation with organizational units largely operating independently. Key characteristics of this stage is a proliferation of technologies and solutions
2. Standardized Technology – Proliferation of different technology platforms during the Business Silos stage increases operating costs. The Technology Standardization stage aims to reduce operating cost by reducing the number of technology platforms.
3. Optimized Core – Business processes and data are standardized throughout the organization in accordance with the operating model of the organization.
4. Business Modularity – Reusable system modules are leveraged to enable business agility.

## Evaluation

Martin Fowler in his Analysis Patterns book states [3]:

To accomplish component reuse for information systems, a common framework must be established. An effective framework must not be too complex or too bulky. It should be widely applicable across a large domain and be based on an effective conceptual model of that domain. Developing such frameworks is difficult, both technically and politically.

In order for a framework to be widely applicable, it needs to be closely aligned with the operating model of the organization. Creating a framework for optimizing business processes in an organization with a Coordination operating model is probably not a smart move.  In a similar vein creating a framework in an organization with a Diversification model may not provide adequate return on investment (ROI).
The architectural maturity stage of the organization has to be able to support building of a framework:

• Building a framework during the Business Silos stage will politically be a difficult sell due to a clash of priorities. In order to be successful at building a framework, the stability and integrity of the framework has to be preserved at all cost. This may mean that the implementation of new features needs to be delayed to maintain framework stability and integrity [4]. This approach is clearly at odds with the typical time-to-market pressures of the Business Silos stage.
• It may be tempting to create a framework at the Standardized Technology stage, but lack of standardized data access and business processes will result in limited opportunities for reuse. Proponents may argue that a framework may be the means through which an organization progresses from the Standardized Technology stage to the Optimized Core stage. Still, in order to be successful, standardization needs to precede framework creation.

## Summary

In closing, here are my general rules of thumb regarding framework development:

• Consider creating a framework only for organizations which has a Coordination, Replication or Unification operating model.
• For a Coordination operating model focus framework design at optimizing data access.
• For a Replication operating model focus framework design at optimizing business processes.
• For a Unification operating model aim framework design at optimizing data access and business processes.
• Create a framework only when the organization has reached the Optimized Core or Business Modularity architectural maturity stage.

If you consider deviating from these guidelines, it will be in your best interest to have a watertight; well documented business case, signed-off by the relevant stakeholders.

# Bibliography

[1]     J. W. Ross, P. Weill and D. C. Robertson, Enterprise Architecture as Strategy: Creating a Foundation for Business Execution, Harvard Business School Press, 2006. http://www.amazon.com/Enterprise-Architecture-Strategy-Foundation-Execution/dp/1591398398
[2]     “Center for Information Systems Research,” MIT Sloan Management, April 2013. [Online]. Available: http://cisr.mit.edu/research/research-overview/classic-topics/enterprise-architecture/.
[3]     M. Fowler, Analysis Patterns: Reusable Object Models, Addison-Wesley, 1997. http://www.amazon.com/Analysis-Patterns-Reusable-Object-Models/dp/0201895420
[4]     J. Tulach, Practical API Design: Confessions of a Java Framework Architect, Apress, 2008. http://www.amazon.com/Practical-API-Design-Confessions-Framework/dp/1430243171