Theory of Constraints in Software Engineering

Theory of Constraints (TOC) is a methodology invented by Dr. Eliyahu M. Goldratt, a scientist, physicist, author, educator and consultant [3]. With the methodology the most important limiting factor, that hinders a goal to be achieved, is identified and then that factor (i.e. constraint) is improved until it is no longer the limiting factor. Since Goldratt introduced the Theory of Constraints in his bestselling 1984 novel, “The Goal”, the methodology has continued to evolve and develop into many different fields including Software Engineering.[2]

The goals of Software Engineering is the creation of software systems that meet the needs of customers and are efficient, maintainable and reliable in addition the systems should meet project schedules and budgets along with being produced in an economical way [1, p21]. Applying analogous set of approaches to the development of software, one can expect the production of software that meets the goals of Software Engineering [1, p22].

This article will introduce how Theory of Constraints can be used in Software Engineering along with guidance to apply the methodology.

Theory of Constraints

Before improving a system or any part of it, one must define system’s global goal and the measurements that will enable him to judge the effect of any subsystem and any local decision on the goal. When measurements have been defined, the factor that limits the system to reach its goal can be found; constraint [5, page 5].

Theory of Constraints (TOC) is a methodology that is used to identify the most important constraints in a system and then improving the constraint until it is no longer the limiting one. According to TOC every complex system consists of multiple linked activities and one of these activities is a constraint upon the whole system.

Theory of Constraints provides powerful set of tools to achieve the ultimate goal of companies: Make profit.

The Five Focusing Steps

TOC provides the Five Focusing Steps to identify and eliminate the constraints, the steps are described in the table below:

The Five Focusing Steps
Identify	Identify the current constraint (the single part of the process that limits the rate at which the goal is achieved).
Exploit	Make quick improvements to the throughput of the constraint using existing resources (i.e. make the most of what you have).
Subordinate	Review all other activities in the process to ensure that they are aligned with and truly support the needs of the constraint.
Elevate	If the constraint still exists (i.e. it has not moved), consider what further actions can be taken to eliminate it from being the constraint. Normally, actions are continued at this step until the constraint has been “broken” (until it has moved somewhere else). In some cases, capital investment may be required.
Repeat	The Five Focusing Steps are a continuous improvement cycle. Therefore, once a constraint is resolved the next constraint should immediately be addressed. This step is a reminder to never become complacent – aggressively improve the current constraint… and then immediately move on to the next constraint.

The Thinking processes

The thinking processes are composed of cause-effect tools along with necessary condition thinking tools and a set of logic rules. Three of the diagrams that are going to be introduced use cause-and-effect logic which strives to first identify the root causes of undesirable effects and then remove the undesirable effects without creating new ones. The other two, that will be introduced, use necessary condition thinking, in order to achieve one condition the other one has to occur [6].

When using the thinking processes the following three questions are answered:

What needs to be changed?
What should it be changed to?
What actions will cause the change?

Examples of tools that have been formalized as a part of the Thinking Processes are described below:

Current Reality Trees (CRT): The CRT is constructed from top-down, where undesirable effects are identified and traced back to their root cause. Dettmer (1997) states that the CRT is designed to achieve the following objectives:

- Provide the basis for understanding complex systems

- Identify undesirable effects exhibited by a system

- Relate undesirable effects through a logical chain of cause and effect to root causes

- Identify, where possible, a core problem that eventually produces 70% or more of the system’s undesirable effects.

- Determine at what points the root causes and/or core problem lie beyond one's span of control or sphere of influence

- Isolate those few causative constraints)that must be addressed in order to realize the maximum improvement of the system

- Identify the one simplest change to make that will have the greatest positive impact on the system. (P.64)

Evaporating Clouds (EC): When the CRT has been used to identify what to change, the EC is used to identify a solution to what should be changed to, to eliminate undesirable effects. The following purposes are sought to be achieved:

- Confirm that the conflict exists

- Identify the conflict perpetuating a major problem

- Resolve conflict

- Avoid compromise

- Create solutions in which both sides win

- Create new ‘breakthrough’ solutions to problems

- Explain in depth why a problem exists

- Identify all assumptions underlying problems and conflicting relationships. (Dettmer, 1997, p.122)

Future Reality Trees (FRT): Once a solution has been identified with the EC method next step is to start building the Future Reality Tree (FRT). The diagram shows the future state and identifies what to change along with its impact on the future of the organization. FRT is designed and considered to test the solution, using the effect-cause-effect method. The FRT is intended to achieve the following purposes:

- Enables effectiveness testing of new ideas before committing resources to implementation

- Determines whether proposed system changes will produce the desired effects without creating negative side effects

- Reveals through negative branches, whether (and where) proposed changes will create new or collateral problems as they solve old problems, and what additional actions are necessary to prevent any such negative side effects from occurring

- Provides a means of making beneficial effects self-sustaining through deliberate incorporation of positive reinforcing loops

- Provides a means of assessing the impacts of localized decisions on the entire system

- Provides an effective tool for persuading decision makers to support a desired course of action

- Serves as an initial planning tool.

Throughput Accounting