The practical applications of traceability Part 1: What’s really going on when you decompose a requirement?
VijaySankar 270000E5JQ Comments (3) Visits (10147)
Jeremy Dick works as Principal Analyst for Integrate Systems Engineering Ltd in a consultancy, research and thought leadership capacity. He has extensive experience in implementing practical requirements processes in significant organizations, including tool customization, training and mentoring. At Integrate, he has been developing the concept of Evidence-based Development, an extension of his previous work on “rich traceability”. Prior to this appointment, he worked for 9 years in Telelogic (now part of IBM Rational) in the UK Professional Services group as both an international ambassador for Telelogic in the field of requirements management, and a high-level consultant for Telelogic customers wishing to implement requirements management processes. During this time, he developed considerable expertise in customizing DOORS using DXL to support advanced engineering processes. His roles in Telelogic included a position in the DOORS product division to assist in the transfer of field knowledge to the product team. Co-author of a book entitled “Requirements Engineering” that has recently reached its 3rd edition, he is recognized internationally for his work on traceability.Jeremy can be reached out at jere
It is a bit of a shock to find myself well into the fourth year on the same project! The nature of my work as a consultant means that it is rare for me to stick with a project beyond the initial phases of defining a requirements management process, establishing effective tool support and training the process enactors. But this time we have been able to stick with the requirements team supporting a large project long enough to see theory put into practice, and to see what it really means to apply the tools and techniques. We have gone well beyond just training, and find ourselves mentoring nearly 300 engineers in the application of DOORS for requirements capture, development and management. This has helped us keep our feet firmly on the ground – rubber on the road – and to walk with those who actually have to do the work.
So what have we learnt?
It is one thing to teach people how to write requirements statements that are clear, unambiguous, testable and traceable; it is quite another thing to help people understand how to take a requirement and develop it. None of the engineers we met on the project had previous experience of how to take a system requirement, for instance, and systematically decompose it through the design into sub-system and component requirements. We had to adapt our training and mentoring to address this skill.
Requirements decomposition establishes one of the essential requirements traceability relationships: how each layer of requirements contributes to the satisfaction of the layer above. This is often known as the satisfaction relationship, or as refinement in SysML. It is this relationship that connects the design to the development of requirements, and that lies at the heart of the ability to perform impact analysis.
Whatever layer you are engaged in – customer, system, sub-system or component – the same basic requirements development process can be applied. These are the process steps we teach for requirements development:
1. Collect and agree your requirements.
2. Design against your requirements.
3. Decompose the requirements to reflect the design.
4. Allocate the decomposed requirements.
The example below illustrates the end result of applying this process on a user requirement decomposed into system requirements. The rounded box contains the design rationale, and refers to a functional model of the product. (If you’ll forgive the shameless plug, such diagrams can be produced using a DOORS extension related to TraceLine. Ask me more if you are interested.)
The example just shown is a classic decomposition pattern. It is actually the decomposition of an overall performance into a combination of capacity and performance attributes of the product. We call this “decomposed”.
Other decomposition patterns are possible. Sometimes no decomposition is necessary, because the system requirement can be satisfied entirely by a single component or sub-system, as in the left-hand example below; or a constraint that will apply universally to all parts, as in the right-hand example. All that changes, perhaps, is the wording of the requirement to indicate the new target. We call this “direct flow”.
You will reach a point in the cascade of decomposed requirements when a requirement is satisfied entirely within the current area without the need to decompose further, as illustrated in the following. In this case, it is important to state the rationale for not flowing the requirement onwards, as otherwise it may be construed as a traceability gap. We call this “not developed further”.
I have seen a number of organisations that capture this flow-down type – Decomposed/Direct Flow/Not developed further – as an attribute of the requirement. We do this because it allows us to cross-check certain things: if you have marked a requirement as “Decomposed” but have not decomposed it, then that does indicate a design gap. However, if you mark the requirement as “Not developed further”, that gives you permission not to trace it further, i.e. not a design gap. (But you would do well to provide rationale!)
As requirements flow down through the layers, the complexity of the design becomes evident in the shape of the requirements graph. In general, satisfaction is a many-to-may relationship between requirements, and the figure below shows how this may be manifest. As the requirements are decomposed, they are refactored through the design.
Some patterns are questionable. Take these for instance:
Why decompose something into three requirements only to reduce them to a single one again? Or why collapse three requirements into one only to re-expand them in the next layer?
These patterns are not necessarily wrong, but they should be targeted for careful review.
So this is what we now teach those engaged in requirements decomposition, flow-down and traceability. It is wrong to assume that people will somehow automatically know how to do this kind of thing. By taking this approach, the flow-down of requirements reflects the design, and a clear satisfaction relationship is expressed in the traceability.