Tuesday, July 28, 2009

Refurbished: Non-Functional Requirements and Levels of Specification

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After an interesting and insightful discussion with Tom Gilb and Sven Biedermann about one of my latest PlanetProject articles I decided to work it over. It is a how-to for a good requirements hierarchy. A good requirements hierarchy is an important prerequisite to a more-conscious and logical design or architecture process. This is because real requirements drive design, not designs in requirements clothes. (Thanks Tom for yet another clarification!)

It seems, in trying to write as short as possible I was kind of swept away from good writing practice and got sloppy with wording. I also found out that there is a philosophical, hence essential difference in how the process of 'requirements decomposition' can be seen.
  1. One school of thought describes requirements decomposition as a process to help us select and evaluate appropriate designs.
  2. The other school describes requirements decomposition as being a form of the design process.
Personally, I subscribe to the second meaning, because of a belief of mine: mankind is very used to solution-thinking, but very new to problem-thinking (Darwin weights in). So most forms of thinking, including requirements decomposition, are outputs of a solution-finding or design process. Or, as Sven put it, requirements decompostion is a shortcut to designing, where 'designing' takes on the meaning suggested by number 1 above.

However, it can be very useful to assume there actually is an essential and clear destinction between requirements decomposition and design processes. The point here is: you need to define it that way, then all is well :-) I didn't define it properly, and thus gave rise to many arguments written and exchanged.

Anyway, I hope the article now sheds even more light on the the constant quarrel about so called 'nonfunctional requirements', i. e. what they are, what they are for, why they are so sparse in the common requirement specification documents.

Most requirement specification document I see these days have lots of required functions, and few (if any) requirements for other attributes of the system, like all the -illities. AND many analysts (including me from time to time) confuse non-functional with scalar. Read the article on PlanetProject to learn more.

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