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Requirements and Functional Decomposition

What is the difference between these two specifications?

1. Security: a team of 10 hackers [profiled elsewhere] per hour attempting to access account holders' credit card information shall be successful no more than an average of once every five years.
2. The system shall require users log in with a user name and password. On the third consecutive unsuccessful log-in attempt using a particular user name, the system will lock the corresponding account.

The first specification is a nonfunctional requirement. The second specification is a functional decomposition of that nonfunctional requirement.

All nonfunctional requirements can be decomposed into functional specifications.

In fact, when an interaction designer fleshes out (defines the particular steps in) a use case, she is functionally decomposing both functional and nonfunctional requirements. She is specifying functional steps that will satisfy the requirements.

Comments

Murray said…
I agree that the 1st specification is a nonfunctional requirement. However, I disagree with you about the 2nd specification.

The 2nd specification looks to me to be a specific design (not requirement) that is trying to satisfy the nonfunctional requirement in the 1st specification.
Roger L. Cauvin said…
Murray, I completely agree that the 2nd specification is design and not a requirement. In fact, the point of the 2nd specification was to show that, as a functional decomposition of a nonfunctional requirement, it is design.

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