It's been said that one cannot manage what one cannot measure.
This applies especially to the so-called "non-functional requirements" that are the traditional meat of software architecture. The questions to ask are simple. They include: how many? in what period? how often? how soon? increasing or decreasing? at what rate? If these cannot be answered then the business need is not understood. In many cases, the answers to these questions should be in the business case for the system being proposed and if they are not, then some hard thinking needs to be done.
Too often, vague adjectives are allowed to stand as statements of intent for a new system: "flexible", "maintainable" and the rest. In every case (yes, even "usable") the phenomena can be quantified, measured, and thresholds set. If this is not done, then there can be no basis for acceptance of the system by its users, no guidance for its builders as they work, no vision for those architecting it.
The criteria must always be given as a range: the least possibly acceptable, the nominal, the most conceivable. If this range cannot be given, then the required system behavior is not understood. As the architecture of the system unfolds, it can be checked against these criteria to see if it is (still) in tolerance. As the performance against some criteria drifts, valuable feedback about the architecture is obtained.
Putting these ranges in place, and checking against them, is a time-consuming and expensive business. If no-one cares enough about the system being "performant" to pay for actual performance trials, then there is a good chance that it doesn't need to be after all.
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