Page 178 - Software and Systems Requirements Engineering in Practice

144 S o f t w a r e & S y s t e m s R e q u i r e m e n t s E n g i n e e r i n g : I n P r a c t i c e

• Response measure No new source code, no extraordinary
downtime, and commencing operation within one calendar
week
Note the quality attribute, measure, and requirement implied by
this scenario:
• The quality attribute is “configurability to accommodate new
sensors.”
• The measure is “the amount of new source code written, the
amount of downtime, and the amount of calendar time to
bring a new sensor online.”
• The requirement is “zero new source code, no extra downtime,
and less than one calendar week.”
Also, notice how the QAS nails down some details that an
unstructured scenario might have left open:
• Since no new source code is permitted, there must be a limit on
the range of new sensor types that can be handled. With enough
programming, any type of sensor could have been handled.
• Shutting the system down to reconfigure it is probably not an
option, because that would require extraordinary downtime.
• Reconfiguration will be done by an expert, not a novice.
• The expert is part of the installation organization, not the
customer organization.
But the most important aspect of the scenario is that it gives a
concrete example of configurability, which is easy for both the
stakeholder and the architecture team to understand.
When eliciting QASs, it is helpful to consider the following types
of scenarios, as a way of bringing out issues that might not have been
considered:

• Normal operations These are the most obvious scenarios.
• System-as-object scenarios In these, the system is a passive
object that is being manipulated by, say, a programmer or an
installer.
• Growth scenarios These scenarios deal with likely or
plausible changes to the requirements in the future, such as a
50 percent increase in capacity requirements. They help
develop a system that is (somewhat) future-proof.
• Exploratory scenarios These are improbable scenarios, such
as the loss of power from an “uninterruptible” power supply.