Definition of Constraint

Definition Last Updated 17-Dec-2015 12:51

A constraint is a criterion associated with a rule that places limits of one of two types on the value of an associated characteristic (observable or tag); the nature of the enclosing rule determines whether the constraint is normative or declarative as follows:

  • normative, i.e. the rule contains a word or phrase1And sometimes people say cannot when they mean may not, which is not the end of the world except when they say they cannot do something they obviously should be able to do as though being asked to break a fundamental law of physics rather than apply a quantum of common sense. If, for them, the rule is fundamental, it probably would be the end of their world if they broke it. Which is sad. such as “shall” or “must not” to say, typically, what conditions can (not), must (not) or should (not) obtain, happen or be done when the criterion is satisfied (true), or
  • declarative, or descriptive, i.e. the rule simply states the fact that a certain characteristic cannot or does not have particular values

Constraint is therefore a secondary concept; criteria are fundamental.

For examples, see the main article.

Scope

Constraint is a defined term of Enterprise Architecture. Constraint is a defined term of Business Analysis.

Discussion

Article Last Updated 17-Dec-2015 12:51

Examples of Declarative or Descriptive Constraints

As an example of a declarative rule consider, “A is taller than B means the same as B is shorter than A.”

Non-trivial declarative constraints should be traceable to standards and the policies that determine their applicability.

As an example of a descriptive rule consider, “The maximum mass of a neutron start is about 2.5 times the mass of the sun2For a serious discussion of this fascinating topic, see e.g. Chamel, N., Haensel, P., Zdunik, J. L. and Fantina, A. F. 2013 On the Maximum Mass of Neutron Starsin the arXiv.”, i.e. the mass of a neutron star is constrained by the laws of physics to be less than this limit. If mass is added to a neutron star whose mass is already maximal it will cease to be a neutron start and will become a black hole.

Descriptive constraints should be traceable to evidential or logical sources.

Example of Normative Constraint

Contrast the above with the statement, “The total power consumption of the data centre cannot exceed 100kW.” This might be a factual description of the data centre’s power supply (e.g. the centre is off-grid and the power supplied from batteries whose theoretical maximum power delivery is 100kW) or it might be a normative constraint on the connection of equipment to the centre’s power supply on the basis that, if the power consumption exceeds 100kW the main circuit breaker will operate (e.g. to prevent a power cable catching fire) and the data centre will cease to operate, which is a bad thing and a disbenefit to be avoided.

Normative constraints should be directly traceable to policies.

Constraints and Requirements

The use of constraints is particularly important in specifying requirements.

Reimbursement of business expenses is usually subject to (normative) constraints: the finance department must not reimburse staff more than €25 for any one meal.

Individual items may be subject to implied constraints through constraints on groups of which they are a part.

For example, the addition of new items of equipment to the data centre may be subject to the constraint that “The total power consumption of the data centre must be less than 250kW at all times.” If a new item is to be added to the data centre, and the forecast total peak power consumption at that time is 249kW, them the item is subject to the implied constraint that its peak power consumption must be less than 1kW.

Related Entries

Condition, Criterion, Requirement, Requirements Analysis, Requirements Development

Notes   [ + ]

1.And sometimes people say cannot when they mean may not, which is not the end of the world except when they say they cannot do something they obviously should be able to do as though being asked to break a fundamental law of physics rather than apply a quantum of common sense. If, for them, the rule is fundamental, it probably would be the end of their world if they broke it. Which is sad.
2.For a serious discussion of this fascinating topic, see e.g. Chamel, N., Haensel, P., Zdunik, J. L. and Fantina, A. F. 2013 On the Maximum Mass of Neutron Starsin the arXiv

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