At its most basic, a software requirement is a property that must be exhibited by something in order to solve some problem in the real world. It may aim to automate part of a task for someone to support the business processes of an organization, to correct shortcomings of existing software, or to control a device—to name just a few of the many problems for which software solutions are possible. The ways in which users, business processes, and devices function are typically complex. By extension, therefore, the requirements on particular software are typically a complex combination from various people at different levels of an organization, and who are in one way or another involved or connected with this feature from the environment in which the software will operate.
An essential property of all software requirements is that they be verifiable as an individual feature as a functional requirement or at the system level as a nonfunctional requirement. It may be difficult or costly to verify certain software requirements. For example, verification of the throughput requirement on a call center may necessitate the development of simulation software. Software requirements, software testing, and quality personnel must ensure that the requirements can be verified within available resource constraints.
Requirements have other attributes in addition to behavioral properties. Common examples include a priority rating to enable tradeoffs in the face of finite resources and a status value to enable project progress to be monitored. Typically, software requirements are uniquely identified so that they can be subjected to software configuration management over the entire life cycle of the feature and of the software.
Product and Process Requirements
A product requirement is a need or constraint on the software to be developed (for example, “The software shall verify that a student meets all prerequisites before he or she registers for a course”).
A process requirement is essentially a constraint on the development of the software (for example, “The software shall be developed using a RUP process”).
Some software requirements generate implicit process requirements. The choice of verification technique is one example. Another might be the use of particularly rigorous analysis techniques (such as formal specification methods) to reduce faults that can lead to inadequate reliability. Process requirements may also be imposed directly by the development organization, their customer, or a third party such as a safety regulator.
Functional and Nonfunctional Requirements
Functional requirements describe the functions that the software is to execute; for example, formatting some text or modulating a signal. They are sometimes known as capabilities or features. A functional requirement can also be described as one for which a finite set of test steps can be written to validate its behavior
Nonfunctional requirements are the ones that act to constrain the solution. Nonfunctional requirements are sometimes known as constraints or quality requirements. They can be further classified according to whether they are performance requirements, maintainability requirements, safety requirements, reliability requirements, security requirements, interoperability requirements or one of many other types of software requirements (see Models and Quality Characteristics in the Software Quality KA).
Emergent Properties
Some requirements represent emergent properties of software—that is, requirements that cannot be addressed by a single component but that depend on how all the software components interoperate. The throughput requirement for a call center would, for example, depend on how the telephone system, information system, and the operators all interacted under actual operating conditions. Emergent properties are crucially dependent on the system architecture.
Quantifiable Requirements
Software requirements should be stated as clearly and as unambiguously as possible, and, where appropriate, quantitatively. It is important to avoid vague and unverifiable requirements that depend for their interpretation on subjective judgment (“the software shall be reliable”; “the software shall be user-friendly”). This is particularly important for nonfunctional requirements. Two examples of quantified requirements are the following: a call center’s software must increase the center’s throughput by 20%; and a system shall have a probability of generating a fatal error during any hour of operation of less than 1 * 10−8 . The throughput requirement is at a very high level and will need to be used to derive a number of detailed requirements. The reliability requirement will tightly constrain the system architecture.
System Requirements and Software Requirements
In this topic, “system” means
an interacting combination of elements to accomplish a defined objective. These include hardware, software, firmware, people, information, techniques, facilities, services, and other support elements,
as defined by the International Council on Software and Systems Engineering (INCOSE)
System requirements are the requirements for the system as a whole. In a system containing software components, software requirements are derived from system requirements
This KA defines “user requirements” in a restricted way, as the requirements of the system’s customers or end users. System requirements, by contrast, encompass user requirements, requirements of other stakeholders (such as regulatory authorities), and requirements without an identifiable human source.
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Published on : 30-May-2018
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