CS302: Software Engineering

In systems engineering and software engineering, requirements analysis encompasses those tasks that go into determining the requirements of a new or altered system, taking account of the possibly conflicting requirements of the various stakeholders, such as users. Requirements analysis is critical to the success of a project.

Systematic requirements analysis is also known as requirements engineering. It is sometimes referred to loosely by names such as requirements gathering, requirements capture, or requirements specification. The term "requirements analysis" can also be applied specifically to the analysis proper (as opposed to elicitation or documentation of the requirements, for instance).

Requirements must be measurable, testable, related to identified business needs or opportunities, and defined to a level of detail sufficient for system design.

Figure 1. Requirements process

Definition of a Software Requirement

At its most basic, a software requirement is a property that must be exhibited in order to solve some problem in the real world. This session refers to requirements on "software" because it is concerned with problems to be addressed by software. Hence, a software requirement is a property that must be exhibited by software developed or adapted to solve a particular problem. The problem may be to automate part of a task of someone who will use the software, to support the business processes of the organization that has commissioned the software, to correct shortcomings of existing software, to control a device, and many more. The functioning of users, business processes, and devices is typically complex. By extension, therefore, the requirements on particular software are typically a complex combination of requirements from different people at different levels of an organization and from the environment in which the software will operate.

An essential property of all software requirements is that they be verifiable. It may be difficult or costly to verify certain software requirements. For example, verification of the throughput requirement on the call center may necessitate the development of simulation software. Both the software requirements and software quality personnel must ensure that the requirements can be verified within the available resource constraints.

Requirements have other attributes in addition to the behavioral properties that they express. Common examples include a priority rating to enable trade-offs 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 over the entire software life cycle.

Product and Process Requirements

A distinction can be drawn between product parameters and process parameters. Product parameters are requirements on 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 parameter is essentially a constraint on the development of the software (for example, "The software shall be written in Ada"). These are sometimes known as process requirements.

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, which 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 Non-functional 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 statements of services the system should provide, how the system should react to particular inputs, and how the system should behave in particular situations.

Nonfunctional requirements are the ones that act to constrain the solution. Nonfunctional requirements are sometimes known as constraints or quality requirements.

Figure 2. Nonfunctional Requirements

They can be further classified according to whether they are performance requirements, maintainability requirements, safety requirements, reliability requirements, or one of many other types of software requirements.

Emergent Properties

Some requirements represent emergent properties of software – that is, requirements which cannot be addressed by a single component, but which depend for their satisfaction 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 whose interpretation depends 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 Systems Engineering.

System requirements are the requirements for the system as a whole. In a system containing software components, software requirements are derived from system requirements.

The literature on requirements sometimes calls system requirements "user requirements." We can define "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.

Source: Hung Vo, https://cnx.org/contents/zx4yYVWR@1.1:Zvb38l8_@6/Requirements-analysis
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