A functional requirement is typically either satisfied or not satisfied, with nothing in between. These include requirements related to the - suitability, reliability, usability, interoperability, verifiability, …. Nonfunctional requirements are typically not either satisfied or not satisfied, unlike functional requirements ; instead, they are satisfied to a greater or lesser degree. Example : A system isn't typically either usable, or not usable, but may be more usable or less usable than some other system. Examples Provide safe, reliable, and quick transportation around campus functional Print a receipt for every transaction functional Be self-explanatory and easy to use. These requirements are satisfied or not by what the system does when it runs. These requirements are satisfied or not by the actions of the people responsible for producing a system. Evolvability, understandability, and testability are developmental quality goals ; but not usability, reliability, robustness, which are non-functional but behavioral. Examples Turn out the lights when the room is unoccupied behavioral Be thoroughly testable so that customers will have a high level of confidence. There are arguably behavioral requirements such as, perhaps, Secretly provide access to internal program state needed to make testing more effective that types of requirements in software engineering more closely related to a nonfunctional requirements such as testability than to any ordinary functional requirements. It seems that every developmental-quality requirement would also be a non-functional requirement, and any functional requirement would also be a behavioral requirement I have not come up with counterexamples, in any case. Partitioning requirements by their form Goals A is something that is desired but that might or might not be achieved, depending on circumstances. Properties A requirement is often assumed to be a property that is expected to be true of the system : The system shall …. The traditional requirements document is a thick book of such properties ; such requirements documents rarely have coffee stains. Such requirements are explicit properties that are expressed in isolation, which means that their intended context is implicit. The property is specifically stated, but the situation in which it applies and its significance in that situation are not stated. It is traditional to use the verb shall for property requirements. An example set of property requirements : thefor a phone message system used in industry to prototype and explore potential new features. Models over time In this form, the requirements as a group are the properties possessed by a model. The system is supposed to behave like the model. Example : a Statechart or other finite state machine. The desired system properties are left implicit in this form of requirements. It is not possible to distinguish which properties of the model are essential properties of the system, and which are accidental properties of the particular model that was chosen. Models are most frequently used to express behaviors that unfold over time. If a model is formal, its behaviors can be explored or verified using software tools. Narratives A narrative about the system can express what is expected from it in a particular situation. Narratives have the advantage of providing context and motivation, and showing how different expectations are related to each other. People are very good at expressing and understanding narratives, and can pick up on a wide variety of implicit or hinted information in a narrative. These relationships are often not understood except by a use case's authors. Regardless of the form in which requirements are expressed, people seem to end up telling, writing, and asking for narratives that motivate, explain, and illustrate what the requirements say. Tables Tabular forms organize requirements often property requirements into tables that show how the individual requirements are related, make it easier to find the requirement you are looking for, and simplify checking the requirements for completeness. The classic example of tabular requirements is the. Audiences Different people want different things from requirements. They use the requirements to estimate the cost, time, and other resources needed to produce the required system. They use the requirements to guide their work. Sources Requirements come types of requirements in software engineering many sources. They provide requirements that they hope will guide the developers to produce what the stakeholders need and want. Experience shows that requirements are the biggest software engineering problem for developers of large, complex systems. The purpose of this tutorial is to help the reader understand why requirements are so difficult to do well, where the state of the art does and does not address current development problems, the strengths and weaknesses of different approaches to requirements, and what help we can expect from ongoing technical developments. Focus of the tutorial is on providing the reader with an understanding of the underlying issues in requirements analysis and specification. It describes the different facets of the requirements problem from the points of view of the many parties involved in system development including customers, contractors, management, regulators, and developers. It discusses the goals of the requirements phase and the problems that can arise in achieving those goals. It describes the characteristics of a disciplined software engineering process and how such a process helps address many of the problems in requirements. It compares a variety of published approaches relative to the goals of a disciplined process. Finally it examines technical trends, including recent work at the Naval Research Laboratory, and discusses where significant advances are likely in the future. Xu+Ziv+2006-apnf We address the research question of transforming dependability requirements into corresponding software architecture constructs, by proposing first that dependability needs can be classified into three types of requirements and second, an architectural pattern that allows requirements engineers and architects to map the three types of dependability requirements into three corresponding types of architectural components. The proposed pattern is general enough to work with existing requirements techniques and existing software architectural styles, including enterprise and product-line architectures.
