Chapter 1: Software Requirements

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Confidentiality, Integrity, and Availability
Directed Acyclic Graph
Functional Size Measurement
International Council on Systems Engineering
Unified Modeling Language
Systems Modeling Language

The Software Requirements knowledge area (KA) is concerned with the elicitation, analysis, specification, and validation of software requirements as well as the management of requirements during the whole life cycle of the software product. It is widely acknowledged amongst researchers and industry practitioners that software projects are critically vulnerable when the requirements-related activities are poorly performed.

Software requirements express the needs and constraints placed on a software product that contribute to the solution of some real-world problem.

The term “requirements engineering” is widely used in the field to denote the systematic handling of requirements. For reasons of consistency, the term “engineering” will not be used in this KA other than for software engineering per se. For the same reason, “requirements engineer,” a term which appears in some of the literature, will not be used either. Instead, the term “software engineer” or, in some specific cases, “requirements specialist” will be used, the latter where the role in question is usually performed by an individual other than a software engineer. This does not imply, however, that a software engineer could not perform the function.

A risk inherent in the proposed breakdown is that a waterfall-like process may be inferred. To guard against this, topic 2, Requirements Process, is designed to provide a high-level overview of the requirements process by setting out the resources and constraints under which the process operates and which act to configure it.

An alternate decomposition could use a product-based structure (system requirements, software requirements, prototypes, use cases, and so on). The process-based breakdown reflects the fact that the requirements process, if it is to be successful, must be considered as a process involving complex, tightly coupled activities (both sequential and concurrent), rather than as a discrete, one-off activity performed at the outset of a software development project.

The Software Requirements KA is related closely to the Software Design, Software Testing, Software Maintenance, Software Configuration Management, Software Engineering Management, Software Engineering Process, Software Engineering Models and Methods, and Software Quality KAs.

Breakdown of Topics for Software Requirements

The breakdown of topics for the Software Requirements KA is shown in Figure 1.1.

1 Software Requirements Fundamentals

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1.1 Definition of a Software Requirement

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.

Figure 1.1: Breakdown of Topics for the Software Requirements KA

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.

1.2 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.

1.3 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).

1.4 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.

1.5 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.

1.6 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) [3].

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.

2 Requirements Process

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This section introduces the software requirements process, orienting the remaining five topics and showing how the requirements process dovetails with the overall software engineering process.

2.1 Process Models

The objective of this topic is to provide an under- standing that the requirements process

  • is not a discrete front-end activity of the software life cycle, but rather a process initiated at the beginning of a project that continues to be refined throughout the life cycle;
  • identifies software requirements as configuration items and manages them using the same software configuration management practices as other products of the software life cycle processes;
  • needs to be adapted to the organization and project context.

In particular, the topic is concerned with how the activities of elicitation, analysis, specification, and validation are configured for different types of projects and constraints. The topic also includes activities that provide input into the requirements process, such as marketing and feasibility studies.

2.2 Process Actors

This topic introduces the roles of the people who participate in the requirements process. This process is fundamentally interdisciplinary, and the requirements specialist needs to mediate between the domain of the stakeholder and that of software engineering. There are often many people involved besides the requirements specialist, each of whom has a stake in the software. The stakeholders will vary across projects, but will always include users/operators and customers (who need not be the same).

Typical examples of software stakeholders include (but are not restricted to) the following:

  • Users: This group comprises those who will operate the software. It is often a heterogeneous group involving people with different roles and requirements.
  • Customers: This group comprises those who have commissioned the software or who represent the software’s target market.
  • Market analysts: A mass-market product will not have a commissioning customer, so marketing people are often needed to establish what the market needs and to act as proxy customers.
  • Regulators: Many application domains, such as banking and public transport, are regulated. Software in these domains must comply with the requirements of the regulatory authorities.
  • Software engineers: These individuals have a legitimate interest in profiting from developing the software by, for example, reusing components in or from other products. If, in this scenario, a customer of a particular product has specific requirements that compromise the potential for component reuse, the software engineers must carefully weigh their own stake against those of the

customer. Specific requirements, particularly constraints, may have major impact on project cost or delivery because they either fit well or poorly with the skill set of the engineers. Important tradeoffs among such requirements should be identified.

It will not be possible to perfectly satisfy the requirements of every stakeholder, and it is the software engineer’s job to negotiate tradeoffs that are both acceptable to the principal stakeholders and within budgetary, technical, regulatory, and other constraints. A prerequisite for this is that all the stakeholders be identified, the nature of their “stake” analyzed, and their requirements elicited.

2.3 Process Support and Management

This section introduces the project management resources required and consumed by the requirements process. It establishes the context for the first topic (Initiation and Scope Definition) of the Software Engineering Management KA. Its principal purpose is to make the link between the pro- cess activities identified in 2.1 and the issues of cost, human resources, training, and tools.

2.4 Process Quality and Improvement

This topic is concerned with the assessment of the quality and improvement of the requirements process. Its purpose is to emphasize the key role the requirements process plays in terms of the cost and timeliness of a software product and of the customer’s satisfaction with it. It will help to orient the requirements process with quality standards and process improvement models for software and systems. Process quality and improvement is closely related to both the Software Quality KA and Software Engineering Process KA, comprising

  • requirements process coverage by process improvement standards and models;
  • requirements process measures and benchmarking;
  • improvement planning and implementation;
  • security/CIA improvement/planning and implementation

3 Requirements Elicitation

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Requirements elicitation is concerned with the origins of software requirements and how the software engineer can collect them. It is the first stage in building an understanding of the problem the software is required to solve. It is fundamentally a human activity and is where the stakehold- ers are identified and relationships established between the development team and the customer. It is variously termed “requirements capture,” “requirements discovery,” and “requirements acquisition.”

One of the fundamental principles of a good requirements elicitation process is that of effective communication between the various stake- holders. This communication continues through the entire Software Development Life Cycle (SDLC) process with different stakeholders at different points in time. Before development begins, requirements specialists may form the conduit for this communication. They must mediate between the domain of the software users (and other stakeholders) and the technical world of the software engineer. A set of internally consistent models at different levels of abstraction facilitate communications between software users/stake holders and software engineers.

A critical element of requirements elicitation is informing the project scope. This involves provid- ing a description of the software being specified and its purpose and prioritizing the deliverables to ensure the customer’s most important business needs are satisfied first. This minimizes the risk of requirements specialists spending time eliciting requirements that are of low importance, or those that turn out to be no longer relevant when the software is delivered. On the other hand, the description must be scalable and extensible to accept further requirements not expressed in the first formal lists and compatible with the previous ones as contemplated in recursive methods.

3.1 Requirements Sources

Requirements have many sources in typical software, and it is essential that all potential sources be identified and evaluated. This topic is designed to promote awareness of the various sources of software requirements and of the frameworks for managing them. The main points covered are as follows:

  • Goals. The term “goal” (sometimes called “business concern” or “critical success factor”) refers to the overall, high-level objectives of the software. Goals provide the motivation for the software but are often vaguely formulated. Software engineers need to pay particular attention to assessing the value (relative to priority) and cost of goals. A feasibility study is a relatively low-cost way of doing this
  • Domain knowledge. The software engineer needs to acquire or have available knowledge about the application domain. Domain knowledge provides the background against which all elicited requirements knowledge must be set in order to understand it. It’s a good practice to emulate an ontological approach in the knowledge domain. Relations between relevant concepts within the application domain should be identified.
  • Stakeholders (see section 2.2, Process Actors). Much software has proved unsatisfactory because it has stressed the requirements of one group of stakeholders at the expense of others. Hence, the delivered software is difficult to use, or subverts the cultural or political structures of the customer organization. The software engineer needs to identify, represent, and manage the “viewpoints” of many different types of stakeholders.
  • Business rules. These are statements that define or constrain some aspect of the structure or the behavior of the business itself. “A student cannot register in next semester’s courses if there remain some unpaid tuition fees” would be an example of a business rule that would be a requirement source for a university’s course-registration software.
  • The operational environment. Requirements will be derived from the environment in which the software will be executed. These may be, for example, timing constraints in real-time software or performance constraints in a business environment. These must be sought out actively because they can greatly affect software feasibility and cost as well as restrict design choices.
  • The organizational environment. Software is often required to support a business process, the selection of which may be conditioned by the structure, culture, and internal politics of the organization. The software engineer needs to be sensitive to these since, in general, new software should not force unplanned change on the business process.

3.2 Elicitation Techniques

4 Requirements Analysis

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This topic is concerned with the process of analyzing requirements to

  • detect and resolve conflicts between requirements;
  • discover the bounds of the software and how it must interact with its organizational and operational environment;
  • elaborate system requirements to derive software requirements.

The traditional view of requirements analysis has been that it be reduced to conceptual modeling using one of a number of analysis methods, such as the structured analysis method. While conceptual modeling is important, we include the classification of requirements to help inform tradeoffs between requirements (requirements classification) and the process of establishing these tradeoffs (requirements negotiation).

Care must be taken to describe requirements precisely enough to enable the requirements to be validated, their implementation to be verified, and their costs to be estimated.

4.1 Requirements Classification

Requirements can be classified on a number of dimensions. Examples include the following:

  • Whether the requirement is functional or nonfunctional (see section 1.3, Functional and Nonfunctional Requirements).
  • Whether the requirement is derived from one or more high-level requirements or an emergent property (see section 1.4, Emergent Properties), or is being imposed directly on the software by a stakeholder or some other source.
  • Whether the requirement is on the product or the process (see section 1.2, Product and Process Requirements). Requirements on the process can constrain the choice of contractor, the software engineering process to be adopted, or the standards to be adhered to.
  • The requirement priority. The higher the priority, the more essential the requirement is for meeting the overall goals of the software. Often classified on a fixed-point scale such as mandatory, highly desirable, desirable, or optional, the priority often has to be balanced against the cost of development and implementation.
  • The scope of the requirement. Scope refers to the extent to which a requirement affects the software and software components. Some requirements, particularly certain nonfunctional ones, have a global scope in that their satisfaction cannot be allocated to a discrete component. Hence, a requirement with global scope may strongly affect the software architecture and the design of many components, whereas one with a narrow scope may offer a number of design choices and have little impact on the satisfaction of other requirements

4.2 Conceptual Modeling

The development of models of a real-world problem is key to software requirements analysis. Their purpose is to aid in understanding the situation in which the problem occurs, as well as depicting a solution. Hence, conceptual models comprise models of entities from the problem domain, configured to reflect their real-world relationships and dependencies. This topic is closely related to the Software Engineering Models and Methods KA.

Several kinds of models can be developed. These include use case diagrams, data flow models, state models, goal-based models, user interactions, object models, data models, and many others. Many of these modeling notations are part of the Unified Modeling Language (UML). Use case diagrams, for example, are routinely used to depict scenarios where the boundary separates the actors (users or systems in the external environment) from the internal behavior where each use case depicts a functionality of the system.

The factors that influence the choice of modeling notation include these:

  • The nature of the problem. Some types of software demand that certain aspects be analyzed particularly rigorously. For example, state and parametric models, which are part of SysML [4], are likely to be more important for real-time software than for information systems, while it would usually be the opposite for object and activity models.
  • The expertise of the software engineer. It is often more productive to adopt a modeling notation or method with which the software engineer has experience.
  • The process requirements of the customer (see section 1.2, Product and Process Requirements). Customers may impose their favored notation or method or prohibit any with which they are unfamiliar. This factor can conflict with the previous factor.

Note that, in almost all cases, it is useful to start by building a model of the software context. The software context provides a connection between the intended software and its external environment.

This is crucial to understanding the software’s context in its operational environment and to identifying its interfaces with the environment.

This subtopic does not seek to “teach” a particular modeling style or notation but rather provides guidance on the purpose and intent of modeling.

4.3 Architectural Design and Requirements Allocation

At some point, the solution architecture must be derived. Architectural design is the point at which the requirements process overlaps with software or systems design and illustrates how impossible it is to cleanly decouple the two tasks. This topic is closely related to Software Structure and Architecture in the Software Design KA. In many cases, the software engineer acts as software architect because the process of analyzing and elaborating the requirements demands that the architecture/design components that will be responsible for satisfying the requirements be identified. This is requirements allocation–the assignment to architecture components responsible for satisfying the requirements.

Allocation is important to permit detailed analysis of requirements. Hence, for example, once a set of requirements has been allocated to a component, the individual requirements can be further analyzed to discover further requirements on how the component needs to interact with other components in order to satisfy the allocated requirements. In large projects, allocation stimulates a new round of analysis for each subsystem. As an example, requirements for a particular braking performance for a car (braking distance, safety in poor driving conditions, smoothness of application, pedal pressure required, and so on) may be allocated to the braking hardware (mechanical and hydraulic assemblies) and an antilock braking system (ABS). Only when a requirement for an antilock braking system has been identified, and the requirements allocated to it, can the capabilities of the ABS, the braking hardware, and emergent properties (such as car weight) be used to identify the detailed ABS software requirements.

Architectural design is closely identified with conceptual modeling (see section 4.2, Conceptual Modeling).

4.4 Requirements Negotiation

Another term commonly used for this subtopic is “conflict resolution.” This concerns resolving problems with requirements where conflicts occur between two stakeholders requiring mutually incompatible features, between requirements and resources, or between functional and nonfunctional requirements, for example. In most cases, it is unwise for the software engineer to make a unilateral decision, so it becomes necessary to consult with the stakeholder(s) to reach a consensus on an appropriate tradeoff. It is often important, for contractual reasons, that such decisions be traceable back to the customer. We have classified this as a software requirements analysis topic because problems emerge as the result of analysis. However, a strong case can also be made for considering it a requirements validation topic (see topic 6, Requirements Validation).

Requirements prioritization is necessary, not only as a means to filter important requirements, but also in order to resolve conflicts and plan for staged deliveries, which means making complex decisions that require detailed domain knowledge and good estimation skills. However, it is often difficult to get real information that can act as a basis for such decisions. In addition, requirements often depend on each other, and priorities are relative. In practice, software engineers perform requirements prioritization frequently without knowing about all the requirements. Requirements prioritization may follow a costvalue approach that involves an analysis from the stakeholders defining in a scale the benefits or the aggregated value that the implementation of the requirement brings them, versus the penalties of not having implemented a particular requirement. It also involves an analysis from the software engineers estimating in a scale the cost of implementing each requirement, relative to other requirements. Another requirements prioritization approach called the analytic hierarchy process involves comparing all unique pairs of requirements to determine which of the two is of higher priority, and to what extent.

4.5 Formal Analysis

Formal analysis concerns not only topic 4, but also sections 5.3 and 6.3. This topic is also related to Formal Methods in the Software Engineering Models and Methods Knowledge Area.

Formal analysis has made an impact on some application domains, particularly those of highintegrity systems. The formal expression of requirements requires a language with formally defined semantics. The use of a formal analysis for requirements expression has two benefits. First, it enables requirements expressed in the language to be specified precisely and unambiguously, thus (in principle) avoiding the potential for misinterpretation. Secondly, requirements can be reasoned over, permitting desired properties of the specified software to be proven. Formal reasoning requires tool support to be practicable for anything other than trivial systems, and tools generally fall into two types: theorem provers or model checkers. In neither case can proof be fully automated, and the level of competence in formal reasoning needed in order to use the tools restricts the wider application of formal analysis.

Most formal analysis is focused on relatively late stages of requirements analysis. It is generally counterproductive to apply formalization until the business goals and user requirements have come into sharp focus through means such as those described elsewhere in section 4. However, once the requirements have stabilized and have been elaborated to specify concrete properties of the software, it may be beneficial to formalize at least the critical requirements. This permits static validation that the software specified by the requirements does indeed have the properties (for example, absence of deadlock) that the customer, users, and software engineer expect it to have.

5 Requirements Specification

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For most engineering professions, the term “specification” refers to the assignment of numerical values or limits to a product’s design goals. In software engineering, “software requirements specification” typically refers to the production of a document that can be systematically reviewed, evaluated, and approved. For complex systems, particularly those involving substantial nonsoftware components, as many as three different types of documents are produced: system definition, system requirements, and software requirements. For simple software products, only the third of these is required. All three documents are described here, with the understanding that they may be combined as appropriate. A description of systems engineering can be found in the Related Disciplines of Software Engineering chapter of this Guide.

5.1 System Definition Document

This document (sometimes known as the user requirements document or concept of operations document) records the system requirements. It defines the high-level system requirements from the domain perspective. Its readership includes representatives of the system users/customers (marketing may play these roles for marketdriven software), so its content must be couched in terms of the domain. The document lists the system requirements along with background information about the overall objectives for the system, its target environment, and a statement of the constraints, assumptions, and nonfunctional requirements. It may include conceptual models designed to illustrate the system context, usage scenarios, and the principal domain entities, as well as workflows.

5.2 System Requirements Specification

Developers of systems with substantial software and nonsoftware components—a modern airliner, for example—often separate the description of system requirements from the description of software requirements. In this view, system requirements are specified, the software requirements are derived from the system requirements, and then the requirements for the software components are specified. Strictly speaking, system requirements specification is a systems engineering activity and falls outside the scope of this Guide.

5.3 Software Requirements Specification

Software requirements specification establishes the basis for agreement between customers and contractors or suppliers (in market-driven projects, these roles may be played by the marketing and development divisions) on what the software product is to do as well as what it is not expected to do.

Software requirements specification permits a rigorous assessment of requirements before design can begin and reduces later redesign. It should also provide a realistic basis for estimating product costs, risks, and schedules.

Organizations can also use a software requirements specification document as the basis for developing effective verification and validation plans.

Software requirements specification provides an informed basis for transferring a software product to new users or software platforms. Finally, it can provide a basis for software enhancement.

Software requirements are often written in natural language, but, in software requirements specification, this may be supplemented by formal or semiformal descriptions. Selection of appropriate notations permits particular requirements and aspects of the software architecture to be described more precisely and concisely than natural language. The general rule is that notations should be used that allow the requirements to be described as precisely as possible. This is particularly crucial for safety-critical, regulatory, and certain other types of dependable software. However, the choice of notation is often constrained by the training, skills, and preferences of the document’s authors and readers.

A number of quality indicators have been developed that can be used to relate the quality of software requirements specification to other project variables such as cost, acceptance, performance, schedule, and reproducibility. Quality indicators for individual software requirements specification statements include imperatives, directives, weak phrases, options, and continuances. Indicators for the entire software requirements specification document include size, readability, specification, depth, and text structure.

6 Requirements Validation

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The requirements documents may be subject to validation and verification procedures. The requirements may be validated to ensure that the software engineer has understood the requirements; it is also important to verify that a requirements document conforms to company standards and that it is understandable, consistent, and complete. In cases where documented company standards or terminology are inconsistent with widely accepted standards, a mapping between the two should be agreed on and appended to the document.

Formal notations offer the important advantage of permitting the last two properties to be proven (in a restricted sense, at least). Different stakeholders, including representatives of the customer and developer, should review the document(s). Requirements documents are subject to the same configuration management practices as the other deliverables of the software life cycle processes. When practical, the individual requirements are also subject to configuration management, generally using a requirements management tool (see topic 8, Software Requirements Tools).

It is normal to explicitly schedule one or more points in the requirements process where the requirements are validated. The aim is to pick up any problems before resources are committed to addressing the requirements. Requirements validation is concerned with the process of examining the requirements document to ensure that it defines the right software (that is, the software that the users expect).

6.1 Requirements Reviews

Perhaps the most common means of validation is by inspection or reviews of the requirements document(s). A group of reviewers is assigned a brief to look for errors, mistaken assumptions, lack of clarity, and deviation from standard practice. The composition of the group that conducts the review is important (at least one representative of the customer should be included for a customer-driven project, for example), and it may help to provide guidance on what to look for in the form of checklists.

Reviews may be constituted on completion of the system definition document, the system specification document, the software requirements specification document, the baseline specification for a new release, or at any other step in the process.

6.2 Prototyping

Prototyping is commonly a means for validating the software engineer’s interpretation of the software requirements, as well as for eliciting new requirements. As with elicitation, there is a range of prototyping techniques and a number of points in the process where prototype validation may be appropriate. The advantage of prototypes is that they can make it easier to interpret the software engineer’s assumptions and, where needed, give useful feedback on why they are wrong. For example, the dynamic behavior of a user interface can be better understood through an animated prototype than through textual description or graphical models. The volatility of a requirement that is defined after prototyping has been done is extremely low because there is agreement between the stakeholder and the software engineer— therefore, for safety-critical and crucial features prototyping would really help. There are also disadvantages, however. These include the danger of users’ attention being distracted from the core underlying functionality by cosmetic issues or quality problems with the prototype. For this reason, some advocate prototypes that avoid software, such as flip-chart-based mockups. Prototypes may be costly to develop. However, if they avoid the wastage of resources caused by trying to satisfy erroneous requirements, their cost can be more easily justified. Early prototypes may contain aspects of the final solution. Prototypes may be evolutionary as opposed to throwaway.

6.3 Model Validation

It is typically necessary to validate the quality of the models developed during analysis. For example, in object models, it is useful to perform a static analysis to verify that communication paths exist between objects that, in the stakeholders’ domain, exchange data. If formal analysis notations are used, it is possible to use formal reasoning to prove specification properties. This topic is closely related to the Software Engineering Models and Methods KA.

6.4 Acceptance Tests

An essential property of a software requirement is that it should be possible to validate that the finished product satisfies it. Requirements that cannot be validated are really just “wishes.” An important task is therefore planning how to verify each requirement. In most cases, designing acceptance tests does this for how end-users typically conduct business using the system.

Identifying and designing acceptance tests may be difficult for nonfunctional requirements (see section 1.3, Functional and Nonfunctional Requirements). To be validated, they must first be analyzed and decomposed to the point where they can be expressed quantitatively.

Additional information can be found in Acceptance/ Qualification/Conformance Testing in the Software Testing KA.

7 Practical Considerations

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The first level of topic decomposition presented in this KA may seem to describe a linear sequence of activities. This is a simplified view of the process.

The requirements process spans the whole software life cycle. Change management and the maintenance of the requirements in a state that accurately mirrors the software to be built, or that has been built, are key to the success of the software engineering process.

Not every organization has a culture of documenting and managing requirements. It is common in dynamic start-up companies, driven by a strong “product vision” and limited resources, to view requirements documentation as unnecessary overhead. Most often, however, as these companies expand, as their customer base grows, and as their product starts to evolve, they discover that they need to recover the requirements that motivated product features in order to assess the impact of proposed changes. Hence, requirements documentation and change management are key to the success of any requirements process.

7.1 Iterative Nature of the Requirements Process

There is general pressure in the software industry for ever shorter development cycles, and this is particularly pronounced in highly competitive, market-driven sectors. Moreover, most projects are constrained in some way by their environment, and many are upgrades to, or revisions of, existing software where the architecture is a given. In practice, therefore, it is almost always impractical to implement the requirements process as a linear, deterministic process in which software requirements are elicited from the stakeholders, baselined, allocated, and handed over to the software development team. It is certainly a myth that the requirements for large software projects are ever perfectly understood or perfectly specified.

Instead, requirements typically iterate towards a level of quality and detail that is sufficient to permit design and procurement decisions to be made. In some projects, this may result in the requirements being baselined before all their properties are fully understood. This risks expensive rework if problems emerge late in the software engineering process. However, software engineers are necessarily constrained by project management plans and must therefore take steps to ensure that the “quality” of the requirements is as high as possible given the available resources. They should, for example, make explicit any assumptions that underpin the requirements as well as any known problems.

For software products that are developed iteratively, a project team may baseline only those requirements needed for the current iteration. The requirements specialist can continue to develop requirements for future iterations, while developers proceed with design and construction of the current iteration. This approach provides customers with business value quickly, while minimizing the cost of rework.

In almost all cases, requirements understanding continues to evolve as design and development proceeds. This often leads to the revision of requirements late in the life cycle. Perhaps the most crucial point in understanding software requirements is that a significant proportion of the requirements will change. This is sometimes due to errors in the analysis, but it is frequently an inevitable consequence of change in the “environment”— for example, the customer’s operating or business environment, regulatory processes imposed by the authorities, or the market into which software must sell. Whatever the cause, it is important to recognize the inevitability of change and take steps to mitigate its effects. Change has to be managed by ensuring that proposed changes go through a defined review and approval process and by applying careful requirements tracing, impact analysis, and software configuration management (see the Software Configuration Management KA). Hence, the requirements process is not merely a front-end task in software development, but spans the whole software life cycle. In a typical project, the software requirements activities evolve over time from elicitation to change management. A combination of topdown analysis and design methods and bottomup implementation and refactoring methods that meet in the middle could provide the best of both worlds. However, this is difficult to achieve in practice, as it depends heavily upon the maturity and expertise of the software engineers.

7.2 Change Management

Change management is central to the management of requirements. This topic describes the role of change management, the procedures that need to be in place, and the analysis that should be applied to proposed changes. It has strong links to the Software Configuration Management KA.

7.3 Requirements Attributes

Requirements should consist not only of a specification of what is required, but also of ancillary information, which helps manage and interpret the requirements. Requirements attributes must be defined, recorded, and updated as the software under development or maintenance evolves. This should include the various classification dimensions of the requirement (see section 4.1, Requirements Classification) and the verification method or relevant acceptance test plan section. It may also include additional information, such as a summary rationale for each requirement, the source of each requirement, and a change history. The most important requirements attribute, however, is an identifier that allows the requirements to be uniquely and unambiguously identified.

7.4 Requirements Tracing

Requirements tracing is concerned with recovering the source of requirements and predicting the effects of requirements. Tracing is fundamental to performing impact analysis when requirements change. A requirement should be traceable backward to the requirements and stakeholders that motivated it (from a software requirement back to the system requirement(s) that it helps satisfy, for example). Conversely, a requirement should be traceable forward into the requirements and design entities that satisfy it (for example, from a system requirement into the software requirements that have been elaborated from it, and on into the code modules that implement it, or the test cases related to that code and even a given section on the user manual which describes the actual functionality) and into the test case that verifies it.

The requirements tracing for a typical project will form a complex directed acyclic graph (DAG) (see Graphs in the Computing Foundations KA) of requirements. Maintaining an up-todate graph or traceability matrix is an activity that must be considered during the whole life cycle of a product. If the traceability information is not updated as changes in the requirements continue to happen, the traceability information becomes unreliable for impact analysis.

7.5 Measuring Requirements

As a practical matter, it is typically useful to have some concept of the “volume” of the requirements for a particular software product. This number is useful in evaluating the “size” of a change in requirements, in estimating the cost of a development or maintenance task, or simply for use as the denominator in other measurements. Functional size measurement (FSM) is a technique for evaluating the size of a body of functional requirements.

Additional information on size measurement and standards will be found in the Software Engineering Process KA.

8 Software Requirements Tools

Tools for dealing with software requirements fall broadly into two categories: tools for modeling and tools for managing requirements.

Requirements management tools typically support a range of activities—including documentation, tracing, and change management—and have had a significant impact on practice. Indeed, tracing and change management are really only practicable if supported by a tool. Since requirements management is fundamental to good requirements practice, many organizations have invested in requirements management tools, although many more manage their requirements in more ad hoc and generally less satisfactory ways (e.g., using spreadsheets).

Further Readings

I. Alexander and L. Beus-Dukic, Discovering Requirements [5].

An easily digestible and practically oriented book on software requirements, this is perhaps the best of current textbooks on how the various elements of software requirements fit together. It is full of practical advice on (for example) how to identify the various system stakeholders and how to evaluate alternative solutions. Its coverage is exemplary and serves as a useful reference for key techniques such as use case modeling and requirements prioritization.

C. Potts, K. Takahashi, and A. Antón, “Inquiry-Based Requirements Analysis” [6].

This paper is an easily digested account of work that has proven to be very influential in the development of requirements handling. It describes how and why the elaboration of requirements cannot be a linear process by which the analyst simply transcribes and reformulates requirements elicited from the customer. The role of scenarios is described in a way that helps to define their use in discovering and describing requirements.

A. van Lamsweerde, Requirements Engineering: From System Goals to UML Models to Software Specifications [7].

Serves as a good introduction to requirements engineering but its unique value is as a reference book for the KAOS goal-oriented requirements modelling language. Explains why goal modelling is useful and shows how it can integrate with mainstream modelling techniques using UML.

O. Gotel and A. Finkelstein, “An Analysis of the Requirements Traceability Problem” [8].

This paper is a classic reference work on a key element of requirements management. Based on empirical studies, it sets out the reasons for and the barriers to the effective tracing of requirements. It is essential reading for an understanding of why requirements tracing is an essential element of an effective software process.

N. Maiden and C. Ncube, “Acquiring COTS Software Selection Requirements” [9].

This paper is significant because it recognises explicitly that software products often integrate third-party components. It offers insights into the problems of selecting off-the-shelf software to satisfy requirements: there is usually a mismatch. This challenges some of the assumptions underpinning much of traditional requirements handling, which tends to assume custom software.


[1] I. Sommerville, Software Engineering, 9th ed., Addison-Wesley, 2011.

[2] K.E. Wiegers, Software Requirements, 2th ed., Microsoft Press, 2003.

[3] INCOSE, Systems Engineering Handbook: A Guide for System Life Cycle Processes and Activities, version 3.2.2, International Council on Systems Engineering, 2012.

[4] S. Friedenthal, A. Morre, and R. Steiner, A Practical Guide to SysML: The Systems Modeling Language, 2nd ed., Morgan Kaufmann, 2012.

[5] I. Alexander and L. Beus-Duekic, Discovering Requirements: How to Specify Products and Services, Wiley, 2009.

[6] C. Potts, K. Takahashi, and A.I. Antón, "Inquiry-Based Requirements Analysis", IEEE Software, International Council on Systems Engineering, vol. 11, no. 2, Mar. 1994, pp. 21-32.

[7] A. van Lamsweerde, Requirements Engineering: From System Goals to UML Models to Software Specifications, Wiley, 2009.

[8] O. Gotel and C.W. Finkelstein, "An Analysis of the Requirements Traceability Problem", Proc 1st Int'l Conf. Requirements Eng., IEEE, 1994.

[9] N.A. Maiden and C. Ncube, "Acquiring COTS Software Selection Requirements", IEEE Software, vol. 15, no. 2, Mar.-Apr. 1998, pp. 46-56.