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Editorial Reviews. About the Author. Klaus Pohl holds a full professorship for Software Systems Download it once and read it on your Kindle device, PC, phones or tablets. Use features like bookmarks, eBook features: Highlight, take notes. Editorial Reviews. About the Author. Klaus Pohl holds a full professorship for Software Systems Download it once and read it on your Kindle device, PC, phones or tablets. Kindle Store; ›; Kindle eBooks; ›; Computers & Technology. Download Ebook: Read Requirements Engineering Fundamentals: A Study Guide for the Certified Professional for Requirements Engineering.
If the defect is fixed during acceptance testing, the effort involved may be up to a times higher [Boehm ].
Symptoms and causes of deficient requirements engineering Symptoms for inadequate requirements engineering are as numerous as their causes. Frequently, requirements are missing or not clearly formu- lated. For instance, if the requirements do not reflect customer wishes precisely or if the requirements are described in an imprecise way and thus allow for several interpretations, the result is often a system that does not meet the expectations of the client or the users.
The most common reason for deficient requirements is the miscon- ception of the stakeholders that much is self-evident and does not need to be stated explicitly. This results in problems in communication among the involved parties that arise from differences in experience and knowledge. To make matters worse, it is often the case that especially the client wishes for quick integration of recent results into a productive system.
The significance of good requirements engineering The increasing importance of software-intensive systems in industrial projects as well as the need to bring more innovative, more individual, and more comprehensive systems to market and the need to do so quicker, better, and with a higher level of quality calls for efficient require- ments engineering. Complete requirements free from defects are the basis for successful system development.
Potential risks have to be identified during requirements engineering and must be reduced as early as possible to allow for successful project progress. Faults and gaps in requirement documents must be discovered early on to avoid tedious change pro- cesses. In order to make a development project succeed, it is necessary to know the requirements for the system and to document them in a suitable man- ner. StakeholdersThe term stakeholder is essential in requirements engineering.
Among other things, stakeholders are the most important sources of requirements. Not considering a stakeholder often results in fragmentally elicited requirements, i.
Stake- holders are those people or organizations that have some impact on the requirements. This could be people that are going to interact with the system e. Definition Requirement 1 A condition or capability needed by a user to solve a problem or achieve an objective. Stakeholder A stakeholder of a system is a person or an organization that has an direct or indirect influence on the requirements of the system.
During requirements elicitation, different techniques are used to obtain requirements from stakeholders and other sources and to refine the requirements in greater detail. During documentation, the elicited requirements are described adequately. Different techniques are used to document the requirements by using natural language or conceptual models see chapters 4, 5, and 6.
In order to guarantee that the predefined quality criteria are met, documented requirements must be validated and negotiated early on see chapter 7. Requirements management is orthogonal to all other activities and comprises any measures that are necessary to structure requirements, to prepare them so that they can be used by different roles, to maintain consistency after changes, and to ensure their imple- mentation see chapter 8.
These core activities can be applied for different levels of requirements abstraction, like stakeholder requirements, system requirements, and soft- ware requirements. Constraints Different project constraints influence requirements engineering.
For instance, people, domain factors, or organizational constraints e. Requirements Engineering 1 Requirements engineering is a systematic and disciplined approach to the specification and management of requirements with the following goals: The goal of such models is to elicit all requirements prior to the actual development.
As a result, in these process models, requirements engineering is understood to be a finite, time-restricted initial phase of system development. Requirementsengineeringas a continuous, collateral process Lightweight process models e. In these process models, requirements engineering is treated as a continuous, comprehen- sive process that comprises and integrates all phases of system develop- ment.
In most cases, one uses a rule- driven medium that is accessible to the communication partner—natural language. For the transmission of information from one individual to another to work properly, a common code is needed.
The sender encodes her mes- sage and the receiver has to decode it. Such a common code is intrinsic to any two people that speak the same language e. The more similar the cultural and educational background, the area of expertise, and the everyday work life, the better the exchange of information works. However, such ideal conditions most often do not exist between stake- holders. It is therefore sensible to agree upon a common language and how this common language is to be used.
This can, for instance, be achieved by means of glossaries see chapter 4 , in which all important terms are explained. Alternatively, this can be done by agreeing upon a formal descriptive language, e. Type of communication medium Another important factor is the type of communication medium. In verbal communication, the success of the communication relies heavily on redundancy e. In written technical communication, for example, information is transmitted with a minimum of redundancy and feedback.
Language comfort In addition to the problems arising from differing domain vocabu- laries and different communication media, it can often be observed that information is not adequately transmitted or not transmitted at all.
This can be traced back to natural transformations that occur during human perception. These transformational effects are, in particular, focusing and simplification and can impact the communication more or less harshly.
Implicit background knowledge Communication—i. The author expects the reader to have some kind of implicit background knowledge. It is the simplifications that arise from language-based knowledge expression that become problematic with regard to requirements, as requirements can become interpretable in different ways.
In chapter 5, natural language-based requirement docu- mentation is discussed in further detail. She is usually the only one who has direct contact with the stakehold- ers and has both the ability and the responsibility to become as familiar as possible with the domain and to understand it as well as possible. The requirements engineer is, in a manner of speaking, a translator that understands the domain as well as its particular language well enough and also possesses enough IT know-how to be aware of the problems the developers face and to be able to communicate with them on the same level.
The requirements engineer therefore has a central role in the project. Seven necessary capabilities of a requirements engineer To be able to fulfill all of her tasks, the requirements engineer needs much more than process knowledge.
Many of the capabilities required must be based on practical experience. The requirements engineer must be able to become familiar with domains that are unknown to her and must understand and analyze complicated problems and relationships. Since stakehold- The requirements engineer has the challenging task of identi- fying the actual needs of a stakeholder.
A core requirement to be able to achieve this is to have good intuition and empathy for people. In addi- tion, she must identify problems that might arise in a group of stakehold- ers and act accordingly.
Requirements Engineering - Fundamentals, Principles, and Techniques | Klaus Pohl | Springer
To elicit the requirements from stakeholders and to interpret them correctly and communicate them in a suitable man- ner, a requirements engineer must have good communication skills. She must be able to listen, ask the right questions at the right time, notice when a statement does not contain the desired information, and make further inquiries when necessary. Different opinions of different stakeholders can be the cause of conflicts during requirements engineering.
The requirements engineer must identify conflicts, mediate between the parties involved, and apply techniques suitable to resolving the conflict.
The requirements engineer must be able to mediate between different opinions and lead discussions.
This holds true for individual conversations as well as group conversations and work- shops. Since the requirements engineer is frequently at the center of attention, she occasionally is exposed to criticism as well. As a result, she needs a high level of self-confidence and the ability to defend herself should strong objections to her opinions arise.
She should never take criticism personally. Among other things, the requirements engineer is, in a matter of speaking, a kind of attorney for the requirements of the stake- holders. She must be able to represent the requirements in team meet- ings and presentations. In addition, she must consolidate differing opinions, facilitate a decision in case of a disagreement, and create con- sensus among the stakeholders.
Usually, these requirements are divided into func- tional requirements, behavioral requirements, and data requirements see chapter 4. Typically, quality requirements are about the perfor- mance, availability, dependability, scalability, or portability of a system. Requirements of this type are frequently classified as non-functional requirements. Requirements of this type can constrain the system itself e.
In contrast to functional and quality requirements, con- straints are not implemented, they are adhered to because they merely limit the solution space available during the development process.
Functional Requirement A functional requirement is a requirement concerning a result of behavior that shall be provided by a function of the system. Quality Requirement A quality requirement is a requirement that pertains to a quality concern that is not covered by functional requirements. Constraint A constraint is a requirement that limits the solution space beyond what is necessary for meeting the given functional requirements and quality requirements.
For example, there are a number of clas- sifications suggested by several standards, e. Other classification schemes describe requirement attributes, such as the level of detail of a requirement, the priority, or the degree of legal obligation of requirements see chapters 4 and 8.
Therefore, the requirements engineer should place special emphasis on the elicitation, documentation, and negotiation of quality requirements during the development process. Typically, many different kinds of desired qualities of the system are assigned to the requirement type quality requirement.
In order to be able to deal with quality requirements in a structured manner, many different classification schemes for quality requirements have been proposed.
However, numerous approaches to document quality requirements by means of models have been suggested over the past couple of years. The requirements engineer is responsible for making sure the quality requirements are as objective and verifiable as possible. Typically, this necessitates that the quality requirements are quantified.
For example, a quality requirement with regard to system performance could specify that a system shall process 95 percent of all queries within 1. This can cause quality requirements to be refined by means of additional functional requirements. This could be the case for a quality requirement that is concerned with system security if a functional requirement speci- fies the exact encryption algorithm to satisfy the need for encryption as demanded by some quality requirement.
Quality requirements are often related to different functional require- ments. As a result, quality requirements should always be kept separated from functional requirements. In other words, quality requirements should not be mixed with functional requirements and should be docu- mented separately, with explicit documentation of their relation to func- tional requirements. The goal of requirements engineering is to document customer requirements as completely as possible in good quality and to identify and resolve problems in the requirements as early as possible.
Successful requirements engineering is based on including the right stakeholders as well as embedding the four core activities of requirements engineering elicitation, documentation, validation and negotiation, and management into the system development process.
At the center of attention is the requirements engineer, who is the primary contact point in requirements engineering and possesses a great deal of domain knowledge and process knowledge as well as a multitude of soft skills. The purpose of defining the system and context bound- aries in requirements engineering is to identify the part of the environment that influences the requirements for the system to be developed.
In order to do that, it is anticipated what the system will be like once it becomes real. By doing so, those parts of the real world which will potentially influence the requirements of the system can be identified.
To be able to specify the requirements for a system correctly and completely, it is necessary to identify the relationships between individual material and immaterial aspects as precisely as possible. The part of reality that is relevant for the requirements of a system is called the system con- text. Context aspects in the system context Among others, the following possible aspects of reality influence the con- text of a system: System Context The system context is the part of the system environment that is relevant for the definition as well as the understanding of the requirements of a system to be developed.
This leads to the system operating on the basis of incom- plete or erroneous requirements, which is often the reason for system fail- ure during operation. Such errors often remain undetected during the val- idation procedures, which determine if the system meets the specified requirements, and occur only during operation, sometimes entailing cata- strophic consequences.
For example, stakeholders, pertinent standards, and legal guidelines demand particular functional properties that the sys- tem to be developed must possess at its interfaces. A requirement is there- fore defined for a specific context and can only be interpreted correctly in regard to this specific context. The better the context of a requirement is understood e. Therefore, a purpose-driven documentation of the system context or information about the system context is of par- ticular importance.
In order to do so, it is necessary to separate the system context from the system to be developed as well as from the parts of reality that are irrelevant for the system see figure When defining the system boundary, a decision has to be made: Which aspects pertain to the system to be developed and which aspects belong in the system context?
When defining the context boundary, the question to be answered is: Which aspects pertain to the system context i. Thus, system and context boundaries define the system context. The sys- tem context comprises all aspects that are relevant with regard to the requirements for the system to be developed. These aspects cannot be altered or modified by the system development process. When the system boundary is defined, the scope of the development i.
We therefore define the system boundary as follows: All aspects that are within the system boundary can thus be altered during system development.
For instance, an existing system that consists of hard- ware and software components and is supposed to be replaced by the new system can be within the system boundary. Aspects within the system con- text can be business processes, technical processes, people and roles, organizational structures, and components of the IT infrastructure.
Figure schematically shows the system context of a system. The system context consists of other systems, groups of stakeholders that in some way use the interfaces of the system to be developed, and additional require- ments sources and their interrelations. System Boundary The system boundary separates the system to be developed from its environment; i.
Sources provide inputs for the system. Sinks receive outputs from the system. Pos- sible sources and sinks of a system are as follows: Using these interfaces, the system provides its functionality to the environment, monitors the environment, influences parameters of the environment, and controls operations of the environment.
Depending on the type of the respective source or sink, the system needs different inter- face types e. The interface type in turn may also impose specific constraints or additional sources of requirements on the system to be developed. Gray zone between system and system context Frequently, the system boundary is not precisely defined until the end of the requirements engineering process. Before that, some or several interfaces as well as desired functions and qualities of the system to be developed are only partially known or not known at all.
We refer to this initially vague separation of the system and its context as the gray zone between the system and the context see figure At the beginning of the requirements engineering process, it may, for example, not be clear whether the system should implement a certain function e. This kind of shifting is caused by the fact that aspects, pertaining at first to the system context, now will be modified during system development.
Such a situation occurs during requirements engineering, for example, if it is not clear in the system con- text whether certain activities of a business process should be imple- mented or supported by the system to be developed or not. In this situa- tion, it is not clear which aspects belong to the system and can thus be changed or modified and which aspects belong to the system context. This causes a corresponding shift of the gray zone between system and system context see figure Figure Gray zone of the system boundary The gray zone shifts, for instance, when interfaces are attributed to the system boundary and the gray zone is extended to comprise aspects of the environment that concern these interfaces.
The context boundary can be defined as follows: In the course of requirements engineering, it is necessary to concretize the boundary between system context and irrelevant environment by analyzing relevant aspects within the environment with regard to their relationships to the system.
Besides the system boundary, the context boundary typically also shifts during requirements engineering. For instance, it may be possible that a law directive that was considered to be relevant for the system to be developed no longer impacts the system or is no longer considered rele- vant.
Figure Gray zone between system context and irrelevant environment Gray zone between system context and irrelevant environment Since the context boundary separates the system context from those parts of reality that are irrelevant to the system, a complete and precise definition of the context boundary for complex systems is virtually impossible. In addition, it may not be possible to clarify for single aspects of the environ- ment whether they influence the system to be developed or are influenced Definition Context Boundary The context boundary separates the relevant part of the environment of a system to be developed from the irrelevant part, i.
These two observations are the reason for the existence of a gray zone with regard to the context boundary see figure Resolving and shifting of the gray zone This gray zone therefore comprises identified aspects of the environ- ment for which it is unclear whether they have a relation to the system or not. In contrast to the gray zone between the system and the system con- text that must be resolved in the course of requirements engineering, it is not necessary to resolve the gray zone between the system context and the irrelevant environment entirely.
When the context is modeled with data flow dia- grams, sources and sinks in the environment of the system that represent the source or destination of data flows or flows of material, energy, money, etc. In use case diagrams, actors such as people or other systems in the system environment and their usage relationships to the system are modeled.
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