Requirements Engineering-Software Engineering-Lecture Slides, Slides of Software Engineering

Download Requirements Engineering-Software Engineering-Lecture Slides and more Slides Software Engineering in PDF only on Docsity! 7/19/2012 1 REQUIREMENTS ENGINEERING Requirements engineering 1 Topics covered Requirements engineering 2  Functional and non-functional requirements  The software requirements document  Requirements specification  Requirements engineering processes  Requirements elicitation and analysis  Requirements validation  Requirements management Requirements engineering Requirements engineering 3  The process of establishing the services that the customer requires from a system and the constraints under which it operates and is developed.  The requirements themselves are the descriptions of the system services and constraints that are generated during the requirements engineering process. Requirements Definition  IEEE - A condition or capability needed by user to solve a problem or achieve an objective  A condition or capability that must be met or possessed by a system or system component to satisfy a contract, standard, specification, or other formally imposed coument.  Jones:94 - The statement of needs by a user that triggers the development of a program or system  Davis:93 - A user need or necessary feature, function, or attribute of a system that can be sensed from a position external to that system Software Engineering - Spring 2009 4 What is a requirement? Requirements engineering 5  It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification.  This is inevitable as requirements may serve a dual function  May be the basis for a bid for a contract - therefore must be open to interpretation;  May be the basis for the contract itself - therefore must be defined in detail;  Both these statements may be called requirements. Requirements abstraction (Davis) Requirements engineering 6 “If a company wishes to let a contract for a large software development project, it must define its needs in a sufficiently abstract way that a solution is not pre-defined. The requirements must be written so that several contractors can bid for the contract, offering, perhaps, different ways of meeting the client organization’s needs. Once a contract has been awarded, the contractor must write a system definition for the client in more detail so that the client understands and can validate what the software will do. Both of these documents may be called the requirements document for the system.” docsity.com 7/19/2012 2 Types of requirements Requirements engineering 7  User requirements  Statements in natural language plus diagrams of the services the system provides and its operational constraints. Written for customers.  System requirements  A structured document setting out detailed descriptions of the system’s functions, services and operational constraints. Defines what should be implemented so may be part of a contract between client and contractor. User and system requirements Requirements engineering 8 Readers of different types of requirements specification Requirements engineering 9 Functional and non-functional requirements Requirements engineering 10  Functional requirements  Statements of services the system should provide, how the system should react to particular inputs and how the system should behave in particular situations.  May state what the system should not do.  Non-functional requirements  Constraints on the services or functions offered by the system such as timing constraints, constraints on the development process, standards, etc.  Often apply to the system as a whole rather than individual features or services.  Domain requirements  Constraints on the system from the domain of operation Functional requirements for the MHC- PMS Requirements engineering 11  A user shall be able to search the appointments lists for all clinics.  The system shall generate each day, for each clinic, a list of patients who are expected to attend appointments that day.  Each staff member using the system shall be uniquely identified by his or her 8-digit employee number. Requirements imprecision Requirements engineering 12  Problems arise when requirements are not precisely stated.  Ambiguous requirements may be interpreted in different ways by developers and users.  Consider the term ‘search’ in requirement 1  User intention – search for a patient name across all appointments in all clinics;  Developer interpretation – search for a patient name in an individual clinic. User chooses clinic then search. docsity.com 7/19/2012 5 Domain requirements problems Requirements engineering 25  Understandability  Requirements are expressed in the language of the application domain;  This is often not understood by software engineers developing the system.  Implicitness  Domain specialists understand the area so well that they do not think of making the domain requirements explicit. Key points Requirements engineering 26  Requirements for a software system set out what the system should do and define constraints on its operation and implementation.  Functional requirements are statements of the services that the system must provide or are descriptions of how some computations must be carried out.  Non-functional requirements often constrain the system being developed and the development process being used.  They often relate to the emergent properties of the system and therefore apply to the system as a whole. REQUIREMENTS ENGINEERING Lecture 2 Requirements engineering 27 The software requirements document Requirements engineering 28  The software requirements document is the official statement of what is required of the system developers.  Should include both a definition of user requirements and a specification of the system requirements.  As far as possible, it should set of WHAT the system should do rather than HOW it should do it. Agile methods and requirements Requirements engineering 29  Many agile methods argue that producing a requirements document is a waste of time as requirements change so quickly.  The document is therefore always out of date.  Methods such as XP use incremental requirements engineering and express requirements as ‘user stories’ (discussed in Chapter 3).  This is practical for business systems but problematic for systems that require a lot of pre-delivery analysis (e.g. critical systems) or systems developed by several teams. Users of a requirements document Requirements engineering 30 docsity.com 7/19/2012 6 Requirements document variability Requirements engineering 31  Information in requirements document depends on type of system and the approach to development used.  Systems developed incrementally will, typically, have less detail in the requirements document.  Requirements documents standards have been designed e.g. IEEE standard. These are mostly applicable to the requirements for large systems engineering projects. The structure of a requirements document Requirements engineering 32 Chapter Description Preface This should define the expected readership of the document and describe its version history, including a rationale for the creation of a new version and a summary of the changes made in each version. Introduction This should describe the need for the system. It should briefly describe the system’s functions and explain how it will work with other systems. It should also describe how the system fits into the overall business or strategic objectives of the organization commissioning the software. Glossary This should define the technical terms used in the document. You should not make assumptions about the experience or expertise of the reader. User requirements definition Here, you describe the services provided for the user. The nonfunctional system requirements should also be described in this section. This description may use natural language, diagrams, or other notations that are understandable to customers. Product and process standards that must be followed should be specified. System architecture This chapter should present a high-level overview of the anticipated system architecture, showing the distribution of functions across system modules. Architectural components that are reused should be highlighted. The structure of a requirements document Requirements engineering 33 Chapter Description System requirements specification This should describe the functional and nonfunctional requirements in more detail. If necessary, further detail may also be added to the nonfunctional requirements. Interfaces to other systems may be defined. System models This might include graphical system models showing the relationships between the system components and the system and its environment. Examples of possible models are object models, data-flow models, or semantic data models. System evolution This should describe the fundamental assumptions on which the system is based, and any anticipated changes due to hardware evolution, changing user needs, and so on. This section is useful for system designers as it may help them avoid design decisions that would constrain likely future changes to the system. Appendices These should provide detailed, specific information that is related to the application being developed; for example, hardware and database descriptions. Hardware requirements define the minimal and optimal configurations for the system. Database requirements define the logical organization of the data used by the system and the relationships between data. Index Several indexes to the document may be included. As well as a normal alphabetic index, there may be an index of diagrams, an index of functions, and so on. Requirements specification Requirements engineering 34  The process of writing down the user and system requirements in a requirements document.  User requirements have to be understandable by end- users and customers who do not have a technical background.  System requirements are more detailed requirements and may include more technical information.  The requirements may be part of a contract for the system development  It is therefore important that these are as complete as possible. Ways of writing a system requirements specification Requirements engineering 35 Notation Description Natural language The requirements are written using numbered sentences in natural language. Each sentence should express one requirement. Structured natural language The requirements are written in natural language on a standard form or template. Each field provides information about an aspect of the requirement. Design description languages This approach uses a language like a programming language, but with more abstract features to specify the requirements by defining an operational model of the system. This approach is now rarely used although it can be useful for interface specifications. Graphical notations Graphical models, supplemented by text annotations, are used to define the functional requirements for the system; UML use case and sequence diagrams are commonly used. Mathematical specifications These notations are based on mathematical concepts such as finite-state machines or sets. Although these unambiguous specifications can reduce the ambiguity in a requirements document, most customers don’t understand a formal specification. They cannot check that it represents what they want and are reluctant to accept it as a system contract Natural language specification Requirements engineering 36  Requirements are written as natural language sentences supplemented by diagrams and tables.  Used for writing requirements because it is expressive, intuitive and universal. This means that the requirements can be understood by users and customers. docsity.com 7/19/2012 7 Guidelines for writing requirements  Invent a standard format and use it for all requirements.  Use language in a consistent way. Use shall for mandatory requirements, should for desirable requirements.  Use text highlighting to identify key parts of the requirement.  Avoid the use of computer jargon.  Include an explanation (rationale) of why a requirement is necessary. 37 Requirements engineering Problems with natural language  Lack of clarity  Precision is difficult without making the document difficult to read.  Requirements confusion  Functional and non-functional requirements tend to be mixed-up.  Requirements amalgamation  Several different requirements may be expressed together. 38 Requirements engineering Example requirements for the insulin pump software system Requirements engineering 39 3.2 The system shall measure the blood sugar and deliver insulin, if required, every 10 minutes. (Changes in blood sugar are relatively slow so more frequent measurement is unnecessary; less frequent measurement could lead to unnecessarily high sugar levels.) 3.6 The system shall run a self-test routine every minute with the conditions to be tested and the associated actions defined in Table 1. (A self-test routine can discover hardware and software problems and alert the user to the fact the normal operation may be impossible.) Structured specifications Requirements engineering 40  An approach to writing requirements where the freedom of the requirements writer is limited and requirements are written in a standard way.  This works well for some types of requirements e.g. requirements for embedded control system but is sometimes too rigid for writing business system requirements.  Limits the terminology that can be used and use templates to specify system requirements Form-based specifications  Definition of the function or entity.  Description of inputs and where they come from.  Description of outputs and where they go to.  Information about the information needed for the computation and other entities used.  Description of the action to be taken.  Pre and post conditions (if appropriate).  The side effects (if any) of the function. 41 Requirements engineering A structured specification of a requirement for an insulin pump Requirements engineering 42 docsity.com 7/19/2012 10 Elicitation and analysis  Sometimes called requirements elicitation or requirements discovery.  Involves technical staff working with customers to find out about the application domain, the services that the system should provide and the system’s operational constraints.  May involve end-users, managers, engineers involved in maintenance, domain experts, trade unions, etc. These are called stakeholders. Problems of requirements analysis  Stakeholders don’t know what they really want.  Stakeholders express requirements in their own terms.  Different stakeholders may have conflicting requirements.  Organisational and political factors may influence the system requirements.  The requirements change during the analysis process. New stakeholders may emerge and the business environment change. The requirements spiral Process activities  Requirements discovery  Interacting with stakeholders to discover their requirements. Domain requirements are also discovered at this stage.  Requirements classification and organisation  Groups related requirements and organises them into coherent clusters.  Prioritisation and negotiation  Prioritising requirements and resolving requirements conflicts.  Requirements documentation  Requirements are documented and input into the next round of the spiral. Requirements discovery  The process of gathering information about the proposed and existing systems and distilling the user and system requirements from this information.  Sources of information include documentation, system stakeholders and the specifications of similar systems. ATM stakeholders  Bank customers  Representatives of other banks  Bank managers  Counter staff  Database administrators  Security managers  Marketing department  Hardware and software maintenance engineers  Banking regulators docsity.com 7/19/2012 11 Stakeholders in the MHC-PMS Requirements engineering 61  Patients whose information is recorded in the system.  Doctors who are responsible for assessing and treating patients.  Nurses who coordinate the consultations with doctors and administer some treatments.  Medical receptionists who manage patients’ appointments.  IT staff who are responsible for installing and maintaining the system. Stakeholders in the MHC-PMS Requirements engineering 62  A medical ethics manager who must ensure that the system meets current ethical guidelines for patient care.  Health care managers who obtain management information from the system.  Medical records staff who are responsible for ensuring that system information can be maintained and preserved, and that record keeping procedures have been properly implemented. Viewpoints  Viewpoints are a way of structuring the requirements to represent the perspectives of different stakeholders. Stakeholders may be classified under different viewpoints.  This multi-perspective analysis is important as there is no single correct way to analyse system requirements. Types of viewpoint  Interactor viewpoints  People or other systems that interact directly with the system. In an ATM, the customer’s and the account database are interactor VPs.  Indirect viewpoints  Stakeholders who do not use the system themselves but who influence the requirements. In an ATM, management and security staff are indirect viewpoints.  Domain viewpoints  Domain characteristics and constraints that influence the requirements. In an ATM, an example would be standards for inter-bank communications. Viewpoint identification  Identify viewpoints using  Providers and receivers of system services;  Systems that interact directly with the system being specified;  Regulations and standards;  Sources of business and non-functional requirements.  Engineers who have to develop and maintain the system;  Marketing and other business viewpoints. LIBSYS viewpoint hierarchy docsity.com 7/19/2012 12 Interviewing  In formal or informal interviewing, the RE team puts questions to stakeholders about the system that they use and the system to be developed.  There are two types of interview  Closed interviews where a pre-defined set of questions are answered.  Open interviews where there is no pre-defined agenda and a range of issues are explored with stakeholders. Interviews in practice  Normally a mix of closed and open-ended interviewing.  Interviews are good for getting an overall understanding of what stakeholders do and how they might interact with the system.  Interviews are not good for understanding domain requirements  Requirements engineers cannot understand specific domain terminology;  Some domain knowledge is so familiar that people find it hard to articulate or think that it isn’t worth articulating. Effective interviewers  Interviewers should be open-minded, willing to listen to stakeholders and should not have pre-conceived ideas about the requirements.  They should prompt the interviewee with a question or a proposal and should not simply expect them to respond to a question such as ‘what do you want’. Scenarios  Scenarios are real-life examples of how a system can be used.  They should include  A description of the starting situation;  A description of the normal flow of events;  A description of what can go wrong;  Information about other concurrent activities;  A description of the state when the scenario finishes. LIBSYS scenario (1) LIBSYS scenario (2) docsity.com 7/19/2012 15 Requirements validation techniques  Requirements reviews  Systematic manual analysis of the requirements.  Prototyping  Using an executable model of the system to check requirements. Covered in Chapter 17.  Test-case generation  Developing tests for requirements to check testability. Requirements reviews  Regular reviews should be held while the requirements definition is being formulated.  Both client and contractor staff should be involved in reviews.  Reviews may be formal (with completed documents) or informal. Good communications between developers, customers and users can resolve problems at an early stage. Review checks  Verifiability. Is the requirement realistically testable?  Comprehensibility. Is the requirement properly understood?  Traceability. Is the origin of the requirement clearly stated?  Adaptability. Can the requirement be changed without a large impact on other requirements? Requirements management  Requirements management is the process of understanding and controlling changes to system requirements.  Requirements are inevitably incomplete and inconsistent  New requirements emerge during the process as business needs change and a better understanding of the system is developed;  Different viewpoints have different requirements and these are often contradictory.  New hardware may be introduced, interoperability requirements, new legislation and regulations may be introduced etc. Requirements evolution Enduring and volatile requirements  Enduring requirements. Stable requirements derived from the core activity of the customer organisation. E.g. a hospital will always have doctors, nurses, etc. May be derived from domain models  Volatile requirements. Requirements which change during development or when the system is in use. In a hospital, requirements derived from health-care policy docsity.com 7/19/2012 16 Requirements classification Requirements management planning  During the requirements engineering process, you have to plan:  Requirements identification  How requirements are individually identified;  A change management process  The process followed when analysing a requirements change;  Traceability policies  The amount of information about requirements relationships that is maintained;  CASE tool support  The tool support required to help manage requirements change; Traceability  Traceability is concerned with the relationships between requirements, their sources and the system design  Source traceability  Links from requirements to stakeholders who proposed these requirements;  Requirements traceability  Links between dependent requirements;  Design traceability  Links from the requirements to the design; A traceability matrix CASE tool support  Requirements storage  Requirements should be managed in a secure, managed data store.  Change management  The process of change management is a workflow process whose stages can be defined and information flow between these stages partially automated.  Traceability management  Automated retrieval of the links between requirements. Requirements change management  Should apply to all proposed changes to the requirements.  Principal stages  Problem analysis. Discuss requirements problem and propose change;  Change analysis and costing. Assess effects of change on other requirements;  Change implementation. Modify requirements document and other documents to reflect change. docsity.com 7/19/2012 17 Change management Key points  The requirements engineering process includes a feasibility study, requirements elicitation and analysis, requirements specification and requirements management.  Requirements elicitation and analysis is iterative involving domain understanding, requirements collection, classification, structuring, prioritisation and validation.  Systems have multiple stakeholders with different requirements. Key points  Social and organisation factors influence system requirements.  Requirements validation is concerned with checks for validity, consistency, completeness, realism and verifiability.  Business changes inevitably lead to changing requirements.  Requirements management includes planning and change management. docsity.com