Chapter 4 – Requirements Engineering Lecture 1 PDF

Chapter 4 – Requirements Engineering Lecture 1 1 Topics covered  Functional and non-functional requirements  The software requirements document  Requirements specification  Requirements engineering processes  Requirements elicitation...

Chapter 4 – Requirements Engineering Lecture 1 1 Topics covered  Functional and non-functional requirements  The software requirements document  Requirements specification  Requirements engineering processes  Requirements elicitation and analysis  Requirements validation 2 Requirements engineering  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. 3 What is a requirement?  It may range from a high-level abstract statement of a service or of a system constraint to a detailed mathematical functional specification. 4 System requirement 5 Types of requirement  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. 6 User and system requirements 7 Functional and non-functional requirements  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. 8 9 Functional requirements  Describe functionality or system services.  Depend on the type of software, expected users and the type of system where the software is used.  Functional user requirements may be high-level statements of what the system should do.  Functional system requirements should describe the system services in detail. 10 Functional requirements for the MHC-PMS  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. 11 Requirements completeness and consistency  In principle, requirements should be both complete and consistent.  Complete  They should include descriptions of all facilities required.  Consistent  There should be no conflicts or contradictions in the descriptions of the system facilities. 12 Non-functional requirements  These define system properties and constraints e.g. reliability, response time and storage requirements. Constraints are I/O device capability, system representations, etc.  Process requirements may also be specified mandating a particular IDE, programming language or development method.  Non-functional requirements may be more critical than functional requirements. If these are not met, the system may be useless. 13 Types of nonfunctional requirement 14 Non-functional requirements implementation  Non-functional requirements may affect the overall architecture of a system rather than the individual components.  For example, to ensure that performance requirements are met, you may have to organize the system to minimize communications between components.  A single non-functional requirement, such as a security requirement, may generate a number of related functional requirements that define system services that are required.  It may also generate requirements that restrict existing requirements. 15 Non-functional classifications  Product requirements  Requirements which specify that the delivered product must behave in a particular way e.g. execution speed, reliability, etc.  Organisational requirements  Requirements which are a consequence of organisational policies and procedures e.g. process standards used, implementation requirements, etc.  External requirements  Requirements which arise from factors which are external to the system and its development process e.g. interoperability requirements, legislative requirements, etc. 16 Examples of nonfunctional requirements in the MHC-PMS Product requirement The MHC-PMS shall be available to all clinics during normal working hours (Mon–Fri, 0830–17.30). Downtime within normal working hours shall not exceed five seconds in any one day. Organizational requirement Users of the MHC-PMS system shall authenticate themselves using their health authority identity card. External requirement The system shall implement patient privacy provisions as set out in HStan-03-2006-priv. 17 Goals and requirements 18 Usability requirements  The system should be easy to use by medical staff and should be organized in such a way that user errors are minimized. (Goal)  Medical staff shall be able to use all the system functions after four hours of training. After this training, the average number of errors made by experienced users shall not exceed two per hour of system use. (Testable non- functional requirement) 19 Metrics for specifying nonfunctional requirements Property Measure Speed Processed transactions/second User/event response time Screen refresh time Size Mbytes Number of ROM chips Ease of use Training time Number of help frames Reliability Mean time to failure Probability of unavailability Rate of failure occurrence Availability Robustness Time to restart after failure Percentage of events causing failure Probability of data corruption on failure Portability Percentage of target dependent statements Number of target systems 20 Key points  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. 21 Chapter 2 – Requirements Engineering Lecture 2 22 The software requirements document 23 Users of a requirements document 24 The structure of a requirements document 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 Here, you describe the services provided for the user. The nonfunctional definition 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. 25 The structure of a requirements document Chapter Description System This should describe the functional and nonfunctional requirements in more requirements detail. If necessary, further detail may also be added to the nonfunctional specification 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. 26 Requirements specification  The process of writing on 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. 27 Ways of writing a system requirements specification Notation Description Natural language The requirements are written using numbered sentences in natural language. Each sentence should express one requirement. Structured natural The requirements are written in natural language on a standard form or language template. Each field provides information about an aspect of the requirement. Design description This approach uses a language like a programming language, but with more languages 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 These notations are based on mathematical concepts such as finite-state specifications 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 28 Requirements and design Natural language specification  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. 30 Guidelines for writing requirements Problems with natural language Example requirements for the insulin pump software system 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.) 33 Structured specifications  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. 34 A structured specification of a requirement for an insulin pump Chapter 4 Requirements 35 engineering A structured specification of a requirement for an insulin pump Chapter 4 Requirements 36 engineering Form-based specifications Form – based specification example 38 Tabular specification Requirements engineering processes 40 Requirements elicitation and analysis 41 Problems of requirements analysis 42 Requirements elicitation and analysis  Software engineers work with a range of system stakeholders to find out about the application domain, the services that the system should provide, the required system performance, hardware constraints, other systems, etc.  Stages include:  Requirements discovery,  Requirements classification and organization,  Requirements prioritization and negotiation,  Requirements specification. 43 Key points  The software requirements document is an agreed statement of the system requirements. It should be organized so that both system customers and software developers can use it.  The requirements engineering process is an iterative process including requirements elicitation, specification and validation.  Requirements elicitation and analysis is an iterative process that can be represented as a spiral of activities – requirements discovery, requirements classification and organization, requirements negotiation and requirements documentation. 44 Chapter 2 – Requirements Engineering Lecture 3 45 Requirements discovery  The process of gathering information about the required and existing systems and distilling the user and system requirements from this information.  Interaction is with system stakeholders from managers to external regulators.  Systems normally have a range of stakeholders. 46 Stakeholders in the MHC-PMS  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. 47 Interviewing  Formal or informal interviews with stakeholders are part of most RE processes.  Types of interview  Closed interviews based on pre-determined list of questions  Open interviews where various issues are explored with stakeholders.  Effective interviewing  Be open-minded, avoid pre-conceived ideas about the requirements and are willing to listen to stakeholders.  Prompt the interviewee to get discussions going using a springboard question, a requirements proposal, or by working together on a prototype system. 48 Interviews in practice Scenarios Scenario for collecting medical history in MHC- PMS 51 Scenario for collecting medical history in MHC- PMS 52 Use cases 53 Use cases for the MHC-PMS 54 Ethnography 55 Requirements validation 56 Requirements checking 57 Requirements validation techniques 58 Requirements reviews 59 Review checks 60 Key points  You can use a range of techniques for requirements elicitation including interviews, scenarios, use-cases and ethnography.  Requirements validation is the process of checking the requirements for validity, consistency, completeness, realism and verifiability.  Business, organizational and technical changes inevitably lead to changes to the requirements for a software system. Requirements management is the process of managing and controlling these changes. 61 References  Sommerville, I. Software Engineering 9th Edition, Addison-Wesley, 2011.  Sommerville, Ian Software engineering / Ian Sommerville. — 9th ed.p. cm. Includes index.ISBN-13: 978-0-13-703515-1  ISBN-10: 0-13-703515-21. Software engineering. I. Title. QA76.758.S657 2011 005.1—dc22 2009053058  Software Engineering: A Practitioner’s Approach, 7/e by Roger S. Pressman Slides copyright © 1996, 2001, 2005, 2009 by Roger S. Pressman Chapter 4 Requirements 62 engineering