[06] Software Requirements

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Software Engineering

Chapter 6

Software requirements

Ku-Yaw Chang

[email protected]

Assistant ProfessorDepartment of Computer Science and Information Engineering

Da-Yeh University
mailto:[email protected]:[email protected]


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2Ku-Yaw Chang Software Requirements

Objectives

Understand the concepts of user requirements

and system requirements and why these

requirements should be written in different ways

Understand the differences between functionaland non-functional software requirements

Understand how requirements may be

organized in a software requirements document


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Preamble

Requirements

Descriptions of the services provided by the system

and its operational constraints

Requirements engineering (RE) The process of finding out, analyzing, documenting

and checking these services and constraints


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Preamble

Different level of descriptions

User requirements

Statements of what services the system is expected to

provide and the constraints under which it must operate

In a natural language plus diagrams

System requirements

Set out the systems functions, services and operational

constraints in detail

Should be precise, also called functional specification

Define exactly what is to be implemented


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Preamble

User requirement definition

1. LIBSYS shall keep track of all data required

by copyright licensing agencies in the UK and

elsewhere


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Preamble

System requirement definition1.1 On making a request for a document from LIBSYS, the

requestor shall be presented with a form that records details ofthe user and the request made.

1.2 LIBSYS request forms shall be stored on the system for fiveyears from the date of the request.

1.3 All LIBSYS request forms must be indexed by user, by thename of the material requested and by the supplier of therequest.

1.4 LIBSYS shall maintain a log of all requests that have been

made to the system.1.5 For material where authors lending rights apply, loan details

shall be sent monthly to copyright licensing agencies that haveregistered with LIBSYS.


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Readers of

different types of specification

User

requirements

Client managers

System end-users

Client engineers

Contractor managersSystem architects

Systemrequirements

System end-users

Client engineersSystem architects

Software developers


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Contents

6.1 Functional and non-functional requirements

6.2 User requirements

6.3 System requirements

6.4 Interface specification

6.5 The software requirements document

6.6 Exercises


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6.1 Functional and non-functional

requirements

Functional requirements Statements of services

the system should provide

how the system should react to particular inputs

how the system should behave in particular situationsNon-functional requirements Constraints on the services or functions offered by the

systemtiming constraints

constraints on the development process, standards, etc.Domain requirements From the application domain of the system and that

reflect characteristics of that domain


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6.1.1 Functional requirements

Examples for a university library system called

LIBSYS

1. The user shall be able to search either all of the initial

set of databases or select a subset from it.2. The system shall provide appropriate viewers for the

user to read documents in the document store.

3. Every order shall be allocated a unique identifier

(ORDER_ID), which the user shall be able to copy tothe accounts permanent storage area.


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6.1.2 Non-functional requirements

Performance

requir e ments

Space

requir ements

Usability

re quir em ents

Effic iency

requir ements

Re lia bility

re quir em ents

Portab ility

requir e ments

Inter oper a bility

requir e ments

Ethical

requir ements

Legislative

requir e ments

Im plem enta tion

requir e ments

Sta ndar ds

requir e ments

Delivery

requir e ments

Safety

requir e ments

Privacy

requir ements

Product

re quir em ents

Org anisa tional

requir e ments

External

requir ements

Non-f unctiona l

requir e ments


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Examples of

non-functional requirementsProduct requirement 8.1 The user interface for LIBSYS shall be

implemented as simple HTML without frames or javaapplets

Organizational requirement 9.3.2 The system development process and

deliverable documents shall conform to the processand deliverables defined in XYZCo-SP-STAN-95.

External requirement 10.6 The system shall not disclose any personal

information about system users apart from their nameand library reference number to the library staff whouse the system.


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Metrics for specifying

non-functional requirementsPrope rty Me asure

Speed Processed transactions/second

User/Event response time

Screen refresh time

Size M Bytes

Number of ROM chips

Ease of use Training timeNumber of help frames

Reliability Mean time to failure

Probability of unavailability

Rate of failure occurrence

Availability

Robustness T ime to restart after failure

Percentage of events causing failure

Probability of data corruption on failure

Portability Percent age of target dependent st atements

Number of target systems


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6.1.3 Domain requirements

Be derived from the application domain Not from the specific needs of system users

Reflect fundamentals of the application domain

Examples There shall be a standard user interface to all

databases that shall be based on the Z39.50 standard.

Because of copyright restrictions, some documentsmust be deleted immediately on arrival. Depending onthe user requirements, these documents will either beprinted locally on the system server for manualforwarding to the user or routed to a network printer.


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Contents






6.6 Exercises


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User requirements

Describe the functional and non-functional

requirements

Be understandable by system users without detailed

technical knowledgeShould

Simple language, simple tables and forms and

intuitive diagrams

Should not Software jargon, structured notations or formal

notations, system implementation


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User requirements

Various problems

Lack of clarity

Requirements confusion

Requirements amalgamation


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A user requirement for

an accounting system in LIBSYS

4.5

LIBSYS shall provide a financial accounting

system that maintains records of all payments

made by users of the system. System managersmay configure this system so that regular users

may receive discounted rates.


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Contents






6.6 Exercises


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6.6 Exercises


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6.6 Exercises


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6.6 Exercises

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