Cs3773 software engineering
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CS3773 Software Engineering. Lecture 02 Requirements Engineering. Requirements Engineering. Requirements engineering is usually the first stage of software life cycle Requirements engineering is the process of understanding and defining functionalities and constraints of proposed systems

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CS3773 Software Engineering

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Cs3773 software engineering

CS3773Software Engineering

Lecture 02

Requirements Engineering


Requirements engineering

Requirements Engineering

  • Requirements engineering is usually the first stage of software life cycle

  • Requirements engineering is the process of understanding and defining functionalities and constraints of proposed systems

  • Requirements engineering process produces a document, software requirements specification (SRS)

    • Customers need a high level specification

    • Software designers and developers need a more detailed specification

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Software requirements

Software Requirements

  • Requirements are desired behaviors

    • Customers “know” what the system shall do

    • Software engineers “know” what to built

  • “Requirements are means of communication with customer and many other stakeholders”

    -- by Helene Wong, PhD thesis, 1994

  • Requirements deal with

    • Objects

    • States

    • Functions

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Software requirements stakeholders

Software Requirements Stakeholders

  • Requirements analysts or system analysts determine requirements

  • Stakeholders contribute to requirements of systems

    • Clients

    • Customers

    • End-users

    • Software engineers

    • Domain experts

    • Lawyers or auditors

    • Market researchers

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Types of requirements

Types of Requirements

  • Functional

    • What is the system supposed to do

    • Mapping from input to output

  • Non-functional (quality)

    • Usability

    • Performance

    • Security

    • Reliability

    • Maintainability

    • Portability

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Types of requirements1

Types of Requirements

  • Process constraints

    • Resources

    • Documentation

    • Standards

  • Design constraints

    • Physical environment

    • Interface

    • Users

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Requirements are important

Requirements Are Important

The hardest single part of building a software system is deciding precisely what to build. No other part of the conceptual work is as difficult as establishing the detailed technical requirements, including all interfaces to people, to machines, and to other software systems. No other part of the work so cripples the resulting system if done wrong. No other part is more difficult to rectify later.

-- by Frederick Brooks, “No silver bullet: essence and accidents of

software engineering”, 1986.

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Requirements are important1

Requirements Are Important

  • 80% of all software errors are requirements errors

    • These are software errors detected after unit testing – i.e., in integration testing, in system testing, and after the software is released

    • Most errors can be traced to unknown, wrong, or misunderstood requirements

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Requirements are important2

Requirements Are Important

  • Requirements usually affect large portions of the implementation; they are rarely encapsulated into modules

  • Requirements errors may be fundamental assumptions built into the design or code

  • Expensive requirements errors are often not fixed; they become “features”

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Requirements are important3

Requirements Are Important

  • Requirements errors are expensive to fix

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Requirements problems

Requirements Problems

  • Over-specification

  • Under-specification (unintended)

  • Contradictory requirements

  • Ambiguous requirements

  • Unknown requirements

  • Bad assumptions about environment

  • Changing requirements

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Characteristics of requirements

Characteristics of Requirements

  • Correct

  • Consistent

  • Complete

  • Concise

  • Traceable

  • Unambiguous

  • Understandable

  • Verifiable

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Requirements engineering process

Requirements Engineering Process

  • Determine the requirements of a system, and specify what behavior is realized

    • Work with customers to elicit the requirements

    • Analyze and model the requirement

    • Document the requirements in a software requirements specification

    • Validate the requirements specification

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Requirements tasks

Requirements Tasks

  • Understand problem from each stakeholder's point of view

  • Extract the essence of the stakeholders' requirements

  • Negotiate a consistent set of requirements with agreement from all stakeholders; set relative priorities

  • Record results in an SRS

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Requirements elicitation

Requirements Elicitation

  • Elicitation is to gather

    • Functions that the system should perform

    • Non-functional requirements that the system should exhibit

  • Elicitation is critical but difficult

    • Customers are not good at describing what they want

    • Software engineers are not good at understanding what customers want

    • Customers and software engineers speak different languages

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Requirements elicitation1

Requirements Elicitation

  • Requirements analysts have to understand the system from each stakeholder's point of view

    • Stakeholders have different views of the system

  • Requirements analysts resolve conflicting views

  • Requirements analysts prioritize requirements

    • Essential requirements

    • Desirable requirements

    • Optional requirements

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Elicitation techniques

Elicitation Techniques

Understand problems

  • For existing system

    • Review documentation

    • Observe current system

    • Questionnaires and Interviews

    • Apprenticeship

  • For new systems - brainstorming

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Analyze existing system

Analyze Existing System

  • What is used, what isn't, what's missing

  • What works well, what doesn't

  • How the system is used, how it was intended to be used, what new ways we want it to be used

  • Risks

    • Users might not be happy with too much change from the old system

    • Might miss real usage patterns

    • Might miss obvious possible improvements

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Analyze existing system review

Analyze Existing System - Review

  • Review all available documentation

    • For an automated system, review its requirements specifications and user manuals, as well as development documentation, internal memos, change histories, etc.

    • For a manual system, review any documented procedures that the workers must follow

  • Gain knowledge of the system before imposing upon other people's time, before bothering the stakeholders

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Analyze existing system observation

Analyze Existing System - Observation

  • Identify what aspects to keep and to understand the system you are about to change

  • System contains a lot of useful functionality that should be included in any future system

  • Documentation rarely describes a system completely and not up to date and

  • Current operation of the system may differ significantly from what is described

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Analyze existing system interview

Analyze Existing System - Interview

  • Questionnaires are useful when information has to be gathered from a large number of people

  • The answers to questions need to be compared or corroborated.

  • Ask problem-oriented questions during interview

  • Interview groups of people together to get synergy

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Analyze existing system apprentice

Analyze Existing System - Apprentice

  • The requirements analyst is the apprentice and the user is the master craftsman.

  • The user can

    • Describe the task precisely

    • Explain why the task is done this way

    • List the exceptions that can occur

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Brainstorm

Brainstorm

  • Brainstorm is used to gather ideas from every stakeholder and prune ideas

  • When you have no idea, or too many ideas, sit down and thrash it out, but with some ground rules

  • Most useful early on, when terrain is uncertain, or when you have little experience, or when novelty is important

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Brainstorm1

Brainstorm

  • Keep the tone informal and non-judgmental

  • Encourage creativity

  • Keep the number of participants “reasonable”, if too many, consider a “playoff”-type filtering

  • Invite back most creative to multiple sessions

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Brainstorm the storm

Brainstorm - the Storm

  • Generate as many ideas as possible

  • Quantity, not quality, is goal at this stage

  • No criticism or debate is permitted

  • Write down all ideas where all can see

  • Participants should NOT self-censor or spend too much time wondering if an idea is practical

  • Original list does not get circulated outside of the meeting

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Brainstorm the calm

Brainstorm – the Calm

  • Go over the list and explain ideas more carefully

  • Categorize into “maybe” and “no” by pre-agreed consensus method

  • Be careful about time

    Meetings tend to lose focus after 90 to 120 minutes

  • Review, consolidate, combine, clarify, and expand

  • Rank the list by priority somehow; choose a winner

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Brainstorm pruning

Brainstorm – Pruning

  • Vote with threshold

    • Each person votes up to n times

    • Keep those ideas with more than m votes

    • Have multiple rounds thereof with smaller n and m

  • Vote with campaign speeches

    • Each person votes up to j < n times

    • Keep those ideas with at least one vote

    • Have multiple rounds thereof with smaller j

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Requirements analysis

Requirements Analysis

  • Understand the desired behavior

    • Interpret the stakeholders' descriptions of requirements

    • Resolve ambiguities, contradictions, loose ends, etc.

  • Build models

    • Use standard notations

    • Help us to understand the requirements

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Requirements what vs how

Requirements: What vs. How

  • Requirements describe purpose and scope of the system

    • What behavior the customer wants

    • Not how the behavior is realized

  • Requirements focus on customer and problems

    • Understand the customer’s needs

    • Describe the background and overview of the problem

  • Requirements represent objects, states, and functions

  • Requirements include assumptions of the environment

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Requirements specification

Requirements Specification

  • Specify requirements

    • Document what is required of the system to be developed

    • State the requirements from the perspective of the

      developers

    • May be a formal document (IEEE-SRS)

  • Requirements document and specification document are

    different

    • Requirements document is a contract

    • Specification is a detailed guideline for developers

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Requirements vs specification

Requirements vs. Specification

  • Requirements document is

    • A complete list on what customers want

    • In terms of environment without reference to system

    • A contract between clients and developers

  • Specification represents

    • System’s behavior in terms of the input and output of a system

    • Which requirements shall be realized by the system

    • How environment entities are controlled by the system

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Cs3773 software engineering

Requirements vs. Specification

Specification

Data structures

and algorithms

Requirements

Interface

Environment

System

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Requirements vs specification1

Requirements vs. Specification

  • Requirements are a collection of statements about phenomena in the environment that we want the system to help make true

  • A specification is a collection of statements that describe a system’s external behavior as observable through the Interface

    • A specification refers only to shared phenomena in the interface and what the system shall do

    • A specification can constrain only shared phenomena that the system itself can control

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Requirements vs specification2

Requirements vs. Specification

  • Example: a turnstile to the park

    • Requirements

      • No one should enter the park without paying an entrance fee

      • For every entrance fee paid, the system should not prevent a corresponding entry

    • Specification

      When a visitor applies a certain amount of force on an

      unlocked turnstile, the turnstile will rotate till a locked

      position

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Requirements validation

Requirements Validation

  • Validate the requirements against stakeholders

    • Reflect accurately customer’s need

    • Also create system-level test plans

  • Validation can be done with techniques

    • Walkthrough

    • Review

    • Prototype

    • Formal inspection

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Specification verification

Specification Verification

  • Verify the specification against requirements

    • Conforms to the requirement definition

    • Build the system right

  • Verification can be done with techniques

    • Simulation

    • Consistency checking

    • Completeness checking

    • Formal verification: model checking or mathematical reasoning

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Software requirements specifications

Software Requirements Specifications

  • Introduction

  • Overall description

  • Specific requirements

  • Requirements table

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Software requirements specification

Software Requirements Specification

Section 0

  • Table of Contents

    Essential for tracing through use cases, classes, state

    diagrams

  • Table of Figures

    Essential for finding each diagram

  • List of Tables

    Essential for finding each table

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Software requirements specification1

Software Requirements Specification

Section 1Introduction

1.1 Purpose of the SRS

e.g., the intended audience

1.2 Scope

1.3 Acronyms, abbreviations, notational conventions

1.4 Overview

e.g., the structure of the rest of the SRS document

1.5 References

Can be put at the end of the document

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Software requirements specification2

Software Requirements Specification

Section 2General description

2.1 Product perspective – the environment

Any hardware and software components that interact

with the system

Overview of the interfaces to other component

A block diagram would be nice

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Software requirements specification3

Software Requirements Specification

Section 2General description

2.2 Product functions

Overview of the system’s main functions

No detail description

At the level of use case names

2.3 User characteristics

Assumptions about the user

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Software requirements specification4

Software Requirements Specification

Section 2General description

2.4 General constraints

e.g., laws, hardware limitations

Any sources of constraints on requirements or design

2.5 Assumptions and Dependencies

Assumptions about the environment

Any environmental conditions that could cause the system to fail

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Software requirements specification5

Software Requirements Specification

Section 3Specific requirements

3.1 Functional requirements

3.1.1 Use case diagrams and detail description in tabular format

Number each use case for future reference.

3.1.2 Class diagrams

3.1.3 State diagrams

3.1.4 Sequence diagrams

In each above section 3.1.x, give English introduction to each diagram to help the reader understand each diagram.

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Software requirements specification6

Software Requirements Specification

Section 3Specific requirements (continued)

3.1 Functional requirements (continued)

3.1.5 Data dictionary in tabular format

  • Classes: purpose

  • Attributes: purpose, range of values

  • Operations: purpose, parameters, pre/post conditions

  • Events: purpose, source, destination, parameters

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Software requirements specification7

Software Requirements Specification

Section 3Specific requirements

3.2 User interface requirements

  • Screen shots

  • Purpose of each button, menu options, etc.

  • List of input/output events

  • How to navigate among windows

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Software requirements specification8

Software Requirements Specification

Section 3Specific requirements

3.3 Non-functional requirements

Reliability

Portability

Security

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Software requirements specification9

Software Requirements Specification

Section 4 Requirements table

  • Requirement number

  • Name

  • Description

  • Related requirements’ numbers and source

  • Related use cases’ numbers

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Reading assignments

Reading Assignments

  • Sommerville’s Book, 8th edition

    • Chapter 7, “Requirements Engineering Process”

  • Sommerville’s Book, 9th edition

    • Chapter 4, “Requirements Engineering”

  • IEEE Std 830-1998, “IEEE Recommended Practice for

    Software Requirements Specification”

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