CS3773 Software Engineering

CS3773Software Engineering

Lecture 02 Requirements Engineering

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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

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

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

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 Requirements

Process constraints– Resources – Documentation – Standards

Design constraints– Physical environment– Interface– Users

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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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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 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 errors are expensive to fix

Stage discovered Relative repair costRequirements 0.1 – 0.2

Design 0.5

Coding 1

Unit test 2

Acceptance test 5

Maintenance 20

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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

Correct Consistent Complete Concise Traceable Unambiguous Understandable Verifiable

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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

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

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 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

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

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

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

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

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

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 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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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

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

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

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

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 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

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 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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Environment

Requirements Data structuresand algorithms

SystemInterface

Specification


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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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Example: a turnstile to the park– Requirements

1. No one should enter the park without paying an entrance fee2. For every entrance fee paid, the system should not prevent

a corresponding entry– Specification

When a visitor applies a certain amount of force on anunlocked turnstile, the turnstile will rotate till a locked position

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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

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

Introduction Overall description Specific requirements Requirements table

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Software Requirements Specification

Section 0– Table of Contents

Essential for tracing through use cases, classes, statediagrams

– Table of FiguresEssential for finding each diagram

– List of TablesEssential for finding each table

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Section 1 Introduction1.1 Purpose of the SRS

e.g., the intended audience1.2 Scope1.3 Acronyms, abbreviations, notational conventions1.4 Overview

e.g., the structure of the rest of the SRS document1.5 References

Can be put at the end of the document

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Section 2 General description2.1 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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Section 2 General description2.2 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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Section 2 General description2.4 General constraints

e.g., laws, hardware limitations Any sources of constraints on requirements or design

2.5 Assumptions and DependenciesAssumptions about the environmentAny environmental conditions that could cause the system to fail

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Section 3 Specific requirements3.1 Functional requirements3.1.1 Use case diagrams and detail description in tabular

format Number each use case for future reference. 3.1.2 Class diagrams3.1.3 State diagrams3.1.4 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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Section 3 Specific 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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Section 3 Specific requirements3.2 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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Section 3 Specific requirements3.3 Non-functional requirements

Reliability Portability Security…

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Section 4 Requirements table– Requirement number– Name– Description– Related requirements’ numbers and source– Related use cases’ numbers

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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”

Documents

CS3773 Software Engineering