Chapter 6

Requirements Engineering

Fall 2011

Preparation for Requirements Engineering

PlanFor

RequirementsActivities

Agreeing onResources,

Process andSchedule for

RequirementsActivities

Obtain andOrganize the Agreed upon

Resources andProcess

1. Prior to actually performing the requirements engineering activities,it is important to plan for the resources and time needed to perform this crucial step in software engineering.

2. Some organizations even perform requirements engineering as a separate ,stand-alone activity and price it separately, with the option of folding the costInto the whole project if they get the call to complete the software project.

Major Requirements Engineering Activities

• Elicitation• Documentation and definitions• Specifications• Prototyping• Analysis• Review and Validations• Agreement and acceptance

Requirements Engineering Activities

RequirementsElicitation

RequirementsAnalysis

RequirementsDefinition

RequirementsPrototyping

RequirementsReview

RequirementsSpecification

RequirementsAgreement &Acceptance

Why is this set of activities important and why should requirements be documented?

• Clear requirements are needed for design and implementation activities.

• Requirements documentation is needed to create test cases and test scenarios - - - especially for large systems where the test team is a separate group of people from the developers.

• Requirements document is needed to control potential scope-creep.

• Requirements document is needed to create user training material, marketing material, and documents for support and maintenance.

• Requirements document provides a way to segment a large project, prioritize releases , and easier project management

Think about agile processes where this crucial step may sometimes becompromised by the novice software engineers.

Requirements Elicitation

• Requirements may:– Be given to the software engineers

• Second or third follow-on release of a “planned” sequences of software product where requirements are already established

• Requirements provided as a part of a request for price quotation for a software development project

– Have to be established by software engineers• Users have an understanding of only the requirements

related to their specific job tasks• The business rationale and goals are not clear• There may be contradicting and incomplete requirements

stated by the users and customers

High Level Requirements Elicitation

• Need to seek out the business and management perceptions and goals for the software project– Business opportunity and business needs– Justification for the project– Scope– Major constraints– Major functionality– Success Factor– User characteristics

6-Dimensions of Detailed Requirements Elicitation

Requirements

Individual Functionality

Business Workflow

Data and Data formats

User Interfaces

Existing & Systems Interfaces Other Constraints: Performance,Reliability, etc.

Requirements Analysis

• Requirements analysis is composed of:

– Categorizing the requirements– Prioritizing the requirements

Requirements Categorization

• By detailed requirements area:

– Individual functionality– Business flow– Data and information needs– User interfaces– Other interfaces to external systems– Various constraints

• Performance• Security• Reliability• Etc.

Requirements Categorization (cont.)

• Using OO’s Use Case, which identifies:

– Basic functionality– Pre-conditions of functionality– Flow of events or scenarios– Post-conditions for the functionality– Error conditions and alternative flows

• Using OO Use Case methodology which identifies:

– Actors (or all external interfaces with the system, including the users)

– Related use cases– Boundary conditions

Requirements Categorization (cont.)

• View Oriented Requirements Definition (VORD) is based on the concept that requirements are viewed differently by different people:

– Identify stakeholders and their viewpoints of the requirements

– Categorize the viewpoints of requirements and eliminating any duplication

– Refine the identified viewpoints of requirements– Map the viewpoints of requirements to the system

and the services that the system must provide

Requirements Prioritization

• Most of the time we have limitations of :

– Time– Resources– Technical capabilities

• We need to prioritize the requirements to satisfy these limitations

Requirements Priorities

• Criteria for prioritization:

– Current user/customer demands or needs– Competition and current market condition– Anticipated future and new customer needs– Sales advantages– Critical problems in existing product

• These are often subjective and requirements should be prioritized with the help of many of the stakeholders.

A Simple Requirements Prioritization List

Req. #Brief Req.description

Req. source Req. priority Req. Status

# 1One page query must respond in less than 1 second

A Major accountMarketingRep.

Priority 1* Accepted forthis release

# 2

Help textmust befield sensitive

Large accountusers

Priority 2 PostponedFor nextrelease

* Priority may be 1, 2, 3, or 4, with 1 being the highest

Requirements Comparison and Prioritization

• Requirements prioritization is an activity of comparing the requirements and placing them in some order relative to each other.

– Done with just experience and gut feel– Done with a little more rigor:

• Sort by priority groups (e.g. previous 2 charts) where the priority groups are based on some prioritization criteria list

• Pair-wise comparison, normalize and compute relative value using the Analytical Hierarchical Process (AHP) – see pages 159-161 of text book

Pairwise Comparison Matrix

17

Req1 Req2 Req3

Req1 1 3 5

Req2 1/3 1 1/2

Req3 1/5 2 1

Sum each column and divide each value in the column by the sum

Req1 Req2 Req3

Req1 .65 .5 .77

Req2 .22 .17 .08

Req3 .13 .33 .15

Pairwise Comparison Matrix

18

• Sum each row: • Row 1 (Req1) = 1.92• Row 2 (Req2) = .47• Row 3 (Req)3 = .61

• Divide each sum by the number of requirements:

• 1.92/3 = .64• 0.47/3 = .15• 0.61/3 = .20

• Results• Req1 highest (64% of total req value)• Req3 second highet (20%)• Req2 lowest (15%)

Only good for small number of requirements

Requirements Definition/Prototyping/Review

• Once the requirements are solicited, analyzed and prioritized, more details must be spelled out. Three major activities which may be intertwined must be performed:

– Requirements definition– Requirements prototyping– Requirements reviewing

Requirements Definitions

• Requirements definitions may be written in different forms:

– Simple Input/process/output descriptions in English– Dataflow diagrams (DFD)– Entity Relations diagram (ERD)– Use Case Diagram from Unified Modeling Language (UML)

• Once the requirements are defined in detail using any of the above forms, they still need to be reviewed by the users/customers and other stakeholders.

Requirement Definition using English and Input-Process-Output Diagram Form

Req. # Input OutputProcess

# 12: customerorder

- Items by type and quantity- Submit request

- Accept the items and respective quantities

- Display acceptance message and- Ask for confirmation message

Requirements Definition using DFD

OrdersOrder Processing

Customer Info DB

Customer credit, address, etc.

Inventory Info.

Packaging

Invoice

Product avail.Info.

Package Data

Packagingdetails

ShippingInstruct.

Customer

Requirements Definition using Entity- Relation-Diagram (ERD)

Author Bookwrites1 m

Cardinality: specifies the number of occurrences of entities

Author Book

Modality: specifies the necessities of relationship to exist

Requirements Definition specifying Entity and Attributes

Employee

Address Name Age

Street City State Zip

(a) Graphical form(a) Tabular form

Employee

- Name

- Address

- Age

- Street- City - State- Zip

Register For SectionAdd Course

Add Section

Add StudentChoose Section

Requirements Definition using Use Case Diagram

Student

Registrar

Requirements Prototyping

• Requirements prototyping mostly address the User Interface (UI) part of the requirement in terms of:– Visual looks (size, shape, position, color)– Flow (control and logical flow)

• The prototyping may be performed in one of the two modes:– Low fidelity : using paper/cardboard to represent

screens and human to move the boards– High fidelity : using automated tools such as

Visual Basic to code the screens and direct the logical flow of these screens

Requirements Specification

• Once the requirements are defined, prototyped and reviewed, it must be placed into a Requirements Specification document

• IEEE /EIA Standard 12207.1-1997 may be purchased from IEEE. It outlines the guideline for 3 major sections of a requirements specification document.– Introduction– High Level description– Detailed descriptions

• Details of each functionality (input-out-process)

• Interfaces, including user interfaces and network interfaces

• Performance requirements (response time, throughput, etc.)

• Design constraints, standards, etc.

• Additional attributes such as security, reliability, etc.

• Any additional unique requirements

Software Requirements Specification

28

The Software Requirements Specification (SRS) specifies the requirements for a Computer Software Configuration Item (CSCI) and the methods to be used to ensure that each requirement has been met. Requirements pertinent to the CSCIs external interfaces may be presented in the SRS or in one or more interface requirements specifications (IRSs). 1.0 Introduction to the Document

1.1 Purpose of the ProjectState the purpose of the system or subsystem to which this document applies.

1.2 Scope of the Project1.3 Acronyms, Abbreviations, Definitions1.4 References1.5 Outline of Rest of SRS

2.0 General Description of Product2.1 Context of Product2.2 Product Functions2.3 User Characteristics2.4 Constraints2.5 Assumptions and Dependencies

Software Requirements Specification

29

3.0 Specific Requirements3.1 External Interface Requirements

3.1.1 User Interfaces3.1.2 Hardware Interfaces3.1.3 Software Interfaces3.1.4Communications Interfaces

3.2 Functional Requirements3.2.1 Class 1

3.2.1.1 Short, imperative sentence stating highest ranked functional requirement.3.2.1.2 DescriptionA full description of the requirement. 3.2.1.3 CriticalityDescribes how essential this requirement is to the overall system. 3.2.1.4 Technical issuesDescribes any design or implementation issues involved in satisfying this requirement. 3.1.2.5 Cost and scheduleDescribes the relative or absolute costs associated with this issue.

Software Requirements Specification

30

3.2.1.6 RisksDescribes the circumstances under which this requirement might not able to be satisfied, and what actions can be taken to reduce the probability of this occurrence.

3.1.2.7 Dependencies with other requirementsDescribes interactions with other requirements.

3.1.2.8 ... others as appropriate3.2.2 Class 23.2.3 Class 3. . .

3.3 Performance Requirements3.4 Design Constraints3.5 Quality Requirements3.6 Safety requirements3.7 Security and Safety Requirements3.8 Computer Resource Requirements3,9 Personnel Requirements3,10 Training Related Requirements3.11 Logistics Related Requirements3.12 Other Requirements

Software Requirements Specification

31

4.0 Notes4.1 Intended Use4.2 Definitions Used in this Document4,3 Abbreviations Used in this Document4.4 Changes from Previous Issue

5.0 Appendices

Requirements “Sign Off”

• Having a requirements specification agreed to and signed off is important:

– Serves as a milestone marker and formally exits a phase of software engineering

– Serves as baseline from which any future changes can be monitored and controlled

Requirements Traceability

• Capability to trace a requirements is needed to ensure that the product has fully implemented the requirements.

• We need to trace requirements:– Requirements from source (people and documents)– Requirements to later steps (design, implementation)

• We also need to link requirements to other “pre-requisite” requirements.

Partially filled Traceability Table

Requirement Design Code Test Other

Reqs.

1 comp. X Module 3 Test Case 32 2,3

2 comp. w Module 5 Test Case 16 1

3 comp. X Module 2 Test Case 27 1

4

5

6

7