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CS 501: Software Engineering

Lecture 7

Requirements I

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Administration

Quiz 1

• Collect after class or from reception at 301 College Avenue

• Average grade was 17 out of 20

Assignment 1: Feasibility study

Due Friday at 5:00 p.m.

Remember to send a copy to your client

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Quiz 1 -- Part of Question 1

What are the visibility requirements of your project? How does your process provide visibility?

Visibility is an aspect of the software development process -- keeping everybody informed of the progress, e.g. with schedules, milestones, presentations, reports, etc.

What risks do you see for your project? How does your process manage risk?

Risks can be categorized by (a) function, (b) timeliness, (c) resources.

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Lectures on Requirements Analysis and Specification

Requirements I: Requirements Analysis

Requirements II: Models -- Scenarios and Use Cases

Requirements III: Informal Methods of Specification

Requirements IV: Formal Methods of Specification

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Requirements Analysis and Definition

From Lecture 2

The requirements analysis and definition establish the system's services, constraints and goals by consultation with users. They are then defined in a manner that is understandable by both users and development staff.

This phase can be divided into:

• Requirements analysis

• Requirements definition

• Requirements specification

Requirements define the function of the system FROM THE CLIENT'S VIEWPOINT.

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Why are Requirements Important?

Causes of failed software projects (Standish Group study, 1994)

Incomplete requirements 13.1%Lack of user involvement 12.4%Lack of resources 10.6%Unrealistic expectations 9.9%Lack of executive support 9.3%Changing requirements & specifications 8.8%Lack of planning 8.1%System no longer needed 7.5%

The commonest mistake is to build the wrong system!

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Requirements in Software Development

Requirements

Operation andMaintenanceImplementation

Design

Feasibility andPlanning

All process models include a

requirements activity

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Requirements

System design

Testing

Operation & maintenance

Program design

Coding

Acceptance & release

Waterfall model with feedback

Feasibility study

Requirements in the Modified Waterfall Model

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Requirements in Iterative Refinement

OutlineDescription

ConcurrentActivities

Requirements

Design

Implementation

InitialVersion

IntermediateVersions

FinalVersion

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Goals During the Requirements Phase

• Understand the requirements in detail (analysis).

• Describe the requirements in a manner that is clear to the client. Ensure that the client understands the description of the requirements and their implications.

• Describe the requirements in a manner that is clear to the people who will design and implement the system.

The Requirements Documentation is the defining document that describes the goals of the system that is being built.

It may form a legal contract between client and software developers.

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The Requirements Process

FeasibilityStudy

RequirementsAnalysis

RequirementsModel

RequirementsSpecification

FeasibilityReport

Documentation ofRequirements

Work with the client to understand the requirements

Organize the requirements in a systematic and comprehensible manner

Requirements Analysis (optional)

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Requirements Analysis

High-level abstract description of requirements:

• Specifies external system behavior

• Comprehensible by customer, management and users

Should reflect accurately WHAT THE CUSTOMER WANTS:

• Services that the system will provide

• Constraints under which it will operate

Often described in a separate document for consultation with the client.

"Our understanding of your requirements is that ..."

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Requirements Documentation (continued on next slide)

The form of documentation varies, but is likely to contain the following:

General

Purpose and scope of system

Objectives and criteria for success

List of terminology, organizations involved, etc.

Description of current system(s)

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Requirements Documentation (continued)

Requirements of proposed system

Overview

Functional Requirements

Usability requirements

Non-functional requirements

System models

Scenarios

Use cases

Models used during analysis

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Requirements Documentation (continued)

Detailed specifications

Business rules, specifications, etc. (e.g., reference to an accounting standard)

Data flow, sources of data, data validation

etc., etc.,

A common fault in requirements documentation is to gloss over the details. This results in misunderstandings between the client and the developers.

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Realism and Verifiability

Requirements must be realistic, i.e., it must be possible to meet them.

Wrong: The system must be capable of x (if no known computer system can do x at a reasonable cost).

Requirements must be verifiable, i.e., it must be possible to test whether a requirement has been met.

Wrong: The system must be easy to use.

Right: After one day's training an operator should be able to input 50 orders per hour.

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Evolution of Requirements

• If the requirements definition is wrong, the system will be wrong.

• With complex systems, understanding of requirements always continues to improve.

Therefore...

• The requirements definition must evolve.

• Its documentation must be kept current (but clearly identify versions).

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New and Old Systems

In requirements analysis it is important to distinguish:

• features of the current system• proposed new features

Clients often confuse the current system with the underlying requirement.

A new system is when there is no existing system.

A replacement system (or subsystem) is when a system is built to replace an existing system.

A legacy system is an existing system that is not being replaced, but must interface to the new system.

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Functional Requirements

Requirements about the functions that the system must perform that will be identified by analyzing the use made of the system

• Functionality

• Data

• User interfaces

Understanding and specifying the functional requirements is the theme of the next three lectures.

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Non-Functional Requirements

Requirements that are not directly related to the functions that the system must perform

• Usability, documentation and training

• Reliability and quality assurance

• Performance

• Supportability

• Implementation and technical constraints

• Interfaces and compatibility

• Operation and physical environment

• Packaging and security

• Legal and business

• Resources

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Examples of Functional and Non-Functional Requirements

Privacy (NSDL digital library)

Functional requirement: Usage data for management of system

Non-functional requirement: Usage data must not identify individuals

Minimizing records (NeXT)

Functional requirement: Retain all required records

Non-functional requirement: Discard all other records

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Non-Functional Requirements

Product requirements

performance, reliability, portability, etc...

Organizational requirements

delivery, training, standards, etc...

External requirements

legal, interoperability, etc...

Marketing and public relations

Example: In the NSDL, the NSF wanted a system that could be demonstrated by the end of 2002

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Example of Non-Functional Requirements

Example: Library of Congress Repository should use systems that the library staff are familiar with:

• Hardware and software systems (IBM/Unix)

• Database systems (Oracle)

• Programming languages (C and C++)

As a federal library:

• Regulations covering government contracting

• Importance of developing a system that will be respected by other major libraries

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Unspoken Requirements

Examples:

• Resistance to change

• Departmental friction

• Management strengths and weaknesses

Discovering the unspoken requirements is often the most difficult part of developing the requirements.

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Requirements Analysis: Interviews with Clients

CLIENT INTERVIEWS ARE THE HEART OF REQUIREMENTS ANALYSIS AND DEFINITION.

Allow plenty of time.

Clients may have only a vague concept of requirements.

• Prepare before you meet with them

• Keep full notes

• If you do not understand, delve further, again and again

• Repeat what you hear

• Small group meetings are often most effective

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Requirements Analysis

1. Identify the stakeholders:

• Who is affected by this system?

ClientSenior managementProduction staffComputing staffCustomersetc., etc., etc.,

Example: Andrew project (Carnegie Mellon and IBM)

• Who can disrupt this project?

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Requirements Analysis

2. Understand the requirements in depth:

• Domain understanding

Examples: Philips light bulbs

• Understanding of the real requirements of all stakeholders

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Viewpoint Analysis

Example: University Admissions System

• Applicants

• University administrationAdmissions officeFinancial aid officeSpecial offices (e.g., athletics, development)

• Computing staffOperationsSoftware development and maintenance

• Academic departments

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Requirements Analysis

3. Organize the requirements:

• Classification into coherent clusters

(e.g., legal requirements)

• Recognize and resolve conflicts

(e.g., functionality v. cost v. timeliness)

Example: Dartmouth general ledger system

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From an Exam Question

An online information system is being developed using a modified version of the Waterfall model. It is likely to be based on Web technology.

(i) How much should the choice of technology be considered during the feasibility study?

(ii) In how much detail should the choice of technology be specified during the requirements phase of the project?

(iii) At what stage should the decision be made to use an Apache Web Server 2.0 with Tomcat 4.1?

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From an Exam Question (Answer)

How much should the choice of technology be considered during the feasibility study?

During the feasibility study it is important to know that the project is technically feasible.

This can be achieved by identifying one possible technical approach and analyzing it sufficiently to show that it is capable of fulfilling the requirements of the system. It can also be used to estimate costs of hardware, software, etc.

However, this is only a possible approach. When the system design is carried out in detail, totally different technology may be chosen (e.g., not Web-based).

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From an Exam Question (Answer)

In how much detail should the choice of technology be specified during the requirements phase of the project?

A requirement is a statement of need as expressed by a client.

The client's requirements are that the system collects certain data, saves it, and carries out specified processes, e.g., displaying it, performing calculations, etc.

The decision of how to store and manipulate the data (e.g., using specific Web technology) is usually not a requirement of the client. It comes later, as part of the design.

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From an Exam Question (Answer)

At what stage should the decision be made to use an Apache Web Server 2.0 with Tomcat 4.1?

This is part of the System Design

*