CS708 Official Handouts

LECTURE 01

INTRODUCTION

Requirements form the basis for all software products Requirements engineering is the process, which enables us to systematically determine the requirements for a software product

Requirement

Something required, something wanted or needed

• Webster’s dictionary

There is a huge difference between wanted and needed and it should be kept in mind all the time

Software Requirements

A complete description of what the software system will do without describing how it will do it is represented by the software requirements Software requirements are complete specification of the desired external behavior of the software system to be built They also represent External behavior of the system Software requirements may be:

• Abstract statements of services and/or constraints
• Detailed mathematical functions

Software requirements may be:

• Part of the bid of contract
• The contract itself
• Part of the technical document, which describes a product

IEEE Definition

A condition or capability that must be met or possessed by a system...to satisfy a contract, standard, specification, or other formally imposed document. (IEEE Std 729)

Sources of Requirements

Stakeholders (People affected in some way by the system), Documents, Existing system, Domain/business area

Levels of Software Requirements

Stakeholders describe requirements at different levels of detail

• “What versus How”
• “One person’s floor is another person’s ceiling”

What versus How

Importance of Software Requirements

The hardest single part of building a software system is deciding what to build...No other part of the work so cripples the resulting system if done wrong. No other part is difficult to rectify later. (Fred Brooks)

Examples of Requirements

The system shall maintain records of all payments made to employees on accounts of salaries, bonuses, travel/daily allowances, medical allowances, etc. The system shall interface with the central computer to send daily sales and inventory data from every retail store The system shall maintain records of all library materials including books, serials, newspapers and magazines, video and audio tapes, reports, collections of transparencies, CD-ROMs, DVDs, etc. The system shall allow users to search for an item by title, author, or by International Standard Book Number The system’s user interface shall be implemented using a web browser The system shall support at least twenty transactions per second The system facilities which are available to public users shall be demonstrable in ten minutes or less

Kinds of Software Requirements

Functional requirements, Non-functional requirements, Domain requirements, Inverse requirements, Design and implementation constraints

Functional Requirements

Statements describing what the system does Functionality of the system Statements of services the system should provide

• Reaction to particular inputs
• Behavior in particular situations

Sequencing and parallelism are also captured by functional requirements Abnormal behavior is also documented as functional requirements in the form of exception handling Functional requirements should be complete and consistent Customers and developers usually focus all their attention on functional requirements

Functional Requirements Example # 1

The system shall solve a quadratic equation using the following formula

x = (-b+sqrt(b2 – 4*a*c))/2*a

Functional Requirements Example # 2

The user shall be able to search either the entire database of patients or select a subset from it (admitted patients, or patients with asthma, etc.)

Functional Requirements Example # 3

The system shall provide appropriate viewers for the user to read documents in the document store

Functional Requirements Example # 4

Every order shall be allocated a unique identifier (ORDER_ID) which the user shall use to access that order

Functional Requirements Example # 5

The system shall allow customers to return non-perishable items within fifteen days of the purchase. A customer must present the original sale receipt to return an item

Comments on Examples

Notice the level of detail in different requirements described above. Some are very detailed compared to others Notice the ambiguity in the requirement, which uses the term ‘appropriate viewers’ This requirement does not mention the formats of documents and types of viewers, which can be used Notice the ambiguity in the requirement for solving the quadratic equation. The requirement does not speak about the possibility when the value of ‘a’ is zero

x = (-b+sqrt(b2 – 4*a*c))/2*a

Incomplete and ambiguous requirements are open to multiple interpretations and assumptions This can lead to the development of poor quality, or faulty, software products