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SWE 513: Software Engineering

Requirements I

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Feedback in the Waterfall Model

Requirements Analysis

System design

Unit & Integration Testing

System Testing

Operation & Maintenance

Program design

Coding

Acceptance Testing

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Iterative Refinement

OutlineDescription

ConcurrentActivities

Requirements

Design

Implementation

InitialVersion

IntermediateVersions

FinalVersion

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From an Old Exam Question

A computing system is likely to need some sort of database

(i) At what stage in the waterfall process, would the decision be made to use a relational database? Give the reasons for your answer.

(ii) At what stage in the waterfall process, would the decision be made to use an Oracle database? Give the reasons for your answer.

(iii) At what stage in the waterfall process would the database schema be specified? Give the reasons for your answer.

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From an Old Exam Question (Answer)

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 the relational database model) is usually not a requirement of the client. It comes later, as part of the design.

However. During the feasibility study it is important to know about relational databases, such as Oracle, and to study their capabilities.

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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 most common mistake is to build the wrong system!

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Evolution of Requirements

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

• 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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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

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The Requirements Process

FeasibilityStudy

RequirementsAnalysis

RequirementsDefinition

RequirementsSpecification

FeasibilityReport System

Models Definition ofRequirements

Specification ofRequirements

RequirementsDocument

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Functional Requirements

Requirements about the functions that the system must perform

• Functionality

• Data

• Interfaces

• Users and human factors

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Example of Functional Requirements

BU Registration System

• Support for complex digital objects. (How many? What size?)

• Access management. (What users? What objects? Policies?)

• Identification. (Which identification system?)

• Information hiding. (Where are the interfaces?)

• Open protocols and formats. (How are these chosen?)

• Integration with existing systems (What legacy systems must be accommodated?).

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Non-Functional Requirements

Requirements about the context in which the system is built

• Documentation and training

• Resources

• Security

• Physical environment

• Quality assurance

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Examples of Functional and Non-Functional Requirements

Privacy

Functional requirement: Usage data for management of system

Non-functional requirement: Usage data must not identify individuals

Minimizing records

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

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Example of Non-Functional Requirements

Example: Library System

• Hardware and software systems (IBM/Unix)• Database systems (Oracle)• Programming languages (C and C++)

• Regulations covering government contracting

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

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Unspoken Requirements

Example:

• Resistance to change

• Departmental friction

• Management strengths and weaknesses

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Requirements Analysis and Definition

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

Described in a Requirements Document that can be understood by the client.

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Requirements Analysis

1. Identify the stakeholders:

• Who is affected by this system?

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

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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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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 don't understand, delve further

• Repeat what you hear

• Small group meetings are often most effective

Clients often confuse the current system with the underlying requirement.

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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)