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SOFTWARE ENGINEERING LECTURE 3

• Today: Requirements Analysis

• Requirements tell us what the system should do - not how it should do it.

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THE BASIC WATERFALL MODEL

Requirements analysis

Maintenance

Testing

Implementation

Design

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PROTOTYPING FOR REQUIREMENT ANALYSIS

Requirements analysis - V&V

Maintenance

- V&V

Testing - V&V

Quick Implementa-tion - V&V

Design - V&V

V&V = Verification and Validation

Quick Design - V&V

Implementation

- V&V

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REQUIREMENTS ANALYSIS

• Aims at producing a requirements specification.• Is generally the most crucial phase of an average

software project - if it succeeds then a complete failure is unlikely.

• The requirements specification can be used as a basis for a contract.

• If so, then the requirements specification will also be eventually used to evaluate if the software fulfills the requirements.

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… REQUIREMENTS ANALYSIS

• First the requirements must somehow be extracted.

• As users generally can not work with formal specifications, natural language specifications must often be used

• Then the requirements must be documented to be used in the later stages of software development. Now a more formal specification method may be possible.

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REQUIREMENT SPECIFICATION DESCRIPTION TECHNIQUES

• Natural language- Understandable for users

• Graphical languages- Simple languages (like ER) work well in practice, but are usually not designed for other purposes than requirement specification.

• General formal languages- Usually much too difficult to understand even for an above average user- You may be able to verify the system, but how can you verify the requirements?

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GOOD REQUIREMENTS SPECIFICATION QUALITIES

• Complete• Accurate• Unambiguous• Verifiable (How can you verify ”user friendliness”?)• Consistent• Modifiable (also the requirements change)• Traceable (where has each requirement come

from?)

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OVERALL STRUCTURE FOR REQ. SPEC. (Ansi/IEEE Standard 830)

• 1. Introduction 1.1. Purpose 1.2. Scope 1.3. Definitions, Acronyms and Abbreviations 1.4. References 1.5. Overview2. General Description 2.1. Product Perspective 2.2. Product Functions 2.3. User Characteristics 2.4. General Constraints 2.5. Assumptions and Dependencies3. Specific Requirements

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ANSI/IEEE: Specific requirements

• 3. Specific requirements 3.1. Functional Requirements 3.2. External Interface Requirements 3.3. Performance Requirements 3.4 Design Constraints 3.4.1. Standards Compliance 3.4.2. Hardware Limitations … 3.5. Attributes 3.5.1. Security 3.5.2. Maintainability … 3.6. Other Requirements 3.6.1. Data Base …

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ANSI/IEEE: FUNCTIONAL REQUIREMENTS

• 3.1. Functional Requirements 3.1.1. Functional Requirement 1 3.1.1.1 Introduction 3.1.1.2 Inputs 3.1.1.3 Processing 3.1.1.4 Outputs 3.1.2 Functional Requirement 2 … 3.1.n Functional Requirement n

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REQUIREMENTS SPECIFICATION - AN EXAMPLE

• A specification for an imaginary library system• From Van Vliet’s book

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TECHNIQUES FOR GETTING THE REQUIREMENTS FROM USERS

• Asking- Interview- Questionnaire- ”Brainstorming” sessions

• Analysing an existing system- We must understand how the new system will differ from any old such system

• Analysing the environment- e.g. process analysis

• Prototyping- Gives best feedback and more formal specifications but can be expensive

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REQUIREMENTS ANALYSIS - What can go wrong?

• Missing specifications- Happens often- Experience helps- Sometimes it is impossible to notice

• Contradictions- Do not document the same thing many times- Integrate different users’ views with the users

• Noise- Do not include material which does not contain relevant information

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REQUIREMENT SPECIFICATION - What can go wrong? (2)

• Documenting a solution rather than the problem- If the users know some information technology, they want to start solving the problem as they express it.- Many formal (also graphical) methods tend to direct the process into this.

• Unrealistic requirements- Although we model the problem rather than the solution, it is good to have some idea of what is possible.

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WHO SHOULD DO REQUIREMENT SPECIFICATION?

• Someone who can communicate with the users• Someone who has experience• Someone who knows similar systems and/or the

application area• Someone who knows what is possible and how

(and how much work is roughly needed).