Software Requirements and the Requirements Engineering Process

Chapters 5 and 6

References Software Engineering. Ian Sommerville. 6th

edition. Pearson. Code Complete. Steve McConnell. (CC) The art of requirements triage. Alan M. Davis.

Computer. IEEE. March 2003. Testing whether requirements are right. Robin F.

Goldsmith, JD. Go Pro Management Inc. NYC Spin Meeting. December 2004. (RG)

Software Requirements. Karl E. Wieger. Windows Press.

Dr. Gotel’s research web page Dr. Barrett’s slides, NYU

What is a requirement? It is about WHAT not HOW Nothing can be said obvious Requirements are the descriptions of the services

provided by a system and its operational constraints It may range from a high level abstract statement to

a detailed mathematical specification It may be as complex as a 500 pages of description It may serve as the basis for a bid for a contract or

the basis for the contract itself

What is requirements engineering? It is the process of discovering,

analyzing, documenting and validating the requirements of the system

Each software development process goes through the phase of requirements engineering

Why requirements? What are the advantages of a complete set of

documented requirements? Ensures the user (not the developer) drives

system functionalities Helps avoiding confusion and arguments Helps minimizing the changes

Changes in requirements are expensive. Changing the requirements costs: 3 x as much during the design phase 5-10 x as much during implementation 10-100 x as much after release [Code Complete,

p30]

Why requirements? 2/3 of finished system errors are

requirements and design errors [RG] A careful requirements process doesn’t

mean there will be no changes later Average project experiences about 25%

changes in the requirements This accounts for 70-80% if the rework of

the project [Code Complete, p40] Important to plan for requirements changes

The case of critical applications

Different levels of abstraction User requirements (abstract +)

Usually the first attempt for the description of the requirements

Services and constraints of the system In natural language or diagrams Readable by everybody Serve business objectives

System requirements (abstract -) Services and constraints of the system in detail Useful for the design and development Precise and cover all cases Structured presentation

Example User requirement: The library system should

provide a way to allow a patron to borrow a book from the library.

System requirement: The library system should provide a withdraw interaction that allows a patron to withdraw a book given the isbn and copy number of the book to be withdrawn. The interaction fails if: the book is already withdrawn, the book is not in the library's collection, the patron has already withdrawn 5 books, the patron owes more than $5, the book is on hold by someone else. Otherwise…(To be completed)

Types of requirements Functional requirements

Services the system should provide What the system should do or not in reaction to particular

situations Non-functional requirements

Constraints on the services or functions offered by the system Examples: Timing constraints, constraints on the development

process (CASE, language, development method…), standards etc

Domain requirements From the application domain of the system May be functional or non-functional Examples: Medicine, library, physics, chemistry

Note: You can have user/system functional/non-functional requirements.

User requirements First attempt to describe functional and non-

functional requirements Understandable by the user

Problems: Lack of clarity - ambiguous language Requirements confusion - functional, non-functional

requirements, design information are not distinguished Requirements amalgamation – several requirements

are defined as a single one Incompleteness – requirements may be missing Inconsistency – requirements may contradict

themselves

User requirements Guideline to minimize the issues:

Separate requirements – separate the requirements, separate functional and non-functional requirements, requirements must be clearly identified (e.g. by a number)

Include a rationale for each requirement – helps clarify reasoning behind the requirements and may be useful for evaluating potential changes in the requirements

Invent or use a standard form/template Distinguish requirements priorities

Example: MoSCoW (Must, Shall, Could, Want/Will (no TBD)) Avoid technical jargon Testable (write test cases) Deliverables

System requirements Elaborate the user requirements to get

a precise, detailed and complete version of them

Used by designers and developers An important aspect is how to

present/write system requirements Natural language is often used!

The difference between user and system requirements must not be thought as a detail

System requirements Example: If sales for current month are below

target sales, then report is to be printed unless difference between target sales and actual sales is less than half of difference between target sales and actual sales in previous month, or if difference between target sales and actual sales for the current month is less than 5%.

Problems: Difficult to read Ambiguity: sales and actual sales, 5% of what? Incomplete: what if sales are above target sales?

Writing system requirements Natural language (informal requirements)

Reviled by academics But widely practiced: everybody can read them, finding a

better notation is hard Structured natural language

Forms/templates are used to bring some rigor to natural language presentations

Graphical notations Using boxes, arrows… but they mean different things to

different people Formal specification

Based on logic, state machines… Hard to understand for many people

An analogy Archimedes (ca. 250 bc)

Any sphere is equal to 4 times the cone which has its base equal to the greatest circle in the sphere and its height equal to the radius of the sphere.

Today V = 4/3 pi r 3

How is this bit of history relevant for software requirements? Formal is better only if everybody understands it It may take a long time to find a good notation Software requirements is an area of research

Non-functional requirements They can be further categorized into:

Product requirements Product behavior Ex: Timing, performance, memory, reliability, portability,

usability Organizational requirements

Policies and procedures in the customer’s and developer’s organizations

Example: Process requirements, implementation requirements, delivery requirements

External requirements Factors externals to the system and the development process Example: Interoperability, legislation, ethics

Non-functional requirements may be more critical than functional requirements. (The system may be useless…)

Non-functional requirements

Performancerequirements

Spacerequirements

Usabilityrequirements

Efficiencyrequirements

Reliabilityrequirements

Portabilityrequirements

Interoperabilityrequirements

Ethicalrequirements

Legislativerequirements

Implementationrequirements

Standardsrequirements

Deliveryrequirements

Safetyrequirements

Privacyrequirements

Productrequirements

Organizationalrequirements

Externalrequirements

Non-functionalrequirements

Non-functional requirements

It is important to be able to test/verify/check non-functional requirementsProperty Measure

Speed Processed transactions/secondUser/Event response timeScreen refresh time

Size K BytesNumber of RAM chips

Ease of use Training timeNumber of help frames

Reliability Mean time to failureProbability of unavailabilityRate of failure occurrenceAvailability

Robustness Time to restart after failurePercentage of events causing failureProbability of data corruption on failure

Portability Percentage of target dependent statementsNumber of target systems

Requirements documents The library system requirements

document (Available in the Blackboard) Very readable Some ambiguities

Examples of templates (Available in the Blackboard): Microsoft template. Karl E. Wiegers.

Software Requirements. Windows Press. Volere template

http://www.volere.co.uk

Requirement engineering 5 important activities:

Feasibility study Requirements elicitation and analysis Requirements documentation Requirements validation Requirements management

Requirement engineeringFeasibility

study

Requirementselicitation and

analysisRequirementsspecification

Requirementsvalidation

Feasibilityreport

Systemmodels

User and systemrequirements

Requirementsdocument

Feasibility study It is done at first to decide whether

or not the project is worthwhile Look at different perspectives:

Market analysis, financial, schedule, technical, resource, legal…

Should make you aware of the risks

Feasibility study Doing the study

Consult information sources: managers, software engineers, end users…

Based on information collection (interviews, surveys, questionnaires…)

Should be short (2-3 weeks) 2 examples of feasibility studies are

posted in the Blackboard

Feasibility study What if the system wasn’t implemented? What are current process problems? Do technical resources exist? What is the risk associated with the technology? Is new technology needed? What skills? How will the proposed project help? How does the proposed project contribute to

the overall objectives of the organization? Have the benefits identified with the system

being identified clearly?

Feasibility study What will be the integration problems? What facilities must be supported by the

system? What is the risk associated with cost and

schedule? What are the potential

disadvantages/advantages? Are there legal issues? Are there issues linked with the fact that this is

an offshore project? …