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 (NOT 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: As a user who found a new job announcement, I want to add a new position to the website so s/he can start working on doing the initial research and apply to it System requirement: A registered user on the academic jobs website should be able to add a new position listing with the name of the school and academic unit, date of posting, date of expiry, application deadline, and contact and application details. The interaction fails if: the position is already listed, the application deadline is in the past, position announcement is expired, or the contact information is missing. If fails, point mistakes to user and ask the user to fix and resubmit.
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 (e.g., one semester), 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 all of 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 N on-functional requirements Pr oduc t O rga nizational E xternal requirements requirements requirements E fficiency Relia bility P or tability Inter operability E thical r equirements requirements requirem ents requirements requirements U sa bility D elivery Implementation Standa rds L egislative requirements requirements requirements requirements requirements Perfor mance Spac e Privacy Safety requirements requirements requirements requirements
Non-functional requirements It is important to be able to test/verify/check non- functional requirements Property Measure Speed Processed transactions/second User/Event response time Screen refresh time Size K Bytes Number of RAM chips Ease of use Training time Number of help frames Reliability Mean time to failure Probability of unavailability Rate of failure occurrence Availability Robustness Time to restart after failure Percentage of events causing failure Probability of data corruption on failure Portability Percentage of target dependent statements Number of target systems
Requirements documents Your project descriptions (Available on the website) Very readable Some ambiguities Examples of templates: 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 engineering Requirem ents F easib ility elicitation and stud y analysis R equirem ents specification F easib ility Requirem ents rep ort va lidation System m odels U ser and sy stem req uirem ents Req uirem ents do cu m ent
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
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