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Types of requirement
• User requirements
• System requirements
• Domain requirements
• Functional requirements
• Non-functional requirements
Types of requirement
• User requirements– Statements in natural language plus diagrams of the
services the system provides and its operational constraints. Written for customers
• System requirements– A structured document setting out detailed descriptions of
the system services. Written as a contract between client and contractor
• Software specification– A detailed software description which can serve as a basis
for a design or implementation. Written for developers
Functional and non-functional requirements
• Domain requirements– Requirements that come from the application domain of
the system and that reflect characteristics of that domain
• Functional requirements– Statements of services the system should provide, how
the system should react to particular inputs and how the system should behave in particular situations.
• Non-functional requirements– constraints on the services or functions offered by the
system such as timing constraints, constraints on the development process, standards, etc.
What are User requirements
User requirements
• Should describe functional and non-functional requirements so that they are understandable by system users who don’t have detailed technical knowledge
• User requirements are defined using natural language, tables and diagrams
What are System requirements
System requirements
– More detailed specifications of user requirements
• Serve as a basis for designing the system
• May be used as part of the system contract
• System requirements may be expressed using system models
What are Domain requirements
Domain requirements
• Derived from the application domain and describe system characteristics and features that reflect the domain
• May be new functional requirements, constraints on existing requirements or define specific computations
• If domain requirements are not satisfied, the system may be unworkable
What is Functional Requirement
Functional Requirements
Describe functionality or system services• Depend on the type of software, expected users
and the type of system where the software is used• Functional user requirements may be high-
level statements of what the system should do BUT functional system requirements should describe the system services in detail
Types of Non-functional requirement
Non-functional requirement types
Performancerequirements
Spacerequirements
Usabilityrequirements
Efficiencyrequirements
Reliabilityrequirements
Portabilityrequirements
Interoperabilityrequirements
Ethicalrequirements
Legislativerequirements
Implementationrequirements
Standardsrequirements
Deliveryrequirements
Safetyrequirements
Privacyrequirements
Productrequirements
Organizationalrequirements
Externalrequirements
Non-functionalrequirements
Non-functional classifications
• Product requirements– Requirements which specify that the delivered product must
behave in a particular way e.g. execution speed, reliability, etc.
• Organisational requirements– Requirements which are a consequence of organisational
policies and procedures e.g. process standards used, implementation requirements, etc.
• External requirements– Requirements which arise from factors which are external to
the system and its development process e.g. interoperability requirements, legislative requirements, etc.
Features of Non-Functional Requirements
1. Non-Functional requirements mostly define the overall attributes of the “resulting” system.
IEEE Standard 830 – 1993
• List of 13 non-functional requirements:
1. Performance 2. Interface3. Operational4. Resource5. Verification6. Acceptance7. Documentation8. Security9. Portability10. Quality11. Reliability12. Maintainability13. Safety
Examples?
Some of these also overlap - - - - - -
IEEE Standard 830 – 1993
• List of 13 non-functional requirements Examples:
1. Performance: 100 transactions per minute2. Interface: capable of importing data with EDI format3. Operational: must not require more than 1 megabyte of main memory4. Resource: will use wireless encryption algorithm that is “better” than WEP 5. Verification: all data updates must be traceable6. Acceptance: must pass a user defined system test bucket 7. Documentation: user manual is needed for novice users only8. Security: user request to access any data must be authorized first 9. Portability: the system must operate with “any” relational db systems10. Quality: the system must install with zero defect11. Reliability: the system must be accessible 99.9 % of the time12. Maintainability: the system must be modifiable (e.g. designed with exits) 13. Safety: the system must not perform “chemical material discard” functions
without “explicit” user authorization.
There may be others that are important such as meeting legal standards thatIs not mentioned in this list
Difficulties in Specifying Non-functional Requirements
Difficulties in Specifying Non-functional Requirements
• The difficulties in gathering Non-Functional Requirements may be attributed to many reasons - - - - - mostly because people tend to focus on the functions and services that they need:
– Certain non-functional requirements are sometimes hard to quantify and therefore hard to express without some “trial and error prototyping”.
• e.g. : usability – Certain non-functional requirements are hard to differentiate between
functional and non-functional• e.g. security
– Certain non-functional requirements are difficult to specify because they can not be well understood or validated until much later
• e.g. reliability or quality
– Certain non-functional requirements may be conflicting• e.g. performance .vs. security .vs. reliability
– Certain non-functional requirements may be difficult to express and validate; may require more refinements.
similar
What are“Critical” Systems
“Critical” Systems
• Critical systems are systems whose failure causes significant economic, human or organizational damage:
– Business Critical System• e-commerce systems such as stock trading, reservations, etc.
– Mission Critical System• Aircraft control, manufacturing process control, etc.
– Safety (life) Critical system• Medical Device control, hazardous materials management, etc.
Requirements for System Criticality
Requirements for System Criticality
• Most of the requirements for these “business,” “mission,” and “safety” criticality deals with non-functional requirements of the a) “complete” system, not just software and b) may be expressed in general ways that need to be decomposed further:
– Performance– Reliability– Security– Usability– Safety
Again, these may be “conflicting” - - - - so what do you do?
Must prioritize the requirements, especially when there are conflicts
Performance Related Requirements
Performance Related Requirements
• These requirements mostly addresses the constraints of speed and sometimes capacity:
– System Response time to end-user such as 1 second response to user requests
– System throughput such as # of transactions per minute (time interval)
– System timing such as collection of data and responding to it within sub-second for real-time system.
– System capacity such as number of simultaneous users that may access an application (instantaneous time)
These should be specified quantitatively.
Reliability Related Requirements
Reliability Related Requirements
• These requirements addresses constraints on run-time behavior of the system:– System Availability such as the system is up certain
percentage of the time.– System Failure rate such as average mean time
between system failures to deliver the user service.
These should also be specified quantitatively.
Security Related Requirements
Security Related Requirements
• These requirements addresses the issues related to unauthorized access: to the system, to specific function, to data:– System Access protection such as firewall
requirement– Application Functional Access & Usage protection
such as authorization and authentication requirement– Data Access and Usage protection such as
authorization and encryption requirement
Security is also an important factor for other requirement suchas safety. A little hard to specify these quantitatively.
Usability Related Requirements
Usability Related Requirements
• These requirements addresses the user interface looks and user inter-actions with the system– Entry and beginners-level knowledge assumption to use the
system – Learning time and experience needed such as hours or number
of lessons to learn the system– Handling and usage such as time to complete certain tasks or
number of errors made before completing certain tasks– Likeness and delight experienced from using the system such
as availability of context sensitive help text or “re-do” capability
Some of these requirements can be and should be specified Quantitatively; delight and likeness are a bit hard to define.
Safety Critical System Requirements
Safety Critical System Requirements
• These requirements addresses everything with the safety of the system and of the usage of the system.
• These requirements deal mostly with the “shall not” requirements such as:– The system shall not allow - - - -– The system will not operate without - - -
Note that safety may be dependent on many of the other requirements:
- insecure system may be open to malicious danger - unreliable system may fail unpredictably and hurt someone - non-responsive system may miss critical data and damage something - difficult to use system may create a critical human error
Requirements measures
Requirements measures
Property MeasureSpeed Processed transactions/second
User/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
Relationship Between Requirement and Design
Requirements and design
• In principle, requirements should state what the system should do and the design should describe how it does this
• In practice, requirements and design are inseparable– A system architecture may be designed to structure the
requirements
– The system may inter-operate with other systems that generate design requirements
– The use of a specific design may be a domain requirement
Relationships between user needs, concerns and NFRs
Relationships between user needs, concerns and NFRs
User’s need
User’s concern Non-functional requirement
Function 1. Ease of use 2. Unauthorised access 3. Likelihood of failure
1. Usability 2. Security 3. Reliability
Performance 1. Resource utilisation 2. Performance verification 3. Ease of interfacing
1. Efficiency 2. Verifiability 3. Interoperability
Change 1. Ease of repair 2. Ease of change 3. Ease of transport ? 4. Ease of expanding or upgrading
capacity or performance ?
1. Maintainability 2. Flexibility 3. Portability 4. Expandability
Concern decompositionCompatibilitySafety
Collision DerailmentPersonalaccident
Hardware Software Physical
Excess speed for track conditions
Track damage
System must be able todetect and avoid excessspeed
Under what conditions can excess speed cause derailment?
What information about track damage is required by the system? How is this provided?
InterfaceExecutionEnvironment
Timing
Will a requirement affectthe performance of theexisting software?
Does a requirement needdata that isn't availablethrough the HST interface?
System must execute in the trustedAda execution environment
Can this function beprovided on the existngexecution environment?
What does 'excess speed' mean in reality?
How to write requirements:
Guidelines for writing requirements
• Invent a standard format and use it for all requirements
• Use language in a consistent way. Use shall for mandatory requirements, should for desirable requirements
• Use text highlighting to identify key parts of the requirement
Avoid the use of computer jargon !!!
Problems with natural language
Problems with natural language
• Lack of clarity – Precision is difficult without making the document difficult
to read
• Requirements confusion– Functional and non-functional requirements tend to be
mixed-up
• Requirements combination– Several different requirements may be expressed together
Alternatives to NL specification
Alternatives to NL specificationNotation DescriptionStructurednaturallanguage
This approach depends on defining standard forms ortemplates to express the requirements specification.
Designdescriptionlanguages
This approach uses a language like a programminglanguage but with more abstract features to specify therequirements by defining an operational model of thesystem.
Graphicalnotations
A graphical language, supplemented by text annotations isused to define the functional requirements for the system.An early example of such a graphical language was SADT(Ross, 1977; Schoman and Ross, 1977). More recently,use-case descriptions (Jacobsen, Christerson et al., 1993)have been used. I discuss these in the following chapter.
Mathematicalspecifications
These are notations based on mathematical conceptssuch as finite-state machines or sets. These unambiguousspecifications reduce the arguments between customerand contractor about system functionality. However, mostcustomers don’t understand formal specifications and arereluctant to accept it as a system contract. I discuss formalspecification in Chapter 9.
What is Requirement Document
The requirements document
• The requirements document is the official statement of what is required of the system developers
• Should include both a definition and a specification of requirements
• It is NOT a design document. As far as possible, it should set of WHAT the system should do rather than HOW it should do it
Users of a requirements document
Users of a requirements
document
Use the requirements todevelop validation tests forthe system
Use the requirementsdocument to plan a bid forthe system and to plan thesystem development process
Use the requirements tounderstand what system is tobe developed
System testengineers
Managers
System engineers
Specify the requirements andread them to check that theymeet their needs. Theyspecify changes to therequirements
System customers
Use the requirements to helpunderstand the system andthe relationships between itsparts
Systemmaintenance
engineers
Requirements document requirements
Requirements document requirements
• Specify external system behaviour• Specify implementation constraints• Easy to change• Serve as reference tool for maintenance• Record forethought about the life cycle of the
system i.e. predict changes• Characterise responses to unexpected events
IEEE requirements standard
• Introduction• General description• Specific requirements• Appendices• Index• This is a generic structure that must be
instantiated for specific systems
Requirements document structure• Introduction• Glossary• User requirements definition• System architecture• System requirements specification• System models• System evolution• Appendices• Index