Requirements
Reference: Software Engineering, by Ian Sommerville, 6th edition, Chapters 5, 6, & 8
CMSC 345, Spring 2003 2
Objectives• To introduce and contrast user and system
requirements• To explain functional and non-functional
requirements• To present guidelines for writing system
requirements• To introduce the concept of use cases for
describing functional requirements
CMSC 345, Spring 2003 3
Requirements Engineering• The process of establishing
– the services that are required of the system, and
– the constraints under which it operates and is developed
• The requirements themselves are the descriptions of both of these items.
CMSC 345, Spring 2003 4
Who’s Who?• Client – the person(s) paying for the
development and will become the owner of the product
• Customer – the person who will buy the product off the shelf (mass marketing), or who has the final say as to whether the product is acceptable (in-house development). May be the same as the client
• Stakeholder – anyone who should have some direct or indirect influence on the system requirements
Reference: Mastering the Requirements Process, Robertson and Robertson
CMSC 345, Spring 2003 5
Types of Requirements• User requirements
– Should describe requirements so that they are understandable by those who do not have detailed technical knowledge. Written mainly for customers (end users)
• System requirements– A structured document setting out detailed descriptions of
the system services and constraints. Written as a contract between client and contractor
• Software design specification– An abstract description of the software design that can
serve as a basis for a more detailed design. Bridges the gap between requirements and design. Written for developers
CMSC 345, Spring 2003 6
Requirements ReadersClient managersSystem end-usersClient engineersContractor managersSystem architects
System end-usersClient engineersSystem architectsSoftware developers
Client engineers (perhaps)System architectsSoftware developers
User requirements
System requirements
Software designspecification
CMSC 345, Spring 2003 7
Functional and Non-functional Requirements
• 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.
CMSC 345, Spring 2003 8
Functional Requirements
• Describe functionality or system services• These services depend on
– the type of software being developed– the expected users of the software
• 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.
CMSC 345, Spring 2003 9
Functional Requirement Examples
• The user shall be able to add or delete problems to/from the problem collection.
• The user shall be able to preview an examination on the monitor.
• A student shall be able to take an examination on-line.
• The system will automatically grade an examination upon completion by the student.
CMSC 345, Spring 2003 10
Requirements Imprecision• Problems arise when requirements are not
precisely stated.• Ambiguous requirements may be interpreted in
different ways by developers and clients.• Requirements should also be verifiable.• Examples:
– The user shall be able to modify the problem collection.
– The user shall be able to use an existing exam.
CMSC 345, Spring 2003 11
Requirements Completeness and Consistency
• In principle, requirements should be both complete and consistent.
• Complete– They should include descriptions of all required
functionality• Consistent
– There should be no conflicts or contradictions in the descriptions of the system functions
• In practice, it is impossible to produce a complete and consistent requirements document.
CMSC 345, Spring 2003 12
Non-functional Requirements• Define system properties and constraints• Examples:
– response time– storage requirements– process requirements (e.g., must use a particular
CASE system, programming language, or development method)
• Non-functional requirements may be more critical than functional requirements. If one is not met, the system may be useless.
CMSC 345, Spring 2003 13
Non-functional Requirements Examples
Product requirements- An examination shall accommodate true/false,
multiple choice, and short answer questions.- An exam question and the space for its answer
must not be divided between two printed pages.
- The system must run under Red Hat Linux, Version 6.2.
- The system must be written in C++, compilable using Microsoft Visual C++, Version 6.0.
CMSC 345, Spring 2003 14
More Non-functional Requirements
User interface requirements- The user interface shall be text-based.- The user interface shall be menu-driven.- The UMBC logo shall always be displayed
in the upper right-hand corner of the screen.
CMSC 345, Spring 2003 15
More Non-functional Requirements
Organizational requirements– The system development process and deliverable
documents shall conform to the process and deliverables defined in XYZCo-SP-STAN-95.
External requirements– The system shall not disclose any personal
information about customers apart from their name and reference number to the operators of the system.
CMSC 345, Spring 2003 16
NFRs Must Be Verifiable, Too!
• Non-verifiable– The system should be easy to use by experienced
controllers and should be organized in such a way that user errors are minimized.
• Verifiable– Experienced controllers shall be able to use all the
system functions after a total of two hours training. After this training, the average number of errors made by experienced users shall not exceed two per day.
CMSC 345, Spring 2003 17
Some Requirements MeasuresProperty 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
CMSC 345, Spring 2003 18
Writing Requirements• Requirements may be written using a natural
language (common for user requirements)– Lack of clarity, possibly ambiguous– Requirements confusion
• Functional and non-functional requirements tend to be mixed-up
– Requirements amalgamation• Several different requirements may be expressed together
• Tables and diagrams may help
CMSC 345, Spring 2003 19
Example:Editor Grid Requirement
2.6 Grid facilities To assist in the positioning of entities on a diagram, the user may turn on a grid in either centimetres or inches, via an option on the control panel. Initially, the grid is off. The grid may be turned on and off at any time during an editing session and can be toggled between inches and centimetres at any time. A grid option will be provided on the reduce-to-fit view but the number of grid lines shown will be reduced to avoid filling the smaller diagram with grid lines.
CMSC 345, Spring 2003 20
Problems!• Difficult to read• Mixes three different requirements:
– Functional requirement (the need for a grid)– Non-functional requirement (grid units)– Non-functional UI requirement (grid
switching)
CMSC 345, Spring 2003 21
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 programming languagebut with more abstract features to specify the requirementsby defining an operational model of the system.
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) havebeen used. I discuss these in the following chapter.
Mathematicalspecifications
These are notations based on mathematical concepts suchas finite-state machines or sets. These unambiguousspecifications reduce the arguments between customer andcontractor 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.
CMSC 345, Spring 2003 22
Some Guidelines for Writing Requirements
• Invent or find a standard format and use it for all requirements.
• Use language in a consistent way. • Use “shall” or “will” for mandatory requirements, “should”
for desirable requirements.• Use text highlighting (e.g., italics) to identify key parts of
the requirement.• Do not use vague phrases (e.g., “around a month,”
“have basic knowledge of”)• Every requirement must be verifiable.• Every requirement should be numbered for traceability.
CMSC 345, Spring 2003 23
Use CasesA way of describing a system’s functional
requirements• Describes the system’s behavior under various conditions
as the system responds to a request from one of the stakeholders called the primary actor.
• The primary actor initiates some interaction with the system to accomplish some goal.
• The system responds, protecting the interests of all of the stakeholders.
• Different sequences of behaviors (scenarios) can unfold, depending on the request and the conditions surrounding the request. The use case gathers these scenarios together.