1. Functional & non-functional Requirements
Lecture 6

2. Tentative Schedule Week# Date Time Topics 2nd OHT Activites7
18 Mar
Lecture#6 Functional & Non Functional Requirements
Lecture,
Tutorial automated tool, Paper Showing
Score card 8
25 Mar
Lecture #7 Requirements for critical systems
Assignment 3 9
1 Apr
Lecture #8 Requirements validation
Assignment Submission 10
8 Apr
Lecture #9 Requirements Management
Assignment 4 11
15 Apr
Lecture#10 Agile Software Requirements
Discussion 12
22 Apr
2nd OHT
Advanced Software Requirements Engineering: MSCS- 20

3. Classification of Requirements
Requirements are commonly classified as (IEEE std 830, 1998):
Functional:
A requirement that specifies an action that a system MUST be able to perform, without considering physical constraints
A requirement that specifies input/output behavior of the system
Non-Functional:
A requirement that specifies system PROPERTIES, such as environmental and implementation constraints, performance, dependencies, maintainability, extensibility and reliability.
Often classified as:
Performance Requirements
External interface requirements
Design constraints
Quality attributes
Advanced Software Requirements Engineering: MSCS- 20

4. Requirements General: Business: Functional:
What is the overall scope of the project from a requirements perspective?
Example: A solution is required that allows users to maintain customers and contact with those customers by salespeople.
Business:
What specific capabilities must the whole solution deliver to the Business?
Example: Users must be able to create a record of contact with a customer by a salesperson.
Functional:
What specific capabilities must the computerised part of the solution deliver?
Example: The system will provide the ability to record details of contacts with a customer by a salesperson.
Advanced Software Requirements Engineering: MSCS- 20

5. Requirements Detailed: Non-Functional: Data: Technical:
What business rules are to be enforced by the solution?
Example: A customer can not be deleted if they have had any contact with salespeople in the last 6 years.
Non-Functional:
What requirements exist that do not fall in to any of the other categories?
Example: The system must support up to 150 concurrent users.
Data:
What rules must the data enforce?
Example: A customer can have many contacts by a salesperson.
Technical:
What technical constraints must the solution abide by? Often documented as a constraint rather than requirement…
Example: The system will be developed using object orientated standards.
Advanced Software Requirements Engineering: MSCS- 20

6. Requirements Overview
A requirement is a statement of one of the following:
What a system must do
A known limitation or constraint on resources or design
How well the system must do what it does
The first definition is for Functional Requirements
The second and third definitions are for Non- Functional Requirements (NFRs)
Curtsy Intel Corporation.
Advanced Software Requirements Engineering: MSCS- 20

7. Examples of Functional and Non-Functional Requirements
VOIP (conference call Project)
Functional Requirements
Non-Functional Requirements
Add Participants
Drop Participants
Count Participants
Mute Microphone
Invite/prompt Operator
Voice/ Video quality
Reliability
Availability
Ease of use
Cost
Localization
Advanced Software Requirements Engineering: MSCS- 20

8. Functional Requirements
A Functional Requirement:
is a statement of what a system must do (#1)
is measured in “yes” or “no” terms
usually employs the word “shall”
Examples:
Add Participant
“The software shall display an option to add a participant”
Invite/prompt Operator
“The software shall invite/prompt the operator if the participant clicks the Operator Help icon.”
Advanced Software Requirements Engineering: MSCS- 20

9. Functional Requirements
Functional requirements define the required behavior of the system to be built.
Statements of services the system should provide
Reaction to particular inputs
Behavior in particular situations
Statements describing what the system does
Functionality of the system
Abnormal behavior is also documented as functional requirements in the form of exception handling
Functional requirements should be complete, unambiguous and consistent
Customers and developers usually focus all their attention on functional requirements
Advanced Software Requirements Engineering: MSCS- 20

10. Non Functional Requirements
Non-functional requirements specify additional properties of the system to be built, other than functionality.
Most non-functional requirements relate to the system as a whole.
They include constraints on timing, performance, reliability, security, maintainability, accuracy, the development process, standards, etc.
Non-functional requirements can be subcategorized such as:
External interface requirements, performance requirements, design constraints, logical database requirements, and “characteristics”
Advanced Software Requirements Engineering: MSCS- 20

11. Non Functional Requirements
A Non-Functional Requirement:
is a known limitation or constraint on resources or design (#2)
usually measured in yes/no terms
can include documentation, marketing collateral, product localization, legal compliance restrictions
typically employs the word “must”
Examples:
Cost
“The retail cost of the software must be between $175 and $199.”
Localization
“The help file must be released in English, French and Spanish.”
Advanced Software Requirements Engineering: MSCS- 20

12. Non Functional Requirements
A Non-Functional Requirement:
is a measure of how well the system must do what it does (#3)
Is measured over an interval or range
usually employs the word “must”
includes the “ilities” (e.g., quality, reliability, scalability, availability)
This type of requirement is problematic within most RE practices.
it is the responsibility of the architect to work with the stakeholders of the system to define and baseline a quality of service measurement for each of the service level requirements.
The architecture must address all the non functional requirements.
Advanced Software Requirements Engineering: MSCS- 20

13. Types of Non-functional requirements
The ‘IEEE-Std ’ lists 13 non-functional requirements to be included in a Software Requirements Document.
Performance requirements
Interface requirements
Operational requirements
Resource requirements
Verification requirements
Acceptance requirements
Documentation requirements
Security requirements
Portability requirements
Quality requirements
Reliability requirements
Maintainability requirements
Safety requirements
Advanced Software Requirements Engineering: MSCS- 20

14. What Functional Requirements Are Not
The functional requirements sections of specifications often tend to be “lists” of statements in the form:
“the system shall…” relating to “capabilities” of the system, not functions.
For example, “the system shall have the capability to update an inventory database.” Or,
“the system shall have the capability to retrieve inventory information and present it to the user.”
Any Problem here?
Both are not functional requirements at all… Why???
The inputs are missing from the statement!
You may say, “It is obvious, the system will update the inventory when it receives an update transaction from the users.”
You may be right, but it isn’t stated explicitly.
The statements are also incomplete:
They do not specify which data elements will be updated, or retrieved, or presented to the user.
They also do not say what the system is required to do if, say, the information to be updated or retrieved is missing from the database.
Because the inputs are missing, the statements are inherently un-testable as well.
So, when the system test team, or the acceptance test team, starts to develop test scenarios, the team members will likewise make assumptions that may vary with the users’ intent.
Advanced Software Requirements Engineering: MSCS- 20

15. Examples and comments (FRs)
The system shall solve a quadratic equation using the following formula
x = (-b+sqrt(b2 – 4*a*c))/2*a
Notice the ambiguity in the requirement for solving the quadratic equation.
The requirement does not speak about the possibility when the value of ‘a’ is zero
x = (-b+sqrt(b2 – 4*a*c))/2*a
The system shall provide appropriate viewers for the user to read documents in the document store
Notice the ambiguity in the requirement, which uses the term ‘appropriate viewers’
This requirement does not mention the formats of documents and types of viewers, which can be used
Incomplete and ambiguous requirements are open to multiple interpretations and assumptions
This can lead to the development of poor quality, or faulty, software products
Advanced Software Requirements Engineering: MSCS- 20

16. Examples and comments (NFRs)
Product Requirements
Usability, efficiency, reliability, portability, proficiency requirements
Organizational Requirements
Standard, implementation, delivery requirements
External Requirements
Interoperability, ethical, legislative, safety requirements
Product Requirements
The system shall allow one hundred thousand hits per minute on the website
The system shall not have down time of more than one second for continuous execution of one thousand hours
Organizational Requirements
The system development process and deliverable documents shall conform to the MIL-STD-2167A
Any development work sub-contracted by the development organization shall be carried out in accordance with Capability Maturity Model
External Requirements
The system shall not disclose any personal information about members of the library system to other members except system administrators
The system shall comply with the local and national laws regarding the use of software tools
Non-functional requirements should be written in a quantitative manner as much as possible, which is not always easy for customers
For some goals, there are no quantitative measures, e.g., maintainability
Advanced Software Requirements Engineering: MSCS- 20

17. Challenges of NFRs NFRs are difficult to express
A number of important issues contribute to the problem of expressing non-functional requirements:
Certain constraints are related to the design solution that is unknown at the requirements stage
Certain constraints, are highly subjective and can only be determined through complex empirical evaluations
Non-functional requirements tend to be related to one or more functional requirements
Non-functional requirements tend to conflict and contradict
There are no ‘universal’ rules and guidelines for determining when non-functional requirements are optimally met.
Advanced Software Requirements Engineering: MSCS- 20

18. Representation
Natural language is unconstrained, informal language as it is used in every day speech and writing (e.g., )
Natural language is the most common medium for expressing requirements in most industries; It is flexible, easy to use and requires no additional training.
Exercise
Write 3-4 natural language non-functional requirements for the purchase of a new car.
Example: The car must be reliable.
Advanced Software Requirements Engineering: MSCS- 20

19. Weak Words in NL
Weak words are subjective or lack a common or precise definition. Examples include:
Fast
Frequently
Intuitive
Feel, Feeling
Effortless
Friendly, User-friendly
Secure
Immediate
Quick, Quickly
Easy, Easily
Timely
Fast
Normal
Reliable
State-of-the-art
Effortless
This is just a partial list. Don’t use weak words – define what you mean using precise, measurable terms
Advanced Software Requirements Engineering: MSCS- 20

20. Un bounded List
An unbounded list is one that lacks a starting point, an end point, or both, Examples include:
At least
Including, but not limited to
Or later
Such as
Unbounded lists are impossible to design for or to test against For example, how would you design and test a system that “must maintain a list of at least 250 users”?
Or, how would you test software that “must install on Windows Vista or later in under 5 seconds”?
Advanced Software Requirements Engineering: MSCS- 20

21. Ambiguity
Ambiguity occurs when a word or statement has multiple meanings or there is doubt about the meaning. These problems (and others) create ambiguity:
Vagueness
Subjectivity
Optionality
Under-specification
Under-reference
Ambiguity leads to differences in interpretation amongst various stakeholders for a requirement.
Advanced Software Requirements Engineering: MSCS- 20

22. Implicit Collections
Often, collections of objects within requirements are not explicitly defined anywhere Without a definition, readers may assume an incorrect meaning
Example:
“The software must support and other network protocols supported by competing applications under Linux.”
What is counted as a “competing application”?
What belongs to the collection of “other network protocols”?
What specific protocols of are included?
“Linux” is also a collection of OS vendors, versions, and revision levels
Advanced Software Requirements Engineering: MSCS- 20

23. References
‘Requirements Engineering: Processes and Techniques’ by G. Kotonya and I. Sommerville, John Wiley & Sons, 1998
Software Engineering 9th Edition, by I. Sommerville,
Software Engineering 8th Edition, by R. Pressman
Specifying Effective Non-Functional Requirements by John Terzakis, Intel Corporation, June 24, 2012, ICCGI Conference, Venice, Italy (special thanks)
Advanced Software Requirements Engineering: MSCS- 20