Non-Functional Requirements
Notes taken from:
Object Oriented Software Engineering text;
Art of Software Architecture (Stephen Albin)
Use Case Analysis (Bittner and Spence);
Use Cases – Requirements in Context (Kulak and Guiney)
Some notes from Rational Software Corporation slides
Other personal notes
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Use-Case Analysis Steps – Here’s
where we are:

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Supplement the Use-Case Descriptions
For each use-case realization
 Find Classes from Use-Case Behavior
 Distribute Use-Case Behavior to Classes
For each resulting analysis class
 Describe Responsibilities
 Describe Attributes and Associations
Non-Functional Requirements –


We have a:
good understanding of the nature of the
analysis classes and their responsibilities
collaborations required to support the
functionality described in the Use Cases via
analysis classes
Need to address the non-functional requirements
IBM-Rational calls these “analysis mechanisms.”
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Non-Functional Requirements

The purpose of “identifying non-functional
requirements” is to get a handle on these
absolutely necessary requirements that are
normally not ‘functional.’

During Analysis, this information is speculative.
Will be refined later.
Philosophy: Capture now; Realize later…
(Essential points in design…)
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

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Non-Functional Requirements –
OOSE Text p.115-
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Types of Requirements
Functional
 Describe
requirements
what the system should do
Non-functional
requirements
 Consists
of Constraints that must be
adhered to during development (design
and implementation)
 Operative word: ‘Constraints.’
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Functional requirements
 What
inputs the system should accept
 What
outputs the system should produce
 What
data the system should store that other
systems might use
 What
 The
computations the system should perform
timing and synchronization of the above
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Non-Functional requirements

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Mostly not unique to a single use case.
Generally thread themselves throughout
some use cases

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Transcend locality
Some of these conflict with each other
Some support ‘efficiency’ of code and
‘inefficiency’ in maintenance…
Let’s contrast functional and nonfunctional requirements 
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Non-functional Requirements –
Verifiable; Measured
All
requirements must be verifiable
 If
not ‘verifiable’ then there is no indications
that these requirements have been satisfied.
Some
must also be measured.
 Some
requirements are measurable, such as
response time; availability time, etc.
 Some may be directly measured; some
measured via simulation.
 We
parse the non-functional requirements
into categories…
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Categories of Non-Functional Requirements
1. Quality Non-Functional Requirements (1 of 2)


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All constrain the design to meet desired quality levels of
usability, efficiency, reliability, maintainability and reusability…
Examples:
Response time – particularly important for processes that
process a lot of data or use networks a great deal.
 Requirements might stipulate < two second response.



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Might use a Timing bar indicating progress…
Response time may be considered a functional requirement
for some ‘real time systems.
Throughput – for computationally-intensive applications or for
heavy transaction-oriented systems, the ‘number of
transactions per minute or number of computations per minute
can/must be measured.
Resource usage – Application cannot use > xxMB of memory
Very practical in a large multiprogramming / multiprocessing
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system.
Categories of Non-Functional Requirements
1. Quality Non-Functional Requirements (2 of 2)

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Reliability – typically Mean Time Between Failures (MTBF)
(number of failures per week; define ‘failure.’)
Availability – Measures the time application is ‘up’ and the
period of down time, when down.
Recovery from failure – Mean Time To Repair: (MTTR) –
critical to some applications; not so for others; checkpoint
restarts; auto-saves, etc. If you specify the detailed
procedure to follow, then this is a functional requirement.
Allowances for Reusability – may specify that a certain
percentage of the system must be designed generically to
support reusability…
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Categories of Non-Functional Requirements
2. Environment and Technology Constraints

Platform



What hardware / software the software must
be able to work on.
Normally indicate ‘least’ platform
 e.g.
Windows XP, Vista; 120 MB free disk
space….
Technology to be used


Programming language; database system. All
individuals are familiar with these languages /
database nuances.
May/may not reduce need to train people
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Categories of Non-Functional Requirements
3. Project Plan and Development Methods

Development Process/Methodology

This may be specified and will greatly impace
development.
SDLC; RUP; Agile Methods, Scrum; XP, FeatureDriven Development (FDD), etc.
 Particular approaches to testing (done by
separate group; ‘Where in supervisory chain?’
 References to these must be included;
not the
details


Cost and Delivery Dates; Traceability


A Bigee!! Usually a very important constraint.
Usually found in the Contract for the system or
in a separate project 36
plan.
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Random Thoughts - Non-functional Requirements
 Not
always clear that a requirement is nonfunctional and functional.
 If requirement is a fundamental part of the
application’s functionality, then it should be stated
as a functional requirement
Think ‘performance’ of a real time / manufacturing app.
 Think ‘reliability’ of a safety-critical application

 If
requirement is a ‘constraint’ on design or some
kind of restriction on design, then the requirement
is almost always ‘non-functional.’
 Certainly the presented list is not exhaustive!
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Quality Models – Many and Varied (1 of 2)


Quality Models from a different source:
from Art of Software Architecture… (p.70-72)
Note the different ‘categories’ for Quality Modeling
according to this source:
 Product Operation Quality:
 Accuracy, Reliability, Efficiency, Integrity, and
Usability
 Product Revision Quality are:
 maintainability, flexibility, testability
 Product Transition Quality are:
 Interface facility, reusability, transferability
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More attributes…(2 of 2)

According to Barry Boehm: Overall quality is a function of
these metrics:


Laurence Best’s Application Architecture identifies:

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Validity, clarity, understandability, modifiability, modularity,
generality, economy, resilience, documentation
Accuracy and comprehensiveness; simplicity of use,
operational flexibility, ease of development, maintenance and
extension, security and control, resource efficiency, recovery
And there are numerous other ‘lists’…..
These are presented to show you that there are different
‘takes’ on quality and what determines quality…
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Looking down on these…(1 of 2)
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Functional models such as use cases, typically drive object-oriented
projects (although they certainly don’t have to…)
Attributes such as modifiability are not easily expressed in
purely object-oriented terms and need supplemental text
descriptions to represent the design.
 Each attribute MUST be considered during design and usually
requires the architect to make multiple passes as a design.

Class design: start by addressing functionality only

Second pass to incorporate modifiability constraints while making sure
that the functionality is not compromised.

Sometimes, multiple passes are needed to incorporate various
requirements and to evaluate the design tradeoffs.
 How can these methods be efficient and yet maintainable?
 Efficient and yet reusable?
 Reusable yet modifiable?
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Looking down on these…(2 of 2)
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Many competing non-functional requirements.
Architect must find a design that realizes a balance among
them.
Example:

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Performance design techniques, however, usually incorporate
fewer levels of indirection and optimizations to data structures in
order to improve operation execution times.
Performance gains may bring decreased maintainability /
reusability features.
Cost is usually competing with everything else and it is common
that usability is sacrificed first!
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Where are Non-Functional
Requirements Captured / Modeled?

Use Case Modeling – Bittner and Spence – p. 43
In the RUP, we place the non-functional requirements in the
Supplementary Specifications
But this is usually considered a ‘standard’…..
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Consider:
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
Legal and Regulatory Requirements


e.g.: The system must be available at least 90% of the time.
Constraints placed on the design and implementation of the system
such as operating system, environments, …


e.g.: The system must be developed in accordance with the Rational
Unified Process
Quality attributes of the system to be built must include usability,
reliability, performance, supportability requirements


e.g.: The customer must be of legal age to purchase alcohol.
Application Development Standards


(Bittner and Spence: different categories!!!) 
e.g.: The system must be written in Java 2….
Others…
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Supplementary Specifications
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 Very misleading to think ‘supplementary specs’ are
inferior or lesser than functional specs.
Many projects have run amuck because they did not
consider these requirements.
May have formed a major part of the overall critical
success criteria.
Consider the following problems / results:



E.g. System must support 300-500 simultaneous users; On
entry into user acceptance testing, system supported eight
simultaneous users; Developers had concentrated on the
user interface and completely ignored this supplementary
spec item.
Upon further review, it was noticed that the developer did not
investigate any of the requirements beyond the scope of the
most basic flow of events.
“Not surprisingly, the system was never deployed.”
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Supplementary Specifications –
Complement to Use Cases

Note use cases can capture non-functional requirements especially
those that apply within the context of a single use case.

Use Supplementary Specs to capture non-functional requirements that
are global in nature or do not vary from one use case to the next.
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Within the Use Case: Typically, if the amount of interaction in use cases
is large, the majority of requirements will be captured here and there may
be just a few, global, non-functional requirements.
Within the Supplementary Specifications: Conversely, if the amount of
interaction in the use case is small, then the majority of requirements may
be captured as non-functional requirements and found in the
Supplementary Specifications.

Keep a balance between using the use cases and non-functional
requirements.

We need both if the overall requirements objectives are to be met.
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Non-Functional Requirements – the Process

Use Cases – Kulak and Guiney…



Identify the non-functional requirements
Validate with appropriate stakeholders
Document the requirements
 Capture its name, its effect, both the system and
business degradation that would result if the
requirements are not satisfied
 Best time to capture them when exploring the
functional requirements
 Document it right away from your sources of
information (interviews, questionnaires, …)
 Ask users about response times, what bothers them
now; what is fine.
 Be certain to differentiate between business-related
concerns which can often be added to a use case
description, with non-functional requirements.
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Non-Functional Requirements – the Process
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Rank the non-functional requirements. Very Important!
Sometimes accommodating non-functional requirement
may cost more than implementing the use case.
There is no free lunch, and some things are not worth
the cost to accommodate.


e.g. Infinite availability; levels of performance. “Don’t let
[a user’s] initial assumptions drive you into an architectural
nightmare.”
Use a template to document the non-functional
requirements, as found in our Use Case book, ~ p. 79.
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Use something like the table below.
You may add ‘column attributes’ as needed.
NonFunctional
Category*
Number
Non Functional
Requirement Name
Description
Applies to Use
Case(s):
Exceptions
You may wish to categorize these (or not). If not, omit column.
36 You may wish a priority column!
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You may also wish to add columns.
Examples
NonFunctional
Category*
Number
Non Functional
Requirement Name
Description
Applies to Use
Cases:
Exceptions
1
Persistency
Student objects need
to be persistent
Where Student
Objects are
Updated
None
2
Security
System Administrator
actor needs to be
authenticated
Those Use
Cases that
access the
Profile Database
None
3
Availability
The Profile Database
must be available
95% of the time
All Use Cases
that require
authentication
None
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Supplementary Specifications



Inserted table(s) into Supplementary Specifications.
The Supplementary Specifications document
constitutes an additional artifact that we will be
using.
Note also that in some cases, the non-functional
requirements are called ‘analysis mechanisms’ (RUP)

Have ‘persistency mechanism’; security ‘mechanism,’ etc.
Means that there will be some process to accommodate
persistency, security, etc.
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Analysis Classes – Use of tags { … }
Non-Functional Requirements Map UML –
•  As analysis classes are identified, it is important to identify the
non-functional requirements that might apply to them
• Classes that must be persistent are mapped to the Persistency
Mechanism and tagged as persistent. Persistency tag: {persistent}
• Classes that are maintained with the Legacy Course Catalog
system are tagged to the Legacy Interface mechanism;
• Classes for which access must be controlled (like who can read
and modify instances of the class) are mapped to a Security
mechanism., etc. {security} or {secure mechanism}
• Distributed classes mapped to a Distribution mechanism, etc.)
(Often ‘control classes’ are distributed.) {distributed}
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Use-Case Analysis Steps



Supplement the Use-Case Descriptions
For each use case realization
For each Analysis Class describe


responsibilities, etc….
…..
Know (now) about analysis classes




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their responsibilities,
the analysis mechanisms / non-functional
requirements that these analysis classes need to
implement (persistence, security, legacy, …) and
the collaborations required to support the
functionality described in the use cases.
Unify Analysis Classes: Now lets review
everything to ensure it is complete and
consistent before moving on….
Checkpoints
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Unify Analysis Classes
The purpose of “Unify Analysis
Classes” activity is to ensure that
each analysis class represents a
single well-defined concept, with
non-overlapping responsibilities.
<<boundary>>
Name of analysis class should capture
role; (e.g. EnrollmentForm)
Description of class should capture
role played by class in the system.
Merge classes that define similar
behavior or represent the same thing.
Merge entity classes that define the
same attributes
Aggregate behaviors of merged
classes.
If you do any of these things, make
sure you update any supplemental
use case descriptions where
necessary.
<<control>>
<<entity>>
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<<entity>>
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Evaluate Your Results
Glossary
Analysis Classes
Supplementary
Specification
Are
Are
Are
Are
Are
the Names appropriate?
the Descriptions accurate?
classes that should be merged actually merged?
appropriate entity classes merged?
aggregations captured?
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Use-Case Model 29
Evaluating and Verifying

 Now, verify analysis classes meet functional
requirements of the system.

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 Trace down through the Use Case specifications for
each Use Case. Are all responsibilities covered? Are
domain / application objects captured? Properties?
 Verify the analysis classes and their relationships are
consistent with collaborations that they may support.


If one class is sending messages to another, there is a
collaboration needed. If one class defines an ‘array’ of objects
from another class, have we modeled this as an aggregation!
etc.
Very important that you evaluate your results at the
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conclusion of Use Case Analysis.
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Use-Case Analysis Steps


Supplement the Use-Case Descriptions
For each use-case realization



For each resulting analysis class





Find Classes from Use-Case Behavior
Distribute Use-Case Behavior to Classes
Describe Responsibilities
Describe Attributes and Associations
Qualify Analysis Mechanisms
Unify Analysis Classes
Checkpoints - Check the ‘quality’ of the model
against criteria that the Designer looks for…
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Checkpoints: Analysis Classes
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Are the classes reasonable?
Does the name of each class clearly reflect the
role it plays?
 Does the class represent a single welldefined abstraction?
 Are all attributes and responsibilities
functionally coupled? (What does this mean to
you?)
 Does the class offer the required behavior?
Are all specific requirements on the class
addressed?
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(continued)
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Checkpoints: Analysis Classes

Note: All checkpoints should be evaluated with regards to
the use cases being developed for the current iteration.

The class should represent a single well-defined abstraction. If not,
consider splitting it.

The class should not define any attributes and responsibilities that
are not functionally coupled to the other attributes or responsibilities
defined by that class.

The classes should offer the behavior the use-case realizations and
other classes require. (These will be methods later…)

The class should address all specific requirements on the class
from the requirement specification – the Use Cases in our case.

Remove any attributes and relationships if they are redundant or
are not needed by the use-case realizations.
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Checkpoints: Use-Case Realizations

Have all the main and/or sub-flows been handled,
including exceptional cases?\

Have all the required objects been found?

 Have all behaviors been unambiguously
distributed to the participating objects?

Have behaviors been distributed to the right
objects?

Where there are several interaction diagrams, are
their relationships clear and consistent? (next
lecture)
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Review: Use-Case Analysis




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
What is the purpose of Use-Case Analysis?
What is an analysis class? Name and describe
the three analysis stereotypes.
What is a use-case realization?
Describe some considerations when allocating
responsibilities to analysis classes.
Describe the role of non-functional
requirements in complementing functional
requirements.
Describe the tie-in of non-functional
requirements to use cases.
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