Software archiecture lecture05

Understanding Quality Attributes

Introduction
• Functionality often takes not only the front seat in
the development scheme but the only seat.
• Systems are frequently redesigned not because they
are functionally deficient—the replacements are
often functionally identical—but because they are
difficult to
▫ maintain, port, or scale, or are too slow, or have been
compromised by network hackers.
• We said that architecture was the first stage in
software creation in which quality requirements
could be addressed.
• It is the mapping of a system's functionality onto
software structures that determines the
architecture's support for qualities

Functionality and Architecture
• Functionality and quality attributes are
orthogonal (independent).
• Functionality is the ability of the system to do
the work for which it was intended
• A task requires that many or most of the
system's elements work in a coordinated
manner to complete the job.
• Software architecture constrains its allocation to
structure when other quality attributes are
important.

Architecture and Quality Attributes
• Satisfactory results are a matter of getting the
big picture (architecture) as well as the details
(implementation) correct.
• Example:
▫ Usability involves both architectural and
nonarchitectural aspects.
▫ Modifiability is determined by how functionality is
divided (architectural) and by coding techniques
within a module (nonarchitectural).
▫ Performance involves both architectural and
nonarchitectural dependencies

Architecture and Quality Attributes
• Within complex systems, quality attributes can
never be achieved in isolation.
▫ The achievement of any one will have an effect
on the achievement of others (both + , -)

Quality attributes
• We will examine the following three classes:
▫ Qualities of the system. We will focus on
availability, modifiability, performance, security,
testability, and usability.
▫ Business qualities (such as time to market) that
are affected by the architecture.
▫ Qualities, such as conceptual integrity, that are
about the architecture itself although they
indirectly affect other qualities, such as
modifiability.

System Quality Attributes
• There are a variety of published taxonomies and
definitions for System Quality Attributes
• From an architect's perspective, there are three
problems with those definitions:
▫ The definitions provided for an attribute are not
operational
▫ A focus of discussion is often on which quality a
particular aspect belongs to.
▫ Each attribute community has developed its own
vocabulary.
• Problem 1 & 2 solve by Quality Attribute Scenarios
• Problem 3 solve by provide a brief discussion of
each attribute

Quality Attribute Scenarios
• A quality attribute scenario is a quality-attribute-
specific requirement.
• It consists of six parts.
▫ Source of stimulus
▫ Stimulus
▫ Environment
▫ Artifact
▫ Response
▫ Response measure

• Source of stimulus.
▫ This is some entity (a human, a computer system,
or any other actuator) that generated the
stimulus.
• Stimulus.
▫ The stimulus is a condition that needs to be
considered when it arrives at a system.
• Environment.
▫ The stimulus occurs within certain conditions. The
system may be in an overload condition or may be
running when the stimulus occurs, or some other
condition may be true.

• Artifact.
▫ Some artifact is stimulated. This may be the
whole system or some pieces of it.
• Response.
▫ The response is the activity undertaken after the
arrival of the stimulus.
• Response measure.
▫ When the response occurs, it should be
measurable in some fashion so that the
requirement can be tested.

General Scenarios VS
Concrete Scenarios
• General quality attribute scenarios (general
scenarios)
▫ those that are system independent and can,
potentially, pertain to any system
• concrete quality attribute scenarios (concrete
scenarios)
▫ those that are specific to the particular system
under consideration

Availability Scenario
• Not every system-specific scenario has all of the
six parts.
• The parts that are necessary are Response &
Response Measure

Quality Attribute Scenarios in Practice
• To do Quality Attribute Scenario Generation in a
disciplined way
▫ We use the quality-attribute-specific tables to
create general scenarios and from these derive
system-specific scenario
▫ Not all of the possible general scenarios are
created
▫ The tables serve as a checklist to ensure that all
possibilities have been considered

Availability
• Availability is concerned with system failure and
its associated consequences
• A system failure occurs when the system no
longer delivers a service consistent with its
specification.
• The availability of a system is calculated by

• From this come terms like 99.9% availability

• Scheduled downtimes are not considered when
calculating availability

Availability General Scenarios
Portion of Scenario Possible Values
Source Internal to the system; external to the system
Stimulus Fault: omission, crash, timing, response
Artifact System's processors, communication channels,
persistent storage, processes
Environment Normal operation;
degraded mode (i.e., fewer features, a fall back
solution)

Availability General Scenarios
Response System should detect event and do one or more of the
following:
record it
notify appropriate parties, including the user and
other systems
disable sources of events that cause fault or failure
according to defined rules
be unavailable for a prespecified interval, where
interval depends on criticality of system
continue to operate in normal or degraded mode
Response Measure Time interval when the system must be available
Availability time
Time interval in which system can be in degraded
mode
Repair time

Sample availability scenario
“An external message is received by a process during
normal operation. The process informs the operator
of the receipt of the message and continues to
operate with no downtime.”

Modifiability
• Modifiability is about the cost of change
▫ What can change ? (the artifact)
▫ When is the change made and who makes it ?
(the environment)

Modifiability General Scenarios
Source End user, developer, system administrator
Stimulus Wishes to add/delete/modify/vary functionality, quality
attribute, capacity
Artifact System user interface, platform, environment; system
that interoperates with target system
Environment At runtime, compile time, build time, design time
Response Locates places in architecture to be modified; makes
modification without affecting other functionality; tests
modification; deploys modification
Response Measure Cost in terms of number of elements affected, effort,
money; extent to which this affects other functions or
quality attributes

Sample modifiability scenario
"A developer wishes to change the user interface to make a
screen's background color blue. This change will be made to
the code at design time. It will take less than three hours to
make and test the change and no side effect changes will
occur in the behavior."

Performance
• Performance is about timing.
• Basically performance is concerned with how
long it takes the system to respond when an
event occurs.
• One of the things that make performance
complicated is the number of event sources and
arrival patterns.

Performance General Scenarios
Source One of a number of independent sources, possibly from
within system
Stimulus Periodic events arrive; sporadic events arrive; stochastic
events arrive
Artifact System
Environment Normal mode; overload mode
Response Processes stimuli; changes level of service
Response Measure Latency, deadline, throughput, jitter, miss rate, data loss

Sample performance scenario
“Users initiate 1,000 transactions per minute
stochastically under normal operations, and these
transactions are processed with an average latency
of two seconds."

Security
• Security is a measure of the system's ability to
resist unauthorized usage while still providing its
services to legitimate users
• Security can be characterized as a system
providing nonrepudiation, confidentiality,
integrity, assurance, availability, and auditing

Security General Scenarios
Portion of Possible Values
Scenario
Source Individual or system that is
correctly identified, identified incorrectly, of unknown
identity
who is
internal/external, authorized/not authorized
with access to
limited resources, vast resources
Stimulus Tries to
display data, change/delete data, access system
services, reduce availability to system services
Artifact System services; data within system
Environment Either
online or offline, connected or disconnected, firewalled or
open

Security General Scenarios
Scenario
Response Authenticates user; hides identity of the user; blocks access to
data and/or services; allows access to data and/or services;
grants or withdraws permission to access data and/or services;
records access/modifications or attempts to access/modify
data/services by identity; stores data in an unreadable format;
recognizes an unexplainable high demand for services, and
informs a user or another system, and restricts availability of
services
Response Time/effort/resources required to circumvent security measures
Measure with probability of success; probability of detecting attack;
probability of identifying individual responsible for attack or
access/modification of data and/or services; percentage of
services still available under denial-of-services attack; restore
data/services; extent to which data/services damaged and/or
legitimate access denied

Sample security scenario
• A correctly identified individual tries to modify
system data from an external site; system maintains
an audit trail and the correct data is restored within
one day.

Testability
• Software testability refers to the ease with which
software can be made to demonstrate its faults
through testing
• At least 40% of the cost of developing well-
engineered systems is taken up by testing
• Testing is done by various developers, testers,
verifiers, or users
• The response measures for testability deal with
▫ how effective the tests are in discovering faults
▫ how long it takes to perform the tests to some
desired level of coverage.

Testability General Scenarios
Scenario
Source Unit developer
Increment integrator
System verifier
Client acceptance tester
System user
Stimulus Analysis, architecture, design, class, subsystem integration
completed; system delivered
Artifact Piece of design, piece of code, complete application
Environment At design time, at development time, at compile time, at deployment
time
Response Provides access to state values; provides computed values; prepares
test environment
Response Measure Percent executable statements executed
Probability of failure if fault exists
Time to perform tests
Length of longest dependency chain in a test
Length of time to prepare test environment

Sample Testability scenario
• A unit tester performs a unit test on a completed
system component that provides an interface for
controlling its behavior and observing its output;
85% path coverage is achieved within three hours.

Usability
• Usability is concerned with
▫ how easy it is for the user to accomplish a desired
task
▫ the kind of user support the system provides
• It can be broken down into following areas:
▫ Learning system features
▫ Using a system efficiently
▫ Minimizing the impact of errors
▫ Adapting the system to user needs
▫ Increasing confidence and satisfaction
• The normal development process detects usability
problems through building prototypes and user
testing

Usability General Scenarios
Scenario
Source End user
Stimulus Wants to
learn system features; use system efficiently; minimize
impact of errors; adapt system; feel comfortable
Artifact System
Environment At runtime or configure time
Response System provides one or more of the following responses:

to support "learn system features"
help system is sensitive to context; interface is familiar to user;
interface is usable in an unfamiliar context

to "adapt system":
customizability; internationalization

Usability General Scenarios
Scenario
Response to support "use system efficiently":
aggregation of data and/or commands; re-use of already
entered data and/or commands; support for efficient navigation
within a screen; distinct views with consistent operations;
comprehensive searching; multiple simultaneous activities

to "minimize impact of errors":
undo, cancel, recover from system failure, recognize and
correct user error, retrieve forgotten password, verify system
resources

to "feel comfortable":
display system state; work at the user's pace
Response Task time, number of errors, number of problems solved, user
Measure satisfaction, gain of user knowledge, ratio of successful
operations to total operations, amount of time/data lost

Sample Usability scenario
• A user, wanting to minimize the impact of an error,
wishes to cancel a system operation at runtime;
cancellation takes place in less than one second

Communicating Concepts Using
General Scenarios
• We use general scenarios to enable stakeholders
to communicate.
• Facilitating the understanding of vocabulary aids
discussions of architectural decisions
• The problem for the architect is to understand
which of these stimuli represent the same
occurrence, which are aggregates of other
stimuli, and which are independent
• Once the relations are clear, the architect can
communicate them to the various stakeholders
using language that each understand.

Communicating Concepts Using
General Scenarios
Quality
Stimulus
Attribute
Availability Unexpected event, nonoccurrence of expected event
Modifiability Request to add/delete/change/vary functionality, platform,
quality attribute, or capacity
Performance Periodic, stochastic, or sporadic
Security Tries to

display, modify, change/delete information, access, or reduce
availability to system services
Testability Completion of phase of system development
Usability Wants to

learn system features, use a system efficiently, minimize the
impact of errors, adapt the system, feel comfortable

Other System Quality Attributes
• A number of other attributes can be found in the
attribute taxonomies in the research literature
and in standard software engineering textbooks
▫ Scalability
▫ Portability
▫ Interoperability
• If some quality is important to your
organization, it is reasonable to create your own
general scenario for it

Business Qualities
• A number of business quality goals frequently
shape a system's architecture
• They need to be made specific with scenarios in
order to make them suitable for influencing the
design process and to be made testable
• Business Qualities
▫ Time to market
▫ Cost and benefit
▫ Projected lifetime of the system
▫ Targeted market
▫ Rollout schedule
▫ Integration with legacy systems

Architecture Qualities
• There are also qualities directly related to the
architecture itself that are important to achieve
▫ Conceptual integrity
is the underlying theme or vision that unifies the
design of the system at all levels
▫ Correctness and completeness
are essential for the architecture to allow for all of
the system's requirements and runtime resource
constraints to be met
▫ Buildability
It refers to the ease of constructing a desired
system and is achieved architecturally by paying
careful attention to the decomposition into modules

Summary
• The qualities presented in this chapter represent
those most often the goals of software
architects.
• Since their definitions overlap, we chose to
characterize them with general scenarios.
• We saw that qualities can be divided into
▫ those that apply to the system
▫ those that apply to the business environment
▫ those that apply to the architecture itself

Software archiecture lecture05

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