Quality Attributes in Software
Architecture
Quality Attributes in Software Architecture
Nov 2011
Gang Tao
Confidential
Levels of Requirement Business
Requirements
Vision and Scope
Documents
User
Requirements
Quality
Attributes
Other
Nonfunctional
Requirements
Use Cases
Functional
Requirements
Software Requirements
Specification
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Quality Attributes
Responsiveness
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Responsiveness
• Responsiveness is defined as how quickly a system
responds to user input. • Long delays can be a major cause of user frustration, or let the user
believe the system is broken or that a command or input has been
ignored.
• Responsiveness is not same as performance.
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Responsive Matters
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Responsiveness
• All Operations Must Be Responsive
Click Me
Under normal IO load, any operation must
return to the user within 3 seconds.
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A Case Study
• getVersion() methods cause serious performance
downgrade.
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What causes the performance issue?
• Complexity
• Indirection
• Repetitions
• Bad Design
• I/O
• … …
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Price of Optimization
• Loss Readability
• Increase Complexity
• Hard to maintain and extend
• Introduce conflict
• … …
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SAP Standard Reference
Scalability
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Scalability
• Scalability refers to a systems ability to handle increased
adversity in its environment in a manner that is graceful and
predictable.
• Adversity comes in two dimensions: • Increased managed content
• Decreased system resources
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Scale Up VS Scale Out
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Scalability Considerations
• Increased Managed Content Must Scale Appropriately
• Maintain Viability as System Resources Decrease
• UI Data Presentation Scaling
• Load Sharing Considerations
Usability
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Usability
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Usability
• Usability is the customer’s ability to get work done with the
system in an efficient and pleasing manner.
• A usable system should build on skills a user already has
and not require new or unique knowledge to use the system.
• Each new function the user encounters should follow a
similar pattern so that once a user has learned one function
others are intuitive to learn.
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Usability Notions
• More efficient to use—takes less time to accomplish a
particular task
• Easier to learn—operation can be learned by observing the
object
• More satisfying to use
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Usability Composition
• Learnability • How easy is it for users to accomplish basic tasks the first time they
encounter the design?
• Efficiency • Once users have learned the design, how quickly can they perform tasks?
• Memorability • When users return to the design after a period of not using it, how easily
can they re establish proficiency?
• Errors • How many errors do users make, how severe are these errors, and how
easily can they recover from the errors?
• Satisfaction • How pleasant is it to use the design?
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Rule One – Don’t Make Me Think
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Rule Two - Know Your Users
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Rule Three – Be Consistent
• Be Consistent • Things that look the same should act the same.
• Things that look different should act different.
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Usability VS. User Experiences
• Free Discussion
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Some References of Usability • [1]《Designing Interfaces》，中文版名为《Designing Interfaces中文版》
• [2]《Usability Engineering》，中文版名为《可用性工程》
• [3]《The Design of Everyday Things》，中文版名为《设计心理学》
• [4]《About Face 3》，中文版名为《交互设计精髓》
• [5]《Designing Web Interfaces》，中文版名为《Web界面设计》
• [6]《Designing the Obvious》，中文版名为《一目了然——Web软件显性设计之路》
• [7]《Communicating Design》，中文版名为《设计沟通十器》
• [8]《Don’t Make Me Think》，中文版名为《点石成金：访客至上的网页设计秘笈》
• [9]《Designing From Both Sides of the Screen》，内外兼修的设计，尚未出版文版
• [10]《UI Design for Programmers》，程序员之用户界面设计，尚未出版中文版
• [11]《The Inmates are Running the Asylum》，中文版名为《交互设计之路——让高科技产品回归人性》
Security
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Security
• Security is defined as being protected against danger or loss
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Security Model
Authentication Authorization
Audit Encryption
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Refereneces
• OWASP Top Ten
• Seven Pernicious Kingdoms
• Sans Top 25
• Common Weakness Enumerations
Accessibility
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Accessibility
• “accessibility“ refers to the possibility for everyone, including
and especially people with disabilities, to access and use
technology and information products.
Serviceability
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Serviceability
• Serviceability refers to the ability to monitor systems, identify
issues, perform root cause analysis, and provide
maintenance in order to solve a problem and restore the
system to a functioning state.
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Serviceability
• Diagnostic Infrastructure Should Not Be Intrusive • Issues that arise on a customer site that require engineering
involvement should be able to be diagnosed without requiring that the
affected system gets taken off line, new binaries get installed, or a
debugger gets installed.
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Serviceability
• Solutions Must Support a Customer-Centric Service
Model • Common issues are able to be root-caused and addressed by
customers or entry level technical service representatives over the
phone.
• Meaningful log messages, alerts, and trouble-shooting trees all must
be considered to be mandatory and complementary components for
servicing a system.
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Cost of Service
Customer Self Service
Web Remote Support, BBS/MSN
Phone/Email Support
On Site Support
Extensibility
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Extensibility
• With extensibility, the design and implementation takes into
consideration future growth.
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Extensibility Considerations
• Design and Implement With Scale in Mind
• Producers of Interfaces Must Support the Ability to
Minimize the Amount of Data Returned to a Client
• Clients Should Minimize Data Traffic and Network
Requests
• Business Logic Must Be Centralized
• Revision Compatibility Focus
Distributability
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Distributability
• Distributability is defined as the ability to manage a group of
systems as one unit and to perform management from any
location in the network.
• Decide whether a component will run in process, in a
separate process on the same machine, or on a remote
machine.
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Layers and Tiers
• Layers are a means of logical separation, and are an
architectural pattern to separate concerns
• Tiers are the physical separation of an application.
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Distributed Architecture
• Web Service
• CORBA
Maintainability
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Maintainability
• Maintainability means the ease with which a system can be
modified in order to correct defects, meet new functionality,
make maintenance easier, or cope with a changing
environment.
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Maintainability Tips
• Adhere to Coding Standards
• Avoid duplication
• Story Telling with your code
• Keep your code simple
Portability
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Portability
• Portability is defined as the ability to reuse features and
utility source code, across multiple Operating systems. • By having an OS abstraction layer that implements platform specific
utilities, such as locks and shared memory.
• Can also be achieved by compiling different segments of code,
depending on the platform the software is to be run on.
Reliability
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Reliability (Availability)
• Reliability is defined as the ability of a system, to function
correctly, under any reasonable circumstance. • Reasonable can include adverse situations, in which unexpected (but
supported) usage occurs.
• For Storage systems, this includes putting extreme I/O load on the
system, but expecting the system to be manageable.
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Availability
MTBF – Mean Time Between Failure
MTTR – Mean Time to Recovery
Availability= MTBF/(MTBF+MTTR)
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Reliability Consideration
• All errors must be handled in a graceful manner
• The Storage System must be manageable, without external
dependencies.
• There should be NO single point of failure
Testability
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Testability
• Software testability is the degree to which a software
artifact (i.e. a software system, software module,
requirements- or design document) supports testing in a
given test context.
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Testability of Software Components • Controllability: The degree to which it is possible to control the state of
the component under test (CUT) as required for testing.
• Observability: The degree to which it is possible to observe (intermediate and final) test results.
• Isolateability: The degree to which the component under test (CUT) can be tested in isolation.
• Separation of concerns: The degree to which the component under test has a single, well defined responsibility.
• Understandability: The degree to which the component under test is documented or self-explaining.
• Automatability: The degree to which it is possible to automate testing of the component under test.
• Heterogeneity: The degree to which the use of diverse technologies requires to use diverse test methods and tools in paralle
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Testability of Requirements
• consistent
• complete
• unambiguous
• quantitative
• verifiable in practice
Compatibility
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Compatibility
• Backward
• Forward
Summary
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Quality Attributes Relationship Matrix
Responsive
ness Reliability Security Usability Extensibility
Reliability
Security
Extensibility
Usability
Responsive
ness
Negative Impact
No Impact
Positive Impact
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Quality Model
• McCall/GE Quality Model
• ISO 9126
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Business Qualities
• Time to Market
• Cost and Benefit
• Project Lifetime
• Target Market
• Rollout schedule
• Integration with Legacy System
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Symptoms of Bad Architecture
• Rigidity • the system is hard to change because every change forces many
other changes.
• Fragility • changes cause the system to break in conceptually unrelated places.
• Immobility • it’s hard to disentangle the system into reusable components.
• Viscosity • doing things correctly is harder than doing things incorrectly.
• Opacity • the code is hard to read and understand. It does not express its intent
well.
