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Quality AttributesSoftware Architecture (707.023)

Roman Kern

KTI, TU Graz

2017-11-15

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Outline

1 Quality Attributes

2 Runtime QA

3 Non-runtime QA

4 Business QA

5 Non-Functional Concepts

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Quality AttributesFocus on non-functional requirements.

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Need to address QAs

Without any need for performance, scalability, ... any implementationof functionality is acceptableHowever, we always need to take into account the broader contextE.g. hardware, technological, organizational, business, ...The functionality must be there but without proper addressing of QA itis worth nothing

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Influence on QAs

Typically, a single component can not address a QA completelyAny QA is influenced bymultiple components and their interactionsE.g. a UI component has a high degree of usability: however, usabilityof the system is compromised if a data management component haspoor performance in accessing the data→ users need to wait long→poor usabilityComponents and their interactions→ software architectureQAs are directly influenced by software architecture

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Runtime Quality AttributesQAs that affect the runtime behaviour.

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PURS

PURS (performance, usability, reliability, security)Performance: time performance, memory, disk, or network utilizationUsability: human factors, easy to learn, easy to use, ...Reliability: availability, safety, ...Security: authentication, data protection, ...

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Performance

Time performance is most obviousMeasured in the number of operations per secondAlso, latency: the time from receiving an input and producing anoutputOther measures: memory, disk, network utilization or throughput

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Performance

Different measures are typically traded off against each otherE.g. increasing throughput may increase latencyTime performancemight be increased with more memoryTrue performance of the system is not only defined by performance ofsingle componentsBut also by their interactions and the overall processes in the system

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Performance factors

1 Choice of algorithms2 Database design3 Communication4 Resource management

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Choice of algorithms

Performance of algorithms is typically measured by their complexity(bigO notation)E.g. linear complexity: O(n)Running time increases in direct proportion to the size of the dataE.g. polynomial complexity: O(n2)It does not scale: double size of the data - running time increased byfactor of 4→ “friendly” runtime complexities:Realistic goal: O(nlog(n))(typically, only theoretically achievable: O(n),O(1))However,O does only describe one aspect of the performance (worstcase behaviour vs. typical behaviour)

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Database design

Performance of database queries can dominate the overallperformanceThe design of the tables has enormous impact on the overallperformanceTechniques to improve it: lazy evaluation, replication, cachingSome additional cost to manage replication and/or cachingIn-memory databases (real-time systems)Developing a new database (search engines)

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Communication

Network overheadPackage data according to a protocol, sending data over networkEach layer means additional overheadThink how to use network: packaging binary data as XML!?Use more compact formats, e.g. JSON vs XML

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Resources management

Overloaded components need to be avoidedA chain is only as strong as its weakest link!E.g. a single-threaded shared resource is in use: all other threads areblockedVery difficult to track down

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Performance Tactics

Generate a response to an event within some time-based constraintTwo basic contributors to response time:

▶ Processing time (e.g. computation)▶ Blocking time (e.g. waiting for other components, resources)

Tactics:1 Control resource demand - reduce demand on resources2 Manage resources - work more efficiently

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Performance Tactics - Control Resource Demand

Increase performance bymanage the demand for resources:Manage the sample rate - reduce the number of events beinggeneratedLimit event response - use queueing systems (in the worst case, throwaway events if queue is full)Prioritize events - ignore low-priority eventsIncrease resource efficiency - invest time to improve algorithmsReduce overhead - e.g. merge components to reduce latency→negative effect onmodifiabilityBound execution times - sacrifice accuracy to achieve higherperformance, e.g. using faster algorithms that just approximate thecorrect behaviour

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Performance Tactics - Manage Resources

Increase performance bymanage resources:Increase resources - faster processors, additional memory, ... (scalevertically)Introduce concurrency - parallel computingMaintain multiple copies of computations - more than onemachine forcomputations (scale horizontally)Maintain multiple copies of data - caches for faster data accessBound queue size - control the number of incoming eventsSchedule resources - introduce scheduling for resources (e.g.priorities)

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Usability

Usability is a very rich fieldIf usability is important you will need a usability expertCombination of many factors: responsiveness, graphical design, userexpectations, confidenceMeasuring with time taken to complete task, error rate, time toresponse, ...

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Responsiveness and data availability

An example of relations between QAsUsability requires that the system responds to user actions within acertain period of timeIf it is a complex system this need translates into performance alongthe path of the user action

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Discussion on relations between QAs

If we support security even if it is not neededVery often QAs exercise opposing forces on the systemSecurity requires a lot of checking: performance will suffer→ usabilitywill sufferA minimalistic approach: develop only what is required!

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Usability Tactics

General advise: separate the user interface... also support modifiabilityTactics:

1 Support user initiative - how easy it is for the user to accomplish a task2 Support system initiative - support of the system for the user

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Usability Tactics

Support user initiativeCancel - allow the user to cancel long running operationsUndo - prove means to rollback the statePause/resume - control the execution of long running operationsAggregate - prevent micro-management by allowing batch operationsonmultiple objects (e.g. select multiple files)

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Usability Tactics

Support system initiativeMaintain task model - assess the user’s context to provide assistance(e.g. word completion in a text editor)Maintain user model - system behaviour should match usersexpectations (even allow the user to control the model), e.g. scrollingspeedMaintain systemmodel - monitor the system’s behaviour, e.g. displayprogressbar for long running operations

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Reliability

In traditional engineering disciplines reliability measures the failurerate of the systemFailure rate specified by mean time to failureMTTFA relatedmeasure: mean time between failuresMTBFMTTR is mean time to repairA is availability

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Reliability

MTBF = MTTF+ MTTRA = MTTF

MTBF

A = MTTFMTTF+MTTR

E.g. expected (theoretically optimal) availability of productionsystems:

1 (always up-and-running)=⇒ MTTF→ ∞

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Reliability

MTBF = MTTF+ MTTRA = MTTF

MTBF

A = MTTFMTTF+MTTR

E.g. expected (theoretically optimal) availability of productionsystems: 1 (always up-and-running)=⇒ MTTF→ ∞

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Reliability

Increasing reliability involves testingHowever, impossible to prove that a system is correct, i.e. withoutbugsAcceptability of errors depends on the nature of a systemPersonal desktop use: bugs are typically toleratedEnterprise level: medium reliability levelHigh-reliable systems: bugs can be fatalFailure analysis: combine the probability of a failure with its effect

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Reliability Tactics

Use higher level programming languagesUse existing proven componentsTactics:

1 Detect faults - identify non-working components2 Recover from faults - what to do in case of faults3 Prevent faults - how to increase availability

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Reliability Tactics

Detect faultsMonitor - explicit component to monitor the systemPing/echo - test the reachability of components by asynchronous testmessagesHeartbeat - fault detection mechanism by periodic messagesTime stamp - detect incorrect sequence of messagesSanity checking - check the validity of messagesCondition monitoring - e.g. checksums for messagesVoting - e.g. three systems do the same thing to detect inconsistenciesException detection - e.g. CPU register for division by zero, read accessto uninitialised memorySelf-test - deliberate health checks by the components themselves

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Reliability Tactics

Recover from faultsActive redundancy - identical system running a backup, instantaneousup-to-date (hot spare)Passive redundancy - identical system running a backup, periodicupdates (warm spare)Spare - backup system needs to be started in case of fault (cold spare)Exception handling - how to deal with exceptional behaviourRollback - go back to an older (good) system stateSoftware upgrade - patch the system (or sub-systems)Retry - repeat failed actionIgnore faulty behaviour - only, if the error messages are spuriousDegradation - explicit state of reduced functionalityReconfiguration - reassign jobs to unaffected components

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Reliability Tactics

Prevent faultsRemoval from service - take components off-line to prevent (or restart)Transactions - prevent race conditions by clear transaction semanticsPredictive model - state of the health of a systemException prevention - e.g. smart pointers (specialised data structure)Increase competence set - improve components to deal withexceptional states

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Security

Increasingly important aspect of systems is securityBecause systems are exposed to threatsEspecially networked systemsAs with other QAs security is a set of related responses to user needs

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Authentication

Requirement for identification of users with a systemUsers present credentials so that the system can identify themTypically username and passwordOther forms: certificates, smart cards, biometric features

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Authorization

After authentication authorization which functions and what data isavailable for usersThis information is captured in an authorization modelAccess control lists (ACL) define who can access and how a resourcemight be accessedE.g. read access, write access, delete access, ...

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Authorization

Drawbacks of ACLsIt is resource based, e.g. a page in a CMSOften, authorization needs to address functions or tasksAlso, managing of ACLs is difficult, e.g. subresources of resourcesAlso, performance problems with checking

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Authorization

Another model: role-based access control (RBAC)Roles are used to managemany-to-many relations between users andpermissionsRoles are used to represent the job functions, e.g. author, teacher,student in an E-learning systemPermissions are modelled as parts of roles, e.g. create page, createtests, ...Users are than assigned to a role and acquire automaticallypermissions of that role

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Security Tactics

Tactics:1 Resist attacks - prevent attacks before they happen2 Detect attacks - identify ongoing attacks3 React to attacks - response in case of attack4 Recover from attacks - actions taken after an attack

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Security Tactics

Resist attacksIdentify actors - identify the source of an external inputAuthenticate actorsAuthorize actorsLimit access - restrict access to resources, e.g. firewallsLimit exposure - restrict the number of access points of the systemEncrypt data - protect the communicationSeparate entities - have components on different machines (or virtualmachines)Change default settings - force users to change the default settings of asystem

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Security Tactics

Detect attacksDetect intrusion - e.g. monitor network traffic and identify maliciousbehaviourDetect service denial - e.g. compare service requests pattern withknown denial-of-service attacksVerify message integrity - e.g. checksums, hash valuesDetect message delays - e.g. monitor time to deliver messages

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Security Tactics

React to attacksRevoke access - restrict access to potentially compromised resourcesLock account - e.g. repeated failed logins as a trigger to lock anaccountInform actors - means to notify relevant actors, e.g. mail to sysadmin

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Security Tactics

Recover from attacksRestore - rollback the system (and its data) to a known state (prior tothe intrusion)Audit trail - keep a record of user and system actions to allowcomputer forensics

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Non-runtime Quality AttributesQA not related to runtime behaviour.

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MeTRiCS

MeTRiCS (maintainability, evolvability, testability, reusability,integrability, configurability, scalability)Maintainability: how easy can you fix bugs and add new featuresEvolvability: how easy your system copes with changesTestability: how easy can you test the system for correctness

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MeTRiCS

Reusability: how easy is to use software elements in other contexts,e.g. a software libraryIntegrability: how easy you canmake the separately developedcomponents of the systemwork correctly togetherConfigurability: how easy can a system be configured for differentinstallations and target groupsScalability: how easy the system copes with a higher performancedemand

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Maintainability

This QA considers the whole lifecycle of a systemWhat happens during system operation?Property that allows a system to bemodified after deployment witheaseE.g. extensible, modified behaviour, fixing errors

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Maintainability

At the design and implementation levelCode commentsObject-oriented principles and design rulesConsistent programming stylesDocumentation

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Maintainability

Maintainability is very important because any software systemwillchange over timeExperience shows that such changes tend to degrade the system overtimeSoftware systems are subject to entropyThe cumulative effect of changes degrades the quality of the system

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Maintainability

The systems tend to becomemessy systemsRegardless of how a nice plan you had at beginningDesign for change - recollect OO design rulesAbstract messy parts of the system so that they can be exchanged

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Maintainability

Don’t be afraid to refactor and rewrite and redesignEach software vendor does this with major versionsCreate throw-away prototypesThink out-of-box and innovateDon’t always follow a hype - very often nothing new in hypesE.g. Web services

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Maintainability Tactics

Tactics:1 Reduce size of components2 Increase cohesion3 Reduce coupling4 Defer binding

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Maintainability Tactics

Reduce size of componentsSplit component - divide big components into smallersub-components

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Maintainability Tactics

Increase cohesionIncrease semantic coherence - if a single component has manyresponsibilities then divide this component into multiple, smallercomponents

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Maintainability Tactics

Reduce couplingEncapsulate - introduce interfaces and APIs, and allow access onlythrough the interfaceUse an intermediary - break dependencies, e.g. by publish-subscribearchitecturesRestrict dependencies - reduce the visibility of components, e.g. n-tierarchitecturesRefactor - avoid duplication by refactoring out common functionalityinto new componentsAbstract common services - for similar, but not identical functionality,build a more general, abstract components

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Maintainability Tactics

Defer bindingParameters - plan components with built in flexibility to anticipatefuture changes and feature requests

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Testability

Means to improve testabilityTest cases: if something fails there is a bugSeparation of the testing framework and the system, i.e. testing withscripts from outsideLogging

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Testability Tactics

Tactics:1 Control and observe system state - make testing easier2 Limit complexity - complex systems are harder to test, thus make them

simpler

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Testability Tactics

Control and observe system stateSpecialised interfaces - additional testing interfaces (e.g. enableverbose output, set state)Record/playback - means to track in internal stateLocalise state storage - store a specific system stateAbstract data sources - e.g. mock object data access instead a “real”databaseSandbox - e.g. by virtualisationExecutable assertions - pre/post conditions with documentation

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Testability Tactics

Limit complexityLimit structural complexity - remove dependencies, reduce couplingLimit non-determinism - limit behaviour complexity by identifyingsources of non-determinism (e.g. unconstrained parallelism)

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Configurability

Ability of a system to vary its operational parameters withoutre-compiling or re-installingE.g. selecting appropriate database drivers, configuring networkparameters, ...Typically, realized by a set of configuration filesE.g. Apache Web server configuration file sets host name, virtual hosts,...

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Configurability

Configurability interacts with other QAs such as testability,maintainability, reliabilityHigh degree of configurability tends to have a negative impact onthose QAsTesting of different system configuration becomesmore difficult→reliability compromisedConfigurable components will be strongly parametrized→ decreasedmaintainability

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Scalability

Ability of a system to increase its capacity without re-compiling orre-installingE.g. serving additional Web pages means only copying these Webpages into a Web server file systemSometimes increasing capacity means increasing hardware, e.g. Webserver clustersManaging user session on the client side, means only providingadditional code-on-demand from the server

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Business Quality AttributesQAs focused on the business quality goals.

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Overview

Two types of business quality attributesRelated to costs and scheduleRelated to marketing

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Overview

Time tomarketTime it takes to ship the (finished) productHighly depends on already existing components... relates to reusability... and the selection of the infrastructure components

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Overview

CostsThe budget to develop the systemArchitecture plays an important role in the actual costs

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Overview

Lifetime of projectThe type of the project plays an important (business) roleProject, that does not need to bemaintained afterwardsProducts, which are planned to bemaintained

▶ Multiple releases in the future▶ ... will require an architecture more focused on evolvability

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Overview

Market and target usersFor the mass market vs. niche marketsExpectations will be different for eachmarket (e.g. level of usability)... need to take this into account

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Overview

Integration with other/legacy systemsWhat is the expected infrastructure?Which other components/systems will be present... need to integrate interfaces to these systems

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Non-Functional ConceptsImplications of cross-cutting concerns.

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Functionality Shared between Software Components

Requirements not covered by individual componentsInstead requirements needed to be addressed by each componentOr constraints imposed on each component⇒ Cross-cutting concerns

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Examples for Cross-Cutting Concerns

LoggingAuditingMonitoringException/error handlingSecurity (e.g. authentication, authorisation)CachingLocalisationConfiguration management

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Cross-Cutting Functionality Implications

Why are cross-cutting aspects a problem?They do need to be consistently used/implemented by all componentsSome components might not allow thisIf the cross-cutting functionality need to be adapted, all componentsneed to bemodifiedE.g. changes due to new security constraints require all components tobe inspected

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Cross-Cutting Concerns on the Implementation Level

Cross-cutting concerns also exist on the implementation levelAddressed by aspect oriented programmingAspects are factored out andmoved away from the functionalityE.g. logging is done in a separate module and at runtime injected intothe method calls

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Cross-Cutting Concerns on the SA Level

Keep cross-cutting functionality in separate components (is possible)E.g. dedicated component for authentication/authorisationUse/develop components that allowmodificationsE.g. Interceptor architectural pattern

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The EndNext: Service architectures

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