Evaluating Software Architectures

RiSE’s SeminarsClement’s book :: Chapters 05

Eduardo Cruz

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Summary

About Software Architecture Quality Attribute Characterizations

Performance Availability Modifiability Characterizations Inspire Questions

Using Quality Attributes Characterizations in ATAM Attribute-Based Architectural Styles

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About software architecture

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What is software architecture ?

It is the manifestation of the earliest design decisions

It is a reusable, transferable abstraction of a system

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Why software architecture is important

Is a vehicle for communication among stakeholders Architectural View Functional View Concurrency View Code View Development view Physical View

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Why evaluate an architecture ?

The sooner, the better Avoid disasters

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Who’s involved

Evaluation team Stakeholders

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Understanding Quality Attributes

Quality Attributes - Prerequisite of an evaluation Common incomplete quality atribute requirements

and architecture documentation

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Architecture Evaluation Focus on Quality Attributes

Evaluate the architecural design decisions to determine if they address the quality attribute scenarios

Understanding Quality Attributes

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Quality attribute parts

AvailabilityModifiabilityPerformanceSecurityTestabilityUsability

Designing the Architecture :: Chapter 7

ARTIFACTSTIMULUS RESPONSE RESPONSEMEASURE

SOURCE OFSTIMULUS ENVIRONMENT

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Functionality and Quality Attributes

Must be considered throughout: design, implementation and deployment Usability, Modifiability, Performance

Architecture itself is unable to achieve quality if attention is not paid to the details

With complex systems, quality attributes can never be achieved in isolation

Software architecture in PracticeChapter 4 – Understanding Quality Attributes

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Quality attribute characterizations

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Quality Attribute Characterizations

Different in its stimuli Performance – Arrival of events Availability – Fault occuring

Different in its responses Modifiability – persons-day required to make a requested

change Security – how many intruders will break into the systems

and what resources they will be able to access

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Performance

Ability of a system to allocate its computational resources to requests for service in a manner that will satisfy timing requirements

Resource types Uni/multi processors, memory, devices

Resource arbitration On-line/Off-line Scheduling, Synchronization

Resource Allocation Load Balancing

Resource consumption Execution Time, Memory Size, Network Bandwith

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Availability

Concerned with system failure and its consequence Failure concerns

How is detected How frequently may occur Consequences How long is out of operation How to prevent

Software architecture in PracticeChapter 4 – Understanding Quality Attributes

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Sample Availability-related questions

Stimuli How are hadware and software failures identified ? Can active as well passive failures be identified ?

Architectural decisions If redundancy is used in the architecture …

what type of redundancy (analytic, replication, functional) ? How many replicas of each critical component and connection

are used ? Responses

Are there levels of service available ? How quickly ca a backup be made/restored ?

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Modifiability

Ability of a system to be changed after is implemented

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Modifiability

Cost of change What to change

Hardware, OS, Performance, number of users When and Who

Compile, build, setup, execution Specify, design, implement, test, deploy

Time and money, measured

Software architecture in PracticeChapter 4 – Understanding Quality Attributes

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Security

System's ability to resist to unauthorized usage while still providing its service to legitimate users

Attack – Attempt to breach security

Access data, modify data, deny service

Characteristics Nonrepudiation

You did (Transaction) Confidentiality

Protected data (income tax) Integrity

Cannot change data Assurance

You are who purport to be (credit card)

Availability Free of DOS

Auditing Keep track of activities

Software architecture in PracticeChapter 4 – Understanding Quality Attributes

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Usability

How easy it is for the user to accomplish a desired task and the kind of support the system provides

Learning system features Using system efficiently Minimizing the impact of errors Adapting the system to user needs Increasing confidence and satisfaction

“Usability is not architectural”

Software architecture in PracticeChapter 4 – Understanding Quality Attributes

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Using Quality Attributes Characterizations in ATAM

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Modifiability Characterization

Stimuli Architectural Decisions Responses

Chage Request

Function

Platform

Quality Attribute

Operating Environment

Indirection

Encapsulation

Separations

Addition

Modification

Deletion

Characterization inspires questions

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Characterization inspires questions

A beginner can make use of existing questions It requires more expertise in a quality attribute to

devise new questions from the characterizations It requires still more expertise to understand how to

respond to the questions

Template for Analysis of an Architectural Approach

Scenario #: A12 Scenario: Detect and recover from HW failure of main switch

Attributes Availability

Environment Normal Operations

Stimulus One of the CPUs fails

Response 0.999999 availability of the swith

Architectural Decisions

Sensitivity Tradeoff Risk Nonrisk

Backup CPU S2 R8

No backup data channel

S3 T3 R9

Heartbeat S5 N12

Reasoning Ensures no common mode failure by using different hardware and operating systems (see Risk R8) Worst case rollover is accomplished in 4 seconds as computing state takes that long at worstGuaranteed to detect failure with 2 seconds based on rates of heartbeat and watchdog

Primary CPU (OS1)

Backup CPU w/ watchdog (OS2)

Primary CPU (OS1)Architecural diagram

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Quality Scenarios

Source of stimulus Some entity: Human, computer

Stimulus Condition that need to be considered when it arrives at a system

Environment Stimulus occurs within certain conditions

Artifact What is stimulated. The whole system or some pieces of it

Response Activity undertaken after the arrival of the stimulus

Response measure Response should be measurable, so that the requirement can be tested.

Software architecture in PracticeChapter 4 – Understanding Quality Attributes

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ABAS – Attribute-Based Architectural Styles

Related architectural and analysis techniques in a package Problem description Stimuli/responses Architectural style Analysis

Another source of inspiration and reference when criating the attribute specific questions

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Business Qualities System Qualities x Business

Qualities Cost, schedule, market,

marketing Time to market

Commercial off-the-shelf (COTS) Cost and benefit

Exceed budget Projected lifetime of the system

Portability/usability Targeted market Rollout schedule

Base functionality w/ features later

Integration with legacy system

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References

[Clements, 2001] P. Clements, R. Kazman, M. Klein, Evaluating Software Architectures: Methods and Case Studies, Addison-Wesley, 2001, pp. 368.

[Bass, 2003] L. Bass, P. Clements, R. Kazman, Software Architecture in Practice, 2nd Edition, Addison-Wesley, 2003, pp. 560.

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