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SYSTEMATIC ELABORATION OFSCALIBILITY REQUIREMENTS THROUGH
GOAL-OBSTACLE ANALYSIS
Leticia Dubo
and David
IEEE
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EXAMPLES OF SCALABILITY
Scenario 1Web search Engine
Scenario 2Air traffic control system
Indicators of scalabilityPerformance, Reliability, Availability, Dependabilit
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ISSUES DURING ELABORATION
Specifying scalability requirements that are precise and testable
Identify and manage scalability-related risks early in the development pro
Dealing with unknowns, uncertainties
Ensure scalability requirements are neither too strong nor too weak
Adequate tradeoffs between scalability and other qualities
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SCALABILITY OBSTACLE
Systematically identifying and resolving potential variations in domain quanthat will cause goal to fail
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GOAL-ORIENTED REQUIREMENTS ENGINEERIN
KAOS Approach - Knowledge Acquisition in Automated Specification or KeObjectives Satisfied
Comprises of modeling language and a constructive elaboration techniques
Approach
Goa
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TERMS AND DEFINITIONS
Goal - statement of intent that a system should satisfy through the cooperaagents.
Agents - active system components, such as humans, hardware devices and components
System refers to the composition of the software under development and
environment.
Goals range from high-level business objectives involving multiple agents tograined technical properties involving fewer agents
Domain Properties and Domain HypothesesDescriptive states about appldomain
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AND/OR REFINEMENT STRUCTURE
Goals are organized in AND/OR refinement structures
AND-refinement
Relates a goal to a set of subgoals
Satisfying all subgoals is a sufficient condition for satisfying the goal
OR-refinement
Relates a goal to a set of alternative AND-refinements
Each AND-refinement represents an alternative way to satisfy the parent g
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KAOS APPROACH
Goal elaboration
o Define initial set of high level goals & objects they refer to
o Define initial set of agents and actions they are capable of
Iteratively do the following:
o Refine goals using AND/OR decompositiono Identify obstacles to goals, and goal conflicts
o Operationalize goals into constraints
o Refine & formalize definitions of objects & actions
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GOAL-OBSTACLE ANALYSIS
Goal obstacle analysis consists of identifying, assessing and resolving obsta
Identifying as many obstacles as possible
Assessing the relative importance of the identified obstacles in terms of thei
Resolving more critical obstacles by modifying existing goals, requirements
assumptions Identify-Assess-Resolve loop is repeated until all obstacles are consideredacceptable.
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UNDERSTANDING SCALABILITYScalability
assumptionsobstacle an
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SCALABILITY REQUIREMENTS
Elements required to define scalability
quality goals of the system,
characteristics of application domain and
acceptable levels of quality goal satisfaction
Requires translation to KAOS Framework
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TRANSLATION TO KAOS FRAMEWORK
Quality goals correspond to goals whose specification includes domain-depobjective functions
Scaling assumptionvariation of characteristics in application domain
Scalability goalsquality goals constrained by scaling assumptions
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SCALING ASSUMPTIONS
Domain assumption specifying how certain characteristics in the application are expected to vary over time
Specification of scaling assumptions
One or more domain quantities whose expected variations are defined in thassumption
The periods of time and classes of system instances over which the assumptiodefined
The range of values each quantity is expected to assume for each system cover each period of time
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EXAMPLE
Assumption - Expected Batch Size Evolution
Category - Scaling Assumption
Definition - From 2011 to 2015,daily batches are expected to contain up tofollowing number of transactions for different bank categories
BANK 2011 UNTIL 2013 UNTIL 2015
Small 10,000 15,000 20,000
Medium 1 million 1.2 million 1.8 million
Large 50 million 55 million 60 million
Merger 80 million 85 million 95 million
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Our definition of scaling assumption covers the case where variations of doquantities refer to a single period of time and a unique class of system instan
Alternative simpler scaling assumption
Assumption - Expected Batch Size Evolution
Category- Scaling Assumption
DefinitionOver the next five years, daily batches of transactions for all bare expected to vary between 10,000 and 95 million transactions
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SUBGOALS FOR GOALS
Subgoal by using the scaling assumption
Goal Achieve [Batch Processed Overnight Under Expected Batch Size Evolu
Category Performance goal, scalability goal
Definition At the end of each day, the batch of transactions submitted by thshould be processed in less than 8 hours, provided that the batch size does noexceed the bounds stated in the scaling assumption Expected Batch Size Evol
Scaling assumptioncould be descriptive or prescriptive
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SCALABILITY GOALS
A scalability goal is a goal whose definition and required levels of goalsatisfaction (as specified by its objective functions) make explicit reference to more scaling assumptions.
The goal Achieve [Batch Processed Overnight Under Expected Batch Size Eis a scalability goal.
Reason - its definition refers to the scaling assumption Expected Batch SizeEvolution.
Comparison of scaling assumptions for air traffic control system and online s
Scalability goals with fixed objectives and varying objectives
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SCALABILITY OBSTACLE ANALYSIS
PROCEDURE
Identifying scalability obstacles that may obstruct the satisfaction of goals,requirements and expectations elaborated so far.
assessing the likelihood and criticality of those obstacles
Resolving obstacles by modifying existing goals
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IDENTIFYING SCALABILITY OBSTACLES
For each goal identify
What defines the goal load.
Agent resources involved in its satisfaction
An obstacle of the form Goal Load exceeds Agent Capacity
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ASSESSING SCALABILITY OBSTACLES
Using a qualitative risk analysis matrix
Likelihood of scalability obstacle estimated
Scale ranges from Very unlikely to almost certain
Criticality estimated from Insignificant to catastrophic
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RESOLVING SCALABILITY OBSTACLES
Obstacle resolution process comprises two activities
Generation of alternative resolutions
Selection of resolutions among generated alternatives
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SCALABILITY OBSTACLE RESOLUTION TACTICS
Goal substitutionfinding alternative goals
Agent substitutionchanging the responsibility assignment
Obstacle preventionIntroduce new goals and assumptions
Obstacle reduction Introduce new goal to reduce obstacles likelihood
Goal restorationIntroduce new goal to restore satisfaction of obstructed
Obstacle mitigationIntroduce new goal to mitigate consequences of an o
Do-NothingLeave the obstacle unresolved
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CONCLUSION
STRENGTHS
By extending KAOS, we facilitate precise specification of testable scalabilitrequirements
Scalability obstacle analysis method provides new obstacle resolution tactic
Elaboration covers achieving scalability as well as goal-resolving technique
WEAKNESS
Statistics from real-time projects not mentioned
No real-time values and experiments included in the elaboration for achievscalability
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QUESTIONS ?