Centralize or Decentralize? A Requirements Engineering Perspective on Internet-Scale Architectures

1. Centralize or Decentralize?A Requirements Engineering Perspective on Internet-Scale Architectures Eric Yu University of Toronto July 2000

2. Themes of this talk • Architectural decisions are (should be) driven by Requirements • Need to make the linkages • more explicit, and • better supported • Need to collect fine-grained design knowledge to support systematic design “Knowledge-based” approach • representational framework • analysis and design techniques • collections of design knowledge • methodologies • tools

3. Non-Functional Requirements* • Designing large-scale systems involves tough tradeoffs among many interacting forces • performance • cost • usability • reliability • security • maintainability • evolvability • time-to-market • ... * also called “-ilities”, Extra-Functional Requirements, Quality Attributes, ...

4. “-ilities” are most often viewed as evaluation criteria for architectures • Most discussions of architectures take these Requirements as evaluation criteria, ie. • present an architectural solution • then argue for its benefits (and drawbacks) with respect to these qualities/ attributes • For example… • <most of the talks in this Workshop> • [Yimam Kobsa] talk shows this approach is too coarse-grained for guiding design (first contrasts decent. and cent., then adopts hybrid.)

5. From [Yimam & Kobsa TWIST2000] presentationAnalysis Background First appr. Alternatives DEMOIR Summary

6. From [Yimam & Kobsa TWIST2000] presentationAnalysis (contd.) Background First appr. Alternatives DEMOIR Summary

7. To centralize or decentralize… ? • Should first ask: What requirements are you trying to address? • Design question: Given the requirements, what are the suitable solutions? • Need to relate architectural solutions ---systematically to--> requirements/ attributes • then use them in the reverse direction during design Examples: • replication --> for speed of global access • distribute data close to source or user --> for local processing • redundancy --> for reliability • centralized management --> to reduce mgmt costs • single database --> to avoid inconsistencies • fewer sites --> to reduce security exposure • But need finer-grained reasoning

8. Need for Requirements Engineering frameworks • Tradeoffs among competing requirements occur at many places and at various stages during requirements analysis and system design decision-making process • Need systematic framework to support: • managing large no. of requirements (Func. & Non-Func.) • detecting & analyzing their interactions • using requirements to guide exploration, pruning & evaluation of design alternatives • dealing with change

9. Security[Ticket Info] Availability [Ticket Info] Confidentiality[Ticket Info] Integrity [Ticket Info] Costs [Ticket System] ... Confidentiality [Ticket Itinerary Info] Confidentiality [Ticket Payment Info] Reliability [Ticket Info Processing] Tamper Resistant [Ticket Info Processing] - ... ... ... Scalability [Ticket System] - Trusted Personnel [Ticket Itinerary Info] Bonded Personnel [Ticket Payment Info] Fault Tolerant Processor [Ticket Info Processor] Redundant Disk Array [Ticket Info Storage] Goal-Oriented Requirements Analysis • Treat requirements as Goals, refine and reduce until operationalized, taking interactions into account [Chung Nixon Yu Mylopoulos 2000 Non-Func. Reqmts for SE], also CACM Jan. 99

10. From viewpoint of Goal-Driven Design... • Centralize vs. Decentralize • refer to broad classes of design techniques or design patterns that have been invented over the years in a number of design areas • transaction processing performance • long-term storage • system availability • security • management functions • Specific techniques for addressing each of these may have classes of solutions that are centralized or decentralized • Each technique tends to address one primary requirement, but typically have impacts on other requirements. Need systematic support to discern, clarify, analyze the interacting issues

11. Knowledge-Based Approach for Requirements-Driven Design • A representational framework (notations, models, languages, ontologies) - expressive enough to deal with the subject matter: reqmts, elaboration steps, design techniques, design steps and process, alternatives, relationships, etc. • Analysis and Design techniques that make use of the semantics of the modelling constructs to support the engineering activities,eg. analyzing interactions among reqmts, generating design options, evaluating implications of design alternatives,... • Collections of reusable design knowledge (KhBs) from case studies to generic knowledge eg. common types of requirements and their possible elaboration, design principles, methods, rules, techniques, patterns of solutions to common design problems, architectures, frameworks, etc.

12. Knowledge-Based Approach for Requirements-Driven Design (cont’d) • Methodologies for guiding the use of the framework, principles, techniques, etc., in various settings • Tools that use the structure & semantics of the knowledge to automate some aspects of the engineering activities, eg. visualization, animation, simulation, verification, support for reasoning (qualitative, quantitative, case-based…) and basic management facilities (maintaining design history, traceability, navigation, query, retrieval, version & change management…)

13. Example: telecom software productDetailed design reasoning in software architecture • task-decomposition • means-ends • contributions to softgoals

15. Requirements and Organizational Issues • Requirements comes from many quarters • in user organization • various kinds of users, • operations personnel • management ... • in development organization • developers • product managers • project managers • quality assurance • marketing … • Tradeoffs involve negotiations among stakeholders (e.g., [Boehm]) • Organizational issues affect technical decisions in significant ways (e.g., [Conway])

16. For Internet-scale systems… organizational issues even more complex • Many distinct economic and legal entities involved in the development, use, and management • Each player has its own interests to pursue • No single overview, or even understanding (e.g., new functionality being added via plug-&-play) • Centralize vs. Decentralize question applies to technical as well as organizational domains

17. Many ways of dividing up the scope of control at various levels • ownership domains • administrative and business management domains • trust domains, from viewpoint of each (class of) stakeholder: • application providers, network operators, user organizations, end-users, intermediaries • developer domains - div. of responsibilities in development • operations management domains - e.g., failure recovery, performance optimization, load balancing, etc. • technical architecture domains at various levels - subsystems, components, modules

18. Domains have intertwined relationships • For example, • trust domains may coincide with administrative domains • ownership domains may overlap with design domains • Alignments are • sometimes intended, other times incidental • usually imperfect • restructured (or may drift) over time. • Complex organizational issues => need extended ontology • goal-oriented --> agent-oriented

19. Agent-Oriented Analysis • Intentional actor as a modelling abstraction to deal with locality and distribution at an intentional level. • Actors have goals, beliefs, abilities, commitments. • Actors depend on each other for goals to be achieved, tasks to be performed, and resources to be furnished.

20. LEGEND Actor Resource Dependency Task Dependency Goal Dependency Softgoal Dependency Depender Dependum Dependee Figure1: Strategic Dependency model of smart card based payment system Example: Smart CardsSome basic relationships among stakeholders

21. Agent-Oriented Analysis (cont’d) • Each actor pursues its own interests, while considering the consequences of its decisions and actions because of its relationships with other actors. • The deliberations of each actor is modelled analogously to the goal-graph structure of NFR framework. • The design space is carved up into many localized spaces. • The intentional relationships among actors define the interfaces among localized spaces. • Actors have limited knowledge about internal rationales of other actors.

22. LEGEND Task Decomposition Link Means-Ends Link Contribution Link Counter Contribution Link Position Resource Task Goal Softgoal Figure 3: Strategic Rationale model of smart card based system Example: Smart CardsDetailed relationships from viewpoint of each player

23. Figure 6: A Strategic Rationale model showing details of selected attacker roles and defender roles Analyzing security & trust in deploying Smart Card technology Attack! Defense!

24. One particular Smart Card deployment configuration: Phone company as Terminal Owner, Data Owner, Card Issuer, Card Manufacturer, and Software Manufacturer When several roles are played by the same player, some attacker-defender pairs disappear.

25. tools

26. Summary • Knowledge-based approach to SE • representational framework - Goals and Agents as key constructs in the ontology • analysis and design techniques • collections of design knowledge • methodologies - Requirements-Driven • tools • Key Challenges: • collecting, organizing knowledge for system design(including various reasons for centralizing vs. decentralizing) • Providing analysis and design support