Enhancing Architecture Evaluation Strategies: A Key to Advancing the Field

1. All hat, no answers Some issues related to the evaluation of architecture John WroclawskiUSC/ISI NSF FIA MeetingMarch 19, 2013

2. Why think about this? • NSF 13-538: “An evaluation plan…” • Guide and inform your work (feedback) • Explain to and convince others • Advance the field – a {hard, transformational} research area in itself.

3. Architecture • “high level design principles that guide the technical development of a system, especially the engineering of its protocols and algorithms” • Two distinct levels: • A specification of system modularity, functional decomposition, interface locations and definitions, etc. – the “what”. • A set of fundamental structuring principles that drive the choices of the first bullet – the “why”.

4. Architecture versus System versus Prototype

5. System “A realized instantiation of a design, that meets specific requirements” • Well studied discipline - multiple design approaches: Waterfall… • Requirementsanalysis • Detailed systemengineering • Detailed component engineering • Construction.. Agile… Some evaluation criteria: • “Meets Requirements” “Performance” “Lifecycle Cost” …

6. Prototype “A partial system development focusing on key components or issues” • Proof of concept • Learn what’s missing • Expose ideas and convince others • Test key system functions quickly • … • Some (meta?) evaluation criteria • “Focused on the key open questions” “Learn what you need to” “Not misleading” “Leverage existing resources” “Quick turnaround” • (See Craig Partridge’s comments in summary from last meeting)

7. Observation • Design objectives, and hence evaluation processes, are significantly different for each of these things. • All are useful. • But: need to tease them apart. Need to discuss each separately.

8. Network* Architecture • Distinction between requirements / properties of the architecture and requirements /properties of a system. • Many systems can share onearchitecture Basic technical requirements and properties Evaluation Coordination in space–multiple independent implementers Coordination and evolution in time –multiple, evolving technologies and uses *or maybe “Internet”…

9. Diversion - “Elegance” • Architects talk about it a lot. Means what? • One view – economy of mechanism, minimalist, “clean” • Another view (or maybe not..): structural principles intuitively apparent –architecture conveys understanding, not just rules. “Feels like it makes sense.” • Why does this matter? Guidance for evolution in space and time. • An evaluation point: are the architecture’s structuringprinciples clear, intuitive, and apparent?

10. Evaluation • Step 1: Clarify the proposed architecture – separate from system and prototype • Step 2: Identify and execute evaluation strategies

11. Evaluation Strategies • Today, we have basically two. • Socratic discourse • Build it, achieve success, and wait 30 years • In fact, this – particularly the first – has worked rather better than my snide tone might suggest • With the right structure and context, people have demonstrated good ability to reason about architectural principles • Telephone vs IP – strengths of each • ATM QoSvs IP QoS – a clear lesson in simplicity, coordination in space, and more • Discussion point: Can we foster this structure and context? How?

12. Evaluation Strategies II • Still, we would like more. Lets talk about two: • … • … • Moving towards Theoretical Frameworks – increased rigor and more structured understanding • Experimental Research – contrasted with prototype deployment

13. Theoretical Frameworks • We do not today have a “theory of architecture” – (very) far from it • But – there are glimmers. From • Game theory • Optimization theory • Various bits of economics • … • A possible synthesis principle for increased architectural evaluability (among other benefits…): • Choose modularities with intent to bring emerging theory into scope • Allows evaluation of sub-architectures using these tools

14. An example:Theoretically Derived Architectural Modularity Network resource allocation formulated as global optimization problem Primal-dual decomposition generates a set of dual problems/algorithms/modules: Local (except scheduling) Tied together through congestion prices (sub)-system architecturetraceable to theoretically provable optimality.. Applications Congestion control Routing Scheduling Channel OLD SLIDE... Utility function U_s{x_s} (strictly concave function of the sending rates) Cross-layer interaction in form of “congestion prices” (cost per unit flow of sending data along a link to a destination) Optimal Cross-Layer Congestion Control, Routing, and Scheduling Design in Ad Hoc Wireless Networks. Lijun Chen, Steven H. Low, Mung Chiang†,John C. Doyle (Caltech and †Princeton)

15. Experimental Research • …as distinct from prototype construction and deployment • Focus on • Worst case experiments • Intentional perturbation • Accelerated evolution • Comparison (implies repeatability?) • Given current state of the art, likely to involve simulation of key system components, modeling of users, etc.

16. Takeaway Points • Clarify the domain – Architecture vs System vs Prototype • Each domain both requires and deserves separate evaluation • Clarify the architecture – both underlying principles and resulting structure • Mixed mode evaluation for a still-empirical discipline • Evaluate through discourse • Evaluate through analysis • Evaluate through experiment Driven by reasoning Driven by increasing rigor