1. Analyzing an Architecture

2. Why analyze an architecture?
Decide whether it solves the problem
Compare to other architectures
Assess what needs to change, given some major requirements change
Find a problem

3. Methods ATAM – Architecture Trade-off Analysis
CBAM – Cost Benefit Analysis
Architecture Review

4. ATAM

5. ATAM Participants Evaluation Team Project Decision Makers
Sponsor, Customer, Management
Architecture Stakeholder
Developers, Users, Testers, etc.

6. Evaluation Team Team Leader Evaluation Leader Scenario Scribe
Proceedings Scribe
Timekeeper
Process Observer
Process Enforcer
Questioner

7. Outputs Concise presentation of the Architecture
Articulation of Business Goals
Quality Requirements – collection of scenarios
Mapping of Architectural decisions to qualities
Identified sensitivity and trade-off points
Set of risks and non-risks
Set of risk themes

8. Phases of ATAM 0 - Partnership and Preparation
1 – Evaluation day
2 TO 3 WEEKS BREAK HERE!
2 – Evaluation 2 – 2 days
4 – Follow-up week

9. Evaluation Steps – Phase 1
1 - Present the ATAM
2 - Present Business Drivers
Most important functions
Constraints – technical, economic, political
Business goals
Major stakeholders
Architectural drivers

10. Evaluation Steps 3 – Present Architecture (~20 slides; 60 minutes)
4 – Identify Architectural Approaches
5 – Quality Attribute Utility Tree

11. Quality attribute utility tree
Top level: quality attributes
Second level: attribute sub-categories (or, scenario groupings if you think bottom-up)
Third level: quality attribute scenarios
For each scenario, two priority rankings (H/M/L):
Importance
Architect’s estimate of difficulty
See pp

12. Scenario priority hierarchy (Don’s)HH
Top
HM, MH
Second level MM
Time permitting
HL, ML, LL
Ignore (you won't have time anyway, and they aren't hard to build)
LH, LM
Why would you implement this in the first place?

13. Evaluation Steps 6 – Analyze Architectural Approaches
Focus on top priority scenarios first, then 2nd level, …
Understand the architectural decisions made for the scenario, and the consequences
For each architectural decision, generate lists of
Sensitivity points (scenarios influenced by the decision)
Tradeoff points (one scenario helped, another one hurt)
For each sensitivity point and each tradeoff, decide whether it is a risk or non-risk

14. Sensitivities and tradeoffs: examples
Configuration files: good for modifiability
Virtual machine: good for portability
Virtual machine: bad for performance
Virtual machine is also a tradeoff point, because it is good for at least one attribute and bad for at least one other

15. Evaluation Steps – Phase 2
7 – Brainstorm and prioritize scenarios
8 - Analyze Architectural Approaches
9 – Present Results

16. CBAM

17. CBAM Goal of CBAM is quantifying decisions
Utility – a measure of value
Cost – to implement a given strategy
Use outputs of ATAM
This is a subjective process!

18. Scenario For each scenario of ATAM: Generate a set of scenarios
The set has a range of responses
The response measure needs to be quantified

19. Assign Utility Values This can be hard!
Look at plots in text (pg. 311)
Start by establishing worst-case and best-case limits (0 and 100)
They also tried to find a current and desired value

20. Priorities Prioritize the scenarios in order first
The add a weight for each one

21. Architectural Strategies
Expected value of the response
(Interpolate on the plot)
Effect on other attributes
Cost to implement

22. Benefit Bi = Sj (bi,j * Wj)
bi,j expected change in utility with respect to scenario j as a result of using strategy i.
W is weight of scenarios

23. ROI
R = B / C

24. Summary ATAM and CBAM provide structure
CBAM can be hard, since quantification is called for!
Next week we consider a less structured alternative.