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Quality Attributes Or, what’s wrong with this:

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Presentation on theme: "Quality Attributes Or, what’s wrong with this:"— Presentation transcript:

1 Quality Attributes Or, what’s wrong with this:
Exterminator kit – place bug on block, strike with mallet

2 Functionality vs Quality Attributes

3 Some Qualities Usability Modifiability Performance Security
Testability Availability Time to market Cost and benefit Projected System lifetime Targeted Market Rollout Schedule Integration / Legacy

4 Architectural Qualities
Conceptual Integrity Correctness and Completeness Buildability

5 Qualities & Trade-offs
The qualities are all good The qualities value is project specific The qualities are not independent

6 Quality Attribute Scenarios
Source of stimulus Stimulus Environment Artifact Response Response measure (see inside front cover) In the environment, the source throws the stimulus and hits the system in the artifact

7 Example from cars Response Measure: Deflection < N% Artifact:
Noise < M dB Artifact: Tires Stimulus: Bumps Environment: Highway driving Source of stimulus: Road Response: Control maintained Smooth ride Low noise

8 Remember One stimulus per scenario One environment per scenario
One artifact per scenario Multiple response measures are OK

9 Measure: No unauthorized
Example from software Response Measure: No unauthorized users, login < 1 min Artifact: User interface Stimulus: Dozens of simultaneous logins Environment: Normal operation Response: Security maintained Acceptable delays Source of stimulus: Shift change

10 If you remember one thing …
To be effective, quality attribute scenarios must be testable (just like any other requirement) Therefore, the Stimulus Artifact Environment Response measure(s) must be clear and specific

11 Activity: define quality attribute scenarios

12 Next step Assume some of the critical quality attribute scenarios have been defined What next?

13 Tactics: how to accomplish a quality attribute scenario
Air-filled tires Big old springs Shock absorbers … (I’m no auto engineer)

14 Tactics for shift change
Separate authentication+authorization from environment setup Show progress indicator(s) Precompute expensive structures Defer at-login-time processing to background Thin clients + shared services Deploy workstations Minimize other load on the system at shift change times What BC&K tactics are these?

15 Tactics for Qualities Tactics are a guide to design!
Tactics are design choices oriented toward achieving qualities Tactics can refine other tactics Patterns package tactics Tactics can interfere! Next week: a way to use quality attribute scenarios and tactics to drive module decomposition

16 Tactics are ways to get the desired response in a scenario
Artifact Response Stimulus Environment

17 Tactics example: performance
Introduce concurrency Max delay < 2 sec Database Prompt results 25 req/sec Normal ops

18 Qualities categorize tactics
Fault Masked or Repair Made Fault Detected (not enough by itself) Availability Fault

19 Availability Recovery Prep and Repeat Fault Detection Recovery-
Reintroduction Prevention Removal from service Transactions Process Monitor Voting Active Redundancy Passive Spare Shadow State ReSync Rollback Echo Ping Exception

20 Tactics can interfere with each other
Modifiability: use an intermediary Performance: reduce computational overhead Modifiability/Performance conflicts are common

21 Patterns package tactics
An architectural pattern usually applies a set of compatible tactics Better yet, mutually reinforcing tactics Or at least, the pattern may give advice on balancing tactics that tend to conflict

22 Example 1: tactics in Money (488)
This is one of Fowler’s Base Patterns Modifiability tactics used include m1. Semantic coherence m2. Anticipate changes m3. Generalize module m5. Abstract common services

23 Example 2: Reactor includes
Modifiability: m3. Generalize module m5. Abstract common services m6. Hide information Performance: p3. Manage event rate p2. Reduce computational overhead p5. Introduce concurrency p8. Scheduling policy

24 Styles Styles (Shaw and Garlan) are recurring partial architectures
Styles are sometimes also called “patterns” Like patterns, they package tactics But they’re not usually linked with a problem A style consists of Set of element types Element topology Set of semantic constraints Set of interaction mechanisms

25 Style example: pipes and filters
Tactics include: m2. anticipate expected changes m5. abstract common services m6. hide information m7. maintain existing interface m8. restrict communication paths m12. polymorphism m13. component replacement p3. manage event rate

26 Style example: Service-Oriented Architecture (SOA)
Tactics include: m2. anticipate expected changes m5. abstract common services m6. hide information m7. maintain existing interface m8. restrict communication paths m12. polymorphism m13. component replacement m14. adherence to defined protocols t2. separate interface from implementation app app service service service service

27 Tools of the architect’s trade
Quality attribute scenarios … A way of defining testable quality requirements Tactics … Bags of tricks you can apply Patterns and styles … Sets of tactics that usually fit together well and are often applied together


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