Listing Vault SE322 · chapter 03 · Quality Attributes

Chapter 03 — Quality Attributes

Quality scenarios, the seven design decision categories, and the tactic catalogues for performance, availability, security, modifiability, testability, usability and interoperability.

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Quality Attributes

18 lists

Six-Part Quality Scenario 6

  1. Source — who or what triggers it
  2. Stimulus — what arrives
  3. Environment — the system state at the time (normal, peak, degraded)
  4. Artifact — the part of the system that is hit
  5. Response — what the system does
  6. Response measure — how the response is measured

General vs Concrete Scenario 2

  1. General — reusable, system-independent template
  2. Concrete — instantiated for this system with real numbers

Seven Categories of Design Decision 7

  1. Allocation of responsibilities — where behavior and knowledge live
  2. Coordination model — how elements interact
  3. Data model — what is stored, where, partitioning, replication and consistency
  4. Management of resources — threads, memory, connections, bandwidth
  5. Mapping among architectural elements — what scales, fails or releases independently
  6. Binding time — compile time, build, startup or run time
  7. Choice of technology — flexibility traded against complexity and cost

Performance — Concerns 5

  1. Latency
  2. Throughput
  3. Capacity
  4. Deadlines
  5. Resource use

Performance — Tactics 2 groups

Control demand

  • Sampling / manage sampling rate
  • Admission control and rate limiting
  • Prioritize events
  • Reduce overhead
  • Bound execution times

Manage resources

  • Increase resources
  • Introduce concurrency
  • Replicate computation or data
  • Bound queue sizes
  • Schedule resources

Performance — Measurement Rules 3

  1. Measure percentile latency (p95, p99), not averages
  2. Measure under representative load, not a quiet machine
  3. Keep headroom — latency rises sharply as utilization nears capacity

Availability — Definition & Arithmetic 3

  1. Readiness for correct service despite faults
  2. Availability = MTBF / (MTBF + MTTR)
  3. 99.9% ≈ 8.8 hours downtime/year · 99.99% ≈ 53 minutes/year

Availability — Tactics 4 groups

Detect faults

  • Ping / echo
  • Heartbeat
  • Exceptions
  • Voting
  • Monitor
  • Timestamp

Recover — preparation & repair

  • Active redundancy
  • Passive redundancy
  • Spare
  • Rollback
  • Retry
  • Exception handling

Recover — reintroduction

  • Shadow operation
  • State resynchronization
  • Escalating restart
  • Non-stop forwarding

Prevent faults

  • Removal from service
  • Transactions
  • Process monitor
  • Predictive model

Fault vs Failure 2

  1. Fault — something has gone wrong internally
  2. Failure — the fault has reached the user; masking it fast enough is a valid strategy

Security — Properties Protected 6

  1. Confidentiality
  2. Integrity
  3. Availability
  4. Authenticity
  5. Accountability
  6. Non-repudiation

Security — Tactic Groups 4

  1. Detect attacks
  2. Resist attacks
  3. React to attacks
  4. Recover from attacks

Security — Core Controls 5

  1. Authenticate subjects
  2. Authorize actions
  3. Limit exposure and apply least privilege
  4. Encrypt data
  5. Audit activity

Threat Modeling Links 4

  1. Assets
  2. Threats
  3. Vulnerabilities
  4. Mitigations

Modifiability — Tactics 4

  1. Reduce the size of a module
  2. Increase cohesion
  3. Reduce coupling (encapsulate, use an intermediary, abstract common services)
  4. Defer binding time

Testability — Requirements 4

  1. Controllability
  2. Observability
  3. Isolation
  4. Repeatability

Usability — Tactics 2 groups

Support user initiative

  • Cancel
  • Undo
  • Pause / resume
  • Aggregate

Support system initiative

  • Maintain a model of the user
  • Maintain a model of the task
  • Maintain a model of the system

Interoperability — Requires 4

  1. Shared syntax
  2. Shared semantics
  3. Shared protocols
  4. Shared identity assumptions

Interoperability — Tactics 3

  1. Discover service
  2. Orchestrate
  3. Tailor interface