Goal of Design

Derive a solution for requirements

• Functionality
• Constraints
• Qualities

Qualiy Attributes

• Availability
• Modifiability
• Security
• Utilisability
• Interoperability
• Performance
• Testability

Availability

Software there and ready to carry out its task when its needed

• Fault --> Failure
• Reliability: Mean Time between Failures
• Recovery: Mean Time to Repair

Scenario:

• Source: Heartbeat monitor
• Stimulus: Server unresponsive
• Artifact: Process, Environment: Normal Operation
• Response: Inform operator continue to operate
• Response Measure: No downtime

Tactics for Availability

• prevent/mitigate failures from faults
• fault --> tactics to control availability

Detect Faults

• Monitor
  • monitors components state of health
• Ping/echo
  • Asynchronous request/response message exchange
• Heartbeat
  • periodic notification
• Timestamp
• Condition Monitoring
• Exception Detection
• Sanity Checking
• Voting
• Self-test

Recover from Faults

• Redundancy
  • Active Redundancy (Hot Spare)
    • all components receive and process identical inputs in parallel
  • Passive Redundancy (Warm Spare)
    • only active member process the input
    • the active member provides spares with periodic state updates
  • Spare (Cold Spare)
    • switches to spare when active member fails
    • power-on-reset procedure
• Preparation & Repair
  • Exception Handling
  • Software Upgrade
  • Ignore Faulty Behavior
  • Reconfiguration
  • Rollback
  • Retry
  • Degradation
• Reintroduction
  • Shadow
  • State Resunchronization
  • Escalating Restart
  • Non-stop Forwarding

Prevent Faults

• Removal from service
• Predictive Model
• Transactions
• Exception Prevention
• Increase competence set

Interoperability

Degree to which parties can usefully exchange meaningful information

• Locate each other
• Understand each other

Scenario:

• Source of Stimulus: Our vehicle information system
• Stimulus: Current location sent
• Artifact: Traffic monitoring system, Environment: systems known prior to run-time
• Response: Traffic monitor, combines current location with other information, overlays on google maps and broadcasts
• Response Measure: Our information included correctly 99.9% of the time

Tactics for Interoperability

• Allow correct handling of Information exchange
  • Information Exchange Request --> Tactics to control Interoperability --> request correctly handled

• Adapter Pattern
  • provides expected interface

Modifiability

• cost and risk of making changes
  • what can change
  • likelihood of the change
  • when change is made
  • who makes it

Scenario:

• Source: developer
• Stimulus: wishs to change the UI
• Artifact: Code, Environment: Design time
• Response: Change Made and Unit Tested
• Response Measure: in 3 hours

Modifiability Tactics

• control complexity (time, cost) of changes
• change arrives --> tactics to control Modifiability --> change made within Time and Budget

• Reduce Coupling
  • Encapsulate
    • Introduces interface: hides internals (e.g. Facade)
  • Intermediary
    • Breaks dependency
  • Restrict Dependencies
    • constraints interactions (e.g. Layering)
  • Refactor
  • Abstract Common Services
• Defer Binding
  • Later binding better modifiability
    • compile/build time, build script or aspects
    • deployment time, configuration-time binding
    • startup/initialization time, resource files
    • run-time, dependency injection, dynamic lookup of services, message brokers, plug-ins.

Performance

Ability to meet timing requirements

• time to respond to events
• which events occurs
• time-based response

Scenario:

• Source: Users
• Stimulus: Initiate Transactions,
• Artifact: System, Environment: Normal Operation
• Responses: transactions are processed
• Response Measure: average latency of two seconds

Performance Tactics

• Generate a response to an event arriving at the system within some time-based constraint
• Event arrives --> tactics to control performance --> response generated within time constraints

• Control Resource Demand
  • Manage sampling rate
  • Limit Event response
  • Prioritize events
  • Reduce Overhead
  • Bound Execution times
  • increase resource efficiency
• Manage resources
  • increase resources
  • maintain multiple copies of computations
  • bound queue sizes
  • increase concurrency
  • maintain multiple copies of Data
  • schedule resources

Security

• ability to protect data and information from attacks
  • unauthorized access to data/services
  • denial of access
• characteristcs
  • confidentiality
  • integrity
  • availability
  • authentication
  • non-repudiation
  • authorization

Scenario:

• source: disgruntled employee from remote location
• Stimulus: attempts to modify pay rate
• Artifact: data within the system, Environment: Normal operations
• Response: System maintains audit trail
• Response Measure: correct data is restored within a day and source of tampering and identified

Security Tactics

Testability

• Ease to have software demonstrate its faults
  • ability to reveal faults
  • control inputs/state
  • observe inputs/state
  • complexity

Scenario:

• source: unit tester
• stimulus: code unit completed
• artifact: code unit, Environment: development
• response: results captured
• response measure: 85% path coverage in thress hours

Testability Tactics

Usability

Degree to which users easily, effectively, efficiently, and satisfactorily perform their tasks

• learning system features
• using a system efficiently
• minimizing the impact of erros
• adapting the system to user needs
• increasing confidence and satisfaction

Scenario:

• source: user
• stimulus: donwloads a new application
• artifact: system, environment: runtime
• response: user uses application productivity
• response measure: within two minutes of experimentation

Usability Tactics

Architectural Tactics

• Fundamental design technique
  • controls a specific quality attribute
  • stimulus --> tactis to control response --> response

Design Decision Categories

• Allocation of responsibilities
• Data model
• Mapping among architectural elements
• Coordination model
• Management of resources
• Binding time decisions
• Choice of technology