Architecture Quality Attributes

Intro

• Qualities that must be accommodated in a systems architecture over and above basic functionality.
• Categories of quality attributes:
  • Observable attributes. e.g:
    • Performance
    • Usability
    • Functionality
    • Security
    • Availability
  • Unobservable attributes. e.g:
    • Modifiability
    • Reusability
    • Testability
    • Portability
    • Certifiability
  • Business attributes.e.g:
    • Time to market
    • Overall cost
    • Longevity
    • Backward compatibility

Quality Attribute Scenarios

• Meaningless to say System is modifiable, reusable etc. How do we justify it ?
• Quality Attributes are best characterised via scenarios.
• SEI format for QA scenarios:
  • Source of stimulus: The entity (human, computer system) that has generated the stimulus.
  • Stimulus: The stimulus is a condition that needs to be considered when it arrives at a system.
  • Environment: The prevailing condition when the stimulus occurs (e.g. system may be overloaded when stimulus occurs or maybe the system is in normal operating conditions)
  • Artifact: Some piece of the system is stimulated. This may be the whole system or some pieces of it.
  • Response: The response is the activity undertaken after the arrival of the stimulus.
  • Response Measure: When the response occurs, it should be measurable so that the requirement can be tested.

• General QA scenarios: These scenarios are system independent and can be applied to any system.
• Specific QA scenarios: These scenarios are derived from the general scenario by instantiating each part. They are specific to the particular system under consideration.
• For e.g. there is a general availability scenario with a range of values that each part (source, stimulus, env etc) can take. From this we can derive concrete, system specific scenarios.

Observable Quality Attributes

Availability

• Availability is concerned with system failure and its associated consequences.
• A fault may become a failure if not corrected or masked.
• Availability of a system is the probability that it will be operational when it is needed.
• Defined as (MTTF)/(MTTF + MTTR). Where MTTF = Mean Time to Failure and MTTR = Mean Time to Repair.

General Scenario

• Source: Internal or external to the system.
• Stimulus: The stimulus is a fault. The fault can be omission (fails to respond), crash (repeatedly suffers omission faults), timing (late or early response), response itself (incorrect)
• Artifact: The part of the system that is required to be highly available (processor, communication channel, storage etc)
• Environment: Normal/Degraded.
• Response: One or more of : Logging, Notifying, Disable source of events, become unavailable, continue to operate in normal or degraded mode.
• Response Measure: Time interval when the system must be available or a repair time.