CompSci331: Lecture 13 Quality Attributes in Software Architecture

• Quality requirements must motivate a buildable, reliable, secure, and fast web-based travel booking system.
• The choice of architecture is determined by quality attributes, not just the system's functionality.
• It's essential to know which quality attributes are important to determine the best or at least acceptable architecture.
• Changing important quality attributes can potentially change the architecture considered most acceptable.
• Software quality reflects how well the software meets requirements and aligns with user needs.
• A quality attribute is a measurable or testable property of a system indicating how well the system meets the needs of stakeholders.
• Quality encompasses the notion of how "good" a product is, by satisfying stakeholder needs.
• A quality attribute represents a specific aspect of quality, integrity, correctness, reliability, maintainability, flexibility, reusability, and modularity.
• There are many architectures for any given set of functional requirements and they differ on non-functional quality attributes.
• The choice of architecture is crucial for realizing quality attributes, the choice gives the possibility of achieving quality attributes.
• The choice of architecture does not guarantee achieving quality attributes and not all quality attributes are affected by architecture.

System Quality Attributes

• Bass, Clements, and Kazman advocated for a certain set of system quality attributes:
• Availability: The system provides the service it is supposed to provide when it is expected to
• Modifiability: Reasonable changes can be made at a reasonable cost.
• Performance: The system responds quickly enough and without using too many resources.
• Security: Authorized people can access the system services while unauthorized people cannot.
• Testability: The system can be artificially exercised to a degree that ensures confidence in correctness.
• Usability: How easy it is for the user to do what is desired.
• Interoperability: The degree to which two or more systems can usefully exchange meaningful information.

Business Quality Attributes

• These can also affect the choice of architecture, though they are more difficult to operationalize:
• Time to market: Is there pressure to deliver quickly?
• Cost and benefit: What does the budget allow?
• System lifetime: How long must the system be useful?
• Market: What does the market consider important?
• Roll-out schedule: In what manner will the system be made available?
• Integration with legacy systems: Must the system have to fit with existing systems?
• Most other system qualities can be expressed in terms of system quality attributes.
• Portability can be expressed as modifiability and the system must be changed to run on different hardware/operating system.
• Maintainability can be expressed as modifiability and the system must be changed to remove the invalid behavior.
• Scaleability can be expressed as modifiability and the system must be changed to have more capacity
• Operability can be expressed as usability and the system must be easy to use by being easy to learn.
• Reliability can be expressed as availability and the system must not fail "too often".
• Integrity can be expressed as security and the Data presented by the system must not be able to be changed in an unauthorized or unexpected manner.
• Describing quality requirements needs to be operational.

Quality Attribute Scenarios

• Quality Attribute Scenarios provide an operational description of the requirements for quality attributes.
• These scenarios identify and categorize the necessary information for making such requirements operational.
• Quality Attribute Scenarios are available in concrete and general forms.
• Determine relevant general QAS and then refine into concrete QAS when using Quality Attribute Scenarios.
• Stimulus: A condition or event that needs to be considered; it occurs at some part of the system.
• Source of stimulus: Some entity generates a stimulus.
• Environment: Conditions under which the stimulus occurs.
• Artifact: The thing that is stimulated.
• Response: What the artifact should do on arrival of the stimulus.
• Response measure: How to measure the response to determine that it is satisfactory.

Fast (Example)

• The stimulus is a form submission which comes from the user within normal working conditions.
• The system loads and displays a page indicating a successful purchase.
• The page should load in less than 2 seconds 95% of the time.
• As another example, the stimulus can be a "form submission" from "the user".
• In this case the environment is that the server is not overloaded and there is no network congestion.
• The system loads and displays a page indicating a successful purchase.
• The page is loaded in less than 2 seconds 95% of the time.
• As yet one more example, the "stimulus" is a "form submission" and comes from "the server" within normal working conditions.
• The server verifies and records the purchase and must process 1000 transactions per second.

Reliable (Example)

• In one specific case, the stimulus is a single CPU failure from a random event within normal working conditions.
• The server notifies the operator with no loss of service .
• In another specific case, the stimulus is requests to a a certain service and comes from the user.
• The artifact is the system which is receiving more than 1000 requests per second.
• The system rejects the request, and reports to operator; and the operator must be notified within 5 minutes.

Secure (Example)

• In one specific case, the stimulus is attempts to change details on the web server from a disillusioned youth.
• The server is the artifact, exists in normal working conditions.
• The response is to notify the operator with details, with no loss of service.
• In another specific case, the stimulus is attempts to change details on the web server from a disillusioned employee.
• The artifact is also the server, and they are in normal working conditions.
• The system must record the access with the identity of person making change available.

Modifiable (Example)

• In one specific case, the stimulus is a request for 100GB extra storage capacity from the system administrator.
• The artifact is also the system, and they are in normal working conditions.
• The system provides extra storage, with no loss of service.
• In another case, the stimulus is a request for a new site made active with 10-100 users, the owner.
• The artifact is "the system" in normal working conditions.
• The new site is activated with the downtime taking no more than 48 hours.
• System quality requirements can be expressed in terms of system quality attributes.
• Quality requirements need to be presented in an operational manner to assess the quality of an architecture.
• Quality Attribute Scenarios provide a means to specify quality requirements operationally.