Predictability

Definition

Predictability

the soundness quality factor representing the degree to which the behavior of an application or component is deterministic (i.e., predictable) for a given set of inputs when in a given state

Classification

As illustrated in the preceding figure, Predictability is part of the following inheritance hierarchy:

• Type: Concrete
• Superclass: Soundness
• Subclasses:
  • None

Responsibilities

The typical responsibilities of Predictability are to:

• Model the degree to which behavior is deterministic.
• Support the analysis and specification of predictability requirements.
• Provide a foundation for evaluating the quality of an architecture.

Subfactors

Predictability is typically decomposed into the following aggregation hierarchy of subfactors:

• TBD

Measures

Predictability is typically measured in terms of the:

• Percentage of behaviors that were deterministic.

Mechanisms

Typical mechanisms for implementing predictability include:

• Deterministic scheduling mechanisms (e.g., use cyclic executive, avoid event/interrupt driven scheduling).
• Avoid concurrency.
• Avoid automatic garbage collection.

Guidelines

The following guidelines have been found to be useful when producing predictability requirements:

• Predictability is important for safety and testability.
• Predictability includes:
  • Values of outputs
  • Functional behavior (e.g., service requests).
  • Timing and sequencing