#dependability
Four fundamental properties can be used to characterize a computing system: functionality, performance, cost, and dependability. The following are key terms and definitions from Avižienis et al. [1].
- The function of the system is what it is intended for as described by the specification.
- Dependability is defined as "the ability to deliver service that can justifiably be trusted.
- The service delivered by a system is its behaviour as perceived by its user(s).
- A user is another system or human that interacts with this system via a system interface.
- A system failure occurs when the delivered service deviates from correct service.
- Correct service is deemed to be delivered when the system implements the expected function.
- A system may fail in two situations—when it does not comply with the specification or because the specification was not adequately defined. When a system fails it is said to provide incorrect service. The duration of incorrect service is called outage.
- An error is the part of the system that may cause a subsequent failure, when it reaches the service interface. An error is detected if it is indicated by an error message or error signal. Errors that are present but not detected are latent errors.
- A fault is the cause an error. A fault is active when it produces an error and dormant otherwise.
The dependability of a system consists of three parts: [[The Threats to Reliability|threats]] to, [[The Attributes of Dependability|attributes]] of, and [[The Means to Attain Dependability|means]] for dependability. These are illustrated in the following figure [1].
![[Pasted image 20230222111308.png]]
[1] J.-C. Laprie and B. Randell, “Algirdas Aviˇz ienis UCLA, Los Angeles, CA, USA and Vytautas Magnus U. Kaunas, Lithuania [email protected]”. Fundamental Concepts of Dependability