dependability

Dependability is the ability of a system to deliver the required service under stated conditions for a specified period of time. It is a broad concept used in engineering, computing, and information technology that encompasses several interrelated attributes, most commonly reliability, availability, safety, and maintainability. In some contexts, dependability also addresses security, integrity, and resilience.

Reliability is the likelihood that a system will perform without failure for a given time. Availability is the fraction of time the system is able to provide service, influenced by both reliability and maintainability. Maintainability refers to how quickly and easily a system can be restored after a fault. Safety concerns the system’s potential to cause unacceptable risk, and security protects against unauthorized access or modification. Resilience describes the capacity to absorb shocks, degrade gracefully, and recover from faults.

Dependability is pursued throughout the system lifecycle by design, production, operation, and maintenance. Techniques include fault prevention (design to avoid faults), fault detection and diagnosis, fault tolerance (redundancy, diversity, and failover), and fault removal (testing and debugging). Architectural choices such as modular design, monitoring, diagnostics, and hot replacement contribute to RAMS: reliability, availability, maintainability, and safety.

Assessment relies on metrics and models. Common measures are mean time between failures (MTBF), mean time to repair (MTTR), mean time to failure (MTTF), and overall availability. Quantitative methods include reliability block diagrams, FMEA, and fault tree analysis. These tools support prediction, optimization, and risk assessment.

Applications span aviation, rail, automotive, medical devices, industrial control systems, data centers, and large software services. Standards and frameworks, such as RAMS concepts and functional-safety norms, guide dependability practice and assurance.