The proportion of time a system, component, or service is operational and capable of performing its required functions within a specified interval; commonly expressed as a percentage (uptime) over that interval.

Principle

Availability is determined by the interplay of reliability (failure rates), maintainability (repair times), redundancy, and external dependencies; it represents operational readiness under stated conditions rather than functional correctness at every instant.

Demonstration

A web service reports 99.95% availability over a month; this means the service was reachable and functioning for the equivalent of all but approximately 21.6 minutes during that month, subject to the provider's availability measurement definitions (e.g., planned maintenance policies).

Misapplication

Treating availability percentage as synonymous with reliability or assuming 100% availability implies no degradation in performance or correctness; ignoring the measurement window, scheduled maintenance exclusions, or differing definitions of 'available' across layers leads to error.

Consequence

Accurately defined and measured availability supports service-level objectives and capacity planning; designing with redundancy and maintainability reduces downtime and the operational impact of failures, though higher availability incurs cost and complexity.

Reversal

Focusing exclusively on availability metrics while ignoring service quality (latency, correctness, throughput) — reversing priorities can create highly available but functionally degraded systems that meet uptime targets yet provide poor user experience.

Boundary

Applies to operational readiness and reachability as defined by the measurement method; excludes subjective notions of usefulness or performance degradation unless those are included in the availability definition. Availability calculations require explicit treatment of scheduled outages and monitoring limitations.

Semantic Tension

Availability overlaps with reliability and uptime but differs in scope and measurement; stakeholders may conflate high availability with high reliability or acceptable performance, causing misaligned expectations and SLOs/SLAs.

Synthesis

Availability is the measurable fraction of time a system is in a ready and reachable state per a defined policy; it integrates failure and repair characteristics plus design choices, must be measured with explicit rules, and complements but does not replace other quality metrics.