Availability vs Reliability
Availability measures whether a service responds (uptime %). Reliability measures whether it responds correctly and within acceptable latency. A service can be available but unreliable if responses are slow or wrong.
Source
- Google SRE Book — Chapter 3: Embracing Risk
Related terms
Uptime
The percentage of time a service is reachable and responding correctly over a given window.
Downtime
Any period during which a service fails to respond, returns errors, or is so degraded it is unusable.
SLA (Service Level Agreement)
A formal contract between a service provider and a customer that defines minimum service levels and the con…
SLO (Service Level Objective)
An internal reliability target — for example, "99.9% of API requests succeed within 500 ms over a rolling 3…
SLI (Service Level Indicator)
The raw measurement that an SLO is calculated from — request success ratio, latency percentile, or freshnes…
MTTR (Mean Time To Resolve / Recover)
Average time between an incident being detected and the service being fully restored.