What is the difference between availability and reliability?

Availability is the percentage of time a system is operational and accessible — usually expressed as uptime percentage (99.9%, 99.99%). It measures whether the system is up right now. A system that crashes every hour but restarts in 1 second has very high availability despite crashing. Formula: Availability = MTTF / (MTTF + MTTR), where MTTF = Mean Time to Failure (average time between failures) and MTTR = Mean Time to Repair (average time to restore service after failure). Improve availability by: reducing failure frequency (better hardware, testing), reducing repair time (automation, monitoring, on-call). Reliability is the probability that a system performs its intended function correctly over a specified period under specified conditions — no failures, no incorrect results, no data corruption. A system can be available (up) but unreliable (returning wrong results). A reliable system produces correct results consistently. Measured by: error rate, success rate, MTTF. Example: a calculator that's always on (100% available) but sometimes returns wrong answers (unreliable). A DNS server that's down for maintenance windows (lower availability) but always returns correct results when up (reliable). Designing for both: reliability (correctness) requires thorough testing, data validation, fault isolation, transactions. Availability requires redundancy, failover, and fast recovery. They often reinforce each other — a reliable system fails less often, improving availability.