What is the difference between availability and reliability?
Why Interviewers Ask This
Interviewers use this question to quickly assess whether a candidate has the foundational knowledge required for System Design development. It reveals whether you understand the building blocks that more complex concepts rely on.
Answer
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.
Pro Tip
If you're unsure about a detail, say so honestly and explain your reasoning. Interviewers respect candidates who can think through uncertainty rather than bluffing.