✏️ Explanatory Question
Availability is the fraction of time the system is able to serve requests.
Reliability is the probability the system performs correctly over a period - no data loss, no wrong answers.
A system can be available but unreliable if it returns stale or corrupted data quickly. It can be reliable but unavailable if it is always correct when up, but frequently down.
Availability is achieved with redundancy, failover and health checks. Reliability is achieved with replication, checksums, retries and idempotent operations.