While building iTicket.AZ — a real-time event ticketing platform — I came across a job posting from a major bank that listed "building scalable, resilient, and fault-tolerant applications" as a core requirement. That made me think: is my backend actually fault-tolerant? Spoiler: it wasn't. Here's what I changed. What does "fault-tolerant" actually mean? A fault-tolerant system keeps running — even in degraded form — when parts of it fail. That means your app doesn't crash just because the database hiccuped, a third-party API timed out, or a job queue backed up. There are four patterns I focused on. Pattern 1 — Retry + Circuit Breaker When a DB write fails, should we silently drop it? No — but we also shouldn't hammer a broken service forever. The retry pattern tries again a few times; the circuit breaker stops calls entirely after too many failures.…