Systems most often fail during peak demand moments like product launches, live events, or ticket drops — exactly when failure is most costly. The core causes are fragile state management, tight service coupling, lack of fault isolation, and inadequate load testing. Resilient systems are built around four principles: assuming failure will happen, isolating blast radius via circuit breakers and rate limiting, designing for high concurrency, and choosing fault-tolerant infrastructure. Practical patterns include stateless application layers, horizontal scaling, asynchronous inter-service communication, and robust observability. Short-term patches introduce hidden complexity; true reliability requires deliberate architectural decisions and realistic load testing.
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