Distributed systems rarely fail as a single unit. More often, one dependency slows down, one shard becomes unavailable, one region loses capacity, or one pool saturates while neighboring components remain healthy. The engineering challenge is not simply availability; it is preventing a local defect from becoming a system-wide event. Google’s SRE guidance describes cascading failure as positive feedback in which lost capacity shifts load onto surviving capacity until additional components fail. Resilience design must break that loop early, cheaply, and at a boundary narrow enough to preserve useful work elsewhere.