Checks-Effects-Interactions

The structural pattern that eliminates reentrancy bugs: every function should perform (1) checks (require statements validating inputs and state), (2) effects (all state mutations), then (3) interactions (external calls and ETH transfers). If state changes always happen before external calls, an attacker's callback sees the post-update state and cannot re-enter into stale-state code paths. CEI is the architecture; ReentrancyGuard is defence-in-depth on top.