coding interview

Four resources, one state machine and three semantics that separate a working API from one that corrupts state under retries. The idempotency key that makes a duplicate POST safe, why cancel returns 202 rather than 200, and the cursor that survives concurrent inserts.

Metrics arrive in time order and are queried by time window, which is the pair of facts the whole design turns on. Sealed segments with a sparse index, pruning that skipped 82 of 84 segments in the executed test, and TTL compaction that drops whole files without rewriting a byte.