SideEffectGuard stores one row per (operation, idempotency_key) in SQLite. BEGIN IMMEDIATE serializes competing reservations. A payload SHA-256 digest prevents accidental reuse of a key for a different request.
Reservation outcomes:
execute: the caller owns a live lease and may perform the side effect;in_progress: another worker owns the lease;duplicate: the operation already succeeded and the stored result is returned;conflict: the same key was used with a different payload.
Failed operations and expired leases may be retried with an incremented attempt number. SQLite coordinates processes on one machine or shared filesystem; use a transactional network database when workers cannot share the same database file.