Idempotency: What, Why and How
A deep dive into idempotency for HTTP APIs, covering the theory and a practical Laravel implementation using the `wendelladriel/laravel-idempotency` package. Explains idempotency keys, request fingerprinting, cached response replay, atomic locks for concurrent requests, and key scoping (user/IP/global). Shows how to apply idempotency via route middleware or PHP attributes, customize per-endpoint settings, design idempotent controllers with proper domain invariants, handle failure status codes (400/409/422), use Artisan commands for observability, and write feature tests for retry contracts. Also covers common mistakes like reusing keys across operations or neglecting shared cache infrastructure.
