backend-patterns

# Backend Patterns Skill **Degree of freedom: MIXED.** Which pattern to apply `[HIGH freedom]`; existing-architecture probes and the validation list `[LOW freedom — run exactly]`. ## How to reason 1. **Observe** — existing server/api/actions, ORM, queues, cache 2. **Interpret** — missing pattern vs duplicate vs wrong layer 3. **Classify** — reuse / add-queue / add-cache / add-rate-limit / edge-fn 4. **Severity** — unauthenticated mutation outranks a missing cache ## Worked example > **Observe:** `createOrder` sends email inline; no Inngest/queue; checkout p95 4s. > **Interpret:** confirmation is post-response work, not request-path. > **Classify:** `after()` or an `order/created` job; keep the write transactional. > **Verify:** order row commits; email/inventory run after response; retries do not double-charge. ## Self-critique before reporting - **Existing first** — searched server/api/actions before adding a second pattern - **Auth + validate** — every mutation checks session and Zod - **Idempotent** — retries on the new path do not double-apply - **Right owner** — which architecture to pick → `audit-backend-architecture` Design scalable, maintainable backend architectures using modern patterns and best practices. > Code examples lean on **Next.js App Router + Supabase/Prisma**. The patterns are > stack-agnostic — adapt ORMs, client libraries, and deploy targets to your detected > ecosystem. ## CRITICAL: Check Existing First [LOW freedom — run exactly] **Before ...