backend-design - SKILL.md Agent Skill

name: backend-design description: Elite Tier Backend standards, including Vertical Slice Architecture, Zero Trust Security, and High-Performance API protocols. allowed-tools: Read, Write, Edit, Glob, Grep, Bash

# Backend Design System

Philosophy: The Backend is the Fortress. Logic is Law. Latency is the Enemy. Core Principle: ISOLATE features. TRUST no one. SCALE linearly.

ANTI-HAPPY PATH MANDATE (CRITICAL): Never assume the ideal scenario. AI-generated code often fails by ignoring edge cases and failure modes. For every business logic slice, you MUST document and test at least three failure scenarios: Race Conditions, Data Integrity violations (e.g., unique constraint overlaps), and Boundary failures. Reject any implementation that only covers the 'Happy Path'. Engineering is the art of handling what shouldn't happen.

## 🚀 ELITE TIER KNOWLEDGE (ARCHITECTURAL PROTOCOLS)

0. The "Vertical Slice" Law (The Anti-Layer Mandate)

CRITICAL: You are FORBIDDEN from creating "Horizontal Layers" (Controllers, Services, Repositories) as primary folders.

The "Feature-First" Protocol: Code must be organized by BUSINESS CAPABILITY, not technical role.

The Slice: A single directory (e.g., features/create-order/) contains EVERYTHING needed for that feature:
- handler.ts (Controller)
- logic.ts (Domain/Service)
- schema.ts (DTO/Validation)
- db.ts (Data Access)
The Benefit: Changing a feature requires touching only ONE folder. No "Shotgun Surgery" across 5 layers.
Shared Kernel: Only truly generic code (Logging, Auth Middleware, Database Connection) goes into shared/.

1. The "Modular Monolith" Mandate

Microservices Ban: Do NOT start with microservices. Start with a Modular Monolith.
Modulith Rules:
- Modules must be isolated (like internal microservices).
- Modules communicate via Events (Sub-Process or Message Bus), NEVER by importing another module's code directly.
- The Outbox Pattern (Guaranteed Delivery):
  - Problem: If DB commit succeeds but Event Bus fails, the system is inconsistent.
  - Mandate: Write events to an outbox table in the SAME transaction as the data change.
  - Relay: A background worker pushes outbox entries to the Message Bus (RabbitMQ/Kafka).
- Data Sovereignty: Module A cannot query Module B's tables. It must ask Module B via API/Event.

2. The "Zero Trust" Security Protocol

Detailed protocols: See security-protocols.md

Quick Rules:

Strict Serialization: NEVER return raw DB entities → Use ResponseDTO
Validation at Gate: Schema validation (Zod/Pydantic) BEFORE logic
Token Sovereignty: PASETO v4 > JWT (Ed25519 if JWT forced)

## 🏗️ Reliability & Performance Contracts

3. The "Sub-100ms" Performance Mandate

The Latency Budget: P50 < 100ms. P99 < 500ms.
UUIDv7 (The Time-Lord Rule):
- Ban: Never use UUIDv4 (Random) for Primary Keys. It fragments B-Tree indexes.
- Mandate: Use UUIDv7 (Time-ordered). It enables clustered index locality (fast inserts) like integers, with the uniqueness of UUIDs.
N+1 Assassin:
- Check: Always inspect ORM queries. Loops triggering DB calls are a "Level 0" error.
- Fix: Use DataLoader pattern or explicit JOIN loading.

4. API Reliability Contracts

RFC 7807 (Problem Details):

Ban: returning { "error": "Something went wrong" }.

Mandate: Return standard Problem JSON:

{
  "type": "https://api.myapp.com/errors/insufficient-funds",
  "title": "Insufficient Funds",
  "status": 403,
  "detail": "Current balance is 10.00, required is 15.00",
  "instance": "/transactions/12345"
}

Idempotency Keys:
- Rule: All critical POST/PATCH (Money, State Change) must accept an Idempotency-Key header.
- Logic: If key exists in Cache (24h TTL), return stored response without re-executing logic.

## 🗄️ Database Integrity & Design

5. Database Integrity & Design

Hard Constraints: Application-level checks are "Suggestions". Database Constraints (Foreign Keys, Unique Indexes, Check Constraints) are "Laws".
Cursor Pagination:
- Ban: OFFSET / LIMIT on large tables (O(N) performance degradation).
- Mandate: Cursor-based pagination (WHERE created_at < cursor LIMIT 20).
Migration Discipline:
- Never alter a column in a way that locks the table for >1s.
- Use "Expand and Contract" pattern for breaking changes.
Concurrency Control:
- Problem: Two users update the same record. The last one wipes the first.
- Mandate: Use Optimistic Locking. Add a version (int) column.
- Logic: Update WHERE id = X AND version = Y. If 0 rows affected, throw StaleObjectException.

6. AI & Vector Readiness

Semantic Storage: Backend must be ready to store embeddings (Vector Types).
Guardrails: Output from LLMs must be sanitized and structure-checked on the server side before returning to frontend.

## 👁️ Observability & Monitoring (The "Glass Box" Protocol)

7. Structured Logging Only

Ban: console.log("User updated"). String logs are useless for machines.
*Mandate: JSON Logs with correlation IDs. { "level": "info", "event": "user_updated", "user_id": "u7-...", "trace_id": "..." }.

8. Distributed Tracing (OpenTelemetry)

Every request MUST carry a traceparent header.
Spans must cover: DB Queries, External API Calls, and Redis operations.

9. Health Checks

Liveness (/health/live): "Am I running?" (Instant, no checks).
Readiness (/health/ready): "Can I take traffic?" (Check DB/Redis connection).

## 🛡️ Resilience Patterns (The "Anti-Fragile" Mandate)

10. Circuit Breakers

Wrap ALL external calls (Payment Gateways, 3rd Party APIs) in a Circuit Breaker.
Logic: After 5 failures, fail fast for 30s. Don't drown the downstream service.

11. Rate Limiting

Protect every public endpoint with a Token Bucket rate limiter (Redis-backed).
Differentiate limits by User Role (Anon: 60/min, Pro: 1000/min).

## 🔧 Workflow Rules

1. The Pre-Flight Checklist

Environment Hardening:
- Verify all process.env variables at startup using a schema (e.g., t3-env or envalid). If a key is missing, crash immediately. Do not start the server in an undefined state. Before writing a single handler:
Define the DTOs: Request Schema (Zod) and Response Schema.
Define the Error States: What can go wrong? (404, 409, 429).
Define the Data Access: What is the most efficient SQL query?

2. The "No Magic" Rule

Avoid "Magical" ORM features (Lazy Loading, Auto-Saving context).
Prefer Explicit over Implicit. "Write the SQL (or Query Builder) if the ORM hides expensive logic."

3. Testing Pyramid

Unit: Test Domain Logic in isolation (mock DB).
Integration: Test Feature Slice with a REAL containerized DB (Testcontainers).
E2E: Test critical flows from the "Outside".

## 📂 Cognitive Audit Cycle Before committing code: 1. **Is the endpoint under a feature slice?** (Not in a generic controller folder). 2. **Is Input Validated with a Schema?** (Zero Trust). 3. **Are DB Indexes used?** (Run `EXPLAIN ANALYZE`). 4. **Is the Primary Key UUIDv7?** (Index Perf). 5. **Are secrets managed properly?** (No hardcoded strings).

🔗 CROSS-SKILL INTEGRATION

Skill	Backend Adds...
`@frontend-design`	API contracts, CORS config, error responses
`@clean-code`	Input validation, no raw SQL, dependency security
`@tdd-mastery`	Integration tests with Testcontainers
`@planning-mastery`	API endpoint task breakdown
`@debug-mastery`	Structured logging, distributed tracing

Command: Use these skills to architect "Fortress-Level" backend systems.