🛠️ API Best Practices — Design, Security, Reliability & Operability

1. API Design & Modeling

• Use nouns for resources: /products, /orders (not /getProducts).
• Use plural resource names: /products instead of /product.
• Keep endpoints RESTful: use HTTP verbs (GET/POST/PUT/PATCH/DELETE) to express actions.
• Design predictable URIs: hierarchical and consistent (/stores/{storeId}/products/{productId}).
• Support filtering, sorting, paging via query parameters (e.g. ?page=1&pageSize=20&sort=price_desc&filter=category:books).

2. Versioning & Compatibility

• Version your API from day one. Common strategies:
  • URI versioning: /v1/products
  • Header versioning: Accept: application/vnd.myapi.v1+json
• Backwards compatibility: never break existing contracts; deprecate gracefully with HTTP 301/410 or response headers.
• Deprecation policy: announce, provide migration guide, and give a sunset period.

3. Contracts & Documentation

• Use OpenAPI / Swagger to document endpoints, models, and examples.
• Provide examples for requests and responses.
• Auto-generate SDKs/clients if you support external consumers.
• Document error formats, rate limits, pagination details, and auth flows.

4. Authentication & Authorization

• Use HTTPS only — TLS everywhere.
• Prefer token-based auth: OAuth2 / OpenID Connect for external APIs; JWTs for internal services with caution.
• Use scopes/roles to limit access.
• Handle token expiry & refresh securely (refresh token rotation for higher security).
• Avoid sending sensitive data in URL (use headers/body).

5. Input Validation & Output Safety

• Validate all inputs (size, types, ranges); fail early with clear error messages.
• Whitelist validation (allow only expected fields) rather than blacklisting.
• Sanitize inputs to avoid injections (SQL/NoSQL/command).
• Use DTOs (request/response models) — don’t expose EF/DB entities directly.
• Map entities to DTOs with tools like AutoMapper or manual mapping.

6. Error Handling & Status Codes

• Use proper HTTP status codes:
  • 200 OK, 201 Created, 204 No Content (successes)
  • 400 Bad Request (validation), 401 Unauthorized, 403 Forbidden, 404 Not Found
  • 409 Conflict (unique constraint), 422 Unprocessable Entity (semantic errors)
  • 429 Too Many Requests, 500 Internal Server Error
• Return structured error payloads (use RFC 7807 application/problem+json):

{
  "type": "https://example.com/probs/invalid-input",
  "title": "Invalid input",
  "status": 400,
  "detail": "Price must be > 0",
  "instance": "/products"
}

• Don’t leak internal details (stack traces, DB errors) in responses.

7. Pagination, Filtering & Sorting

• Always paginate large collections. Default page size + max limit (e.g., default=20, max=100).
• Return paging metadata (total, page, pageSize), and consider Link headers for next/prev.
• Allow flexible filtering but restrict expensive operations server-side.

8. Caching & Performance

• Use HTTP caching: Cache-Control, ETag, and conditional GET (If-None-Match) for GET endpoints.
• Cache frequently-read resources (CDN, reverse proxy, in-memory).
• Compress responses (gzip/ Brotli).
• Avoid N+1 queries — eager load or use optimized queries.
• Add database indexes for frequent filters/sorts.

9. Transactions & Consistency

• Keep endpoints idempotent where possible (PUT for full updates, idempotency keys for POST).
• Use Unit of Work / transactions inside service layer for multi-step updates.
• Prefer eventual consistency for distributed systems, and document it.

10. Rate Limiting & Throttling

• Apply rate limits to protect services (global and per-user).
• Expose limit headers so clients know remaining quota:
  • X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset
• Return 429 Too Many Requests for exceeded limits.

11. Logging, Monitoring & Observability

• Log structured data (JSON logs) with correlation IDs.
• Include request id / correlation id in logs and responses (X-Request-Id).
• Expose metrics & health checks (/health, Prometheus metrics).
• Use distributed tracing (OpenTelemetry) for cross-service traces.

12. Security Hardening

• Enforce HTTPS & HSTS.
• Validate and limit file uploads (size/type).
• Use secure headers (CSP, X-Content-Type-Options, X-Frame-Options).
• Mask sensitive data in logs (PII, passwords, tokens).
• Rotate secrets and store them in secret managers (Azure Key Vault, AWS Secrets Manager).
• Scan dependencies for vulnerabilities and patch regularly.

13. Testing

• Unit tests for business logic and service layer.
• Integration tests with an in-memory or test DB.
• Contract tests for API consumers (Pact).
• Load/performance tests to identify bottlenecks.

14. CI/CD & Releases

• Automate builds, tests, and deployment.
• Run static analysis & security scans in pipeline.
• Deploy with blue/green or canary strategies for safer rollouts.
• Automate DB migrations (careful with destructive changes).

15. Documentation & Developer Experience

• Publish API docs (OpenAPI) and examples.
• Provide SDKs or code samples for common languages.
• Offer client-friendly error messages and examples.
• Provide a sandbox environment for integration testing.

16. Data Privacy & Compliance

• Comply with GDPR / local regulations: data minimization, consent, deletion endpoints.
• Implement retention policies and secure backups.
• Log access to sensitive data and audit when necessary.

17. Practical Tips & Patterns

• Prefer POST for complex queries (when query string becomes huge), but document clearly.
• Use PATCH for partial updates; prefer JSON Patch or a custom patch model.
• Use ETags and optimistic concurrency for updates to prevent lost updates.
• Support content negotiation if you plan multiple formats (JSON/MsgPack).
• Offer health checks and readiness probes for orchestrators (Kubernetes).

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Quick Checklist (copy to your README)

• [ ] HTTPS enforced
• [ ] OpenAPI spec published
• [ ] Versioning implemented
• [ ] Authentication & scopes in place
• [ ] Input validation & DTOs used
• [ ] Proper status codes + problem+json errors
• [ ] Pagination, filtering, sorting supported
• [ ] Rate limiting & caching applied
• [ ] Structured logging + correlation id
• [ ] Health checks & metrics exposed
• [ ] Automated tests in CI pipeline
• [ ] Secrets in secret manager