API Design for Scalability

Why API design matters

A well-designed API reduces developer friction, prevents costly breaking changes, and makes it possible to grow traffic without rewriting core systems. Good API design is both an engineering and product decision: it impacts developer experience, client integrations, and operational cost.

1. Choose a clear architectural style

Common choices are REST and GraphQL. REST is simple, cache-friendly, and works well for resource-oriented systems. GraphQL is great if clients need flexible queries and you want to reduce overfetching.

• REST: predictable, easy to cache, great for public APIs and simple CRUD.
• GraphQL: flexible responses, reduces round-trips but requires careful rate-limiting and complexity control.

2. Contracts & schemas (single source of truth)

Define your API contract using OpenAPI/Swagger (REST) or a strict GraphQL schema. Treat the contract as the primary artifact — it should drive client SDK generation, tests, and integration checks.

# example (OpenAPI snippet)
paths:
  /users:
    get:
      summary: List users
      responses:
        '200':
          description: OK

3. Versioning strategy

Never break existing clients. Options:

• URI versioning — `/v1/resource` (explicit, cacheable).
• Header versioning — `Accept: application/vnd.myapp.v2+json` (clean URLs, but less visible).
• Backward-compatible changes — prefer additive changes (new fields) and deprecate old fields clearly in docs.

4. Pagination, filtering & sorting

Large lists must be paginated. Cursor-based pagination (aka keyset) scales better than offset for high-traffic endpoints.

# cursor style
GET /v1/items?limit=50&cursor=eyJpZCI6IjEwMDAifQ

5. Idempotency & retries

For non-idempotent operations (payments, orders), require an idempotency key so clients can safely retry. Design endpoints to be idempotent where possible.

# HTTP example
POST /v1/payments
Idempotency-Key: 8df3a7e2-...

6. Error handling & status codes

Use consistent error shapes. Include machine-readable fields (`code`, `message`, `details`) and human-readable messages. Use proper HTTP status codes (4xx for client errors, 5xx for server errors).

{
  "error": {
    "code": "USER_NOT_FOUND",
    "message": "User with id 123 not found",
    "details": {}
  }
}

7. Caching & performance

Make frequent read endpoints cacheable. Use Cache-Control, CDN edge caching, and ETags for conditional requests. Cache invalidation needs clear strategies — prefer short TTLs for dynamic data or event-driven cache invalidation.

8. Rate limiting & throttling

Protect your platform with rate limits and fair-usage policies. Provide `Retry-After` headers and visible quotas in developer dashboards for paid tiers.

9. Observability: logs, metrics, traces

Instrument your API: structured logs, request/response metrics, and distributed traces (OpenTelemetry). Alert on error budgets, latency regressions, and unusual traffic patterns.

10. Security & best practices

• Always use TLS (HTTPS).
• Prefer OAuth 2.0 / JWT for auth; rotate secrets regularly.
• Validate inputs against schemas (Zod / Joi).
• Use scopes/roles for fine-grained access control.

Practical pattern: Versioned Express route + schema check

Minimal Node/Express pattern showing versioning + request validation (pseudo-code):

// express v1 route (pseudo)
import express from "express";
import { z } from "zod";

const router = express.Router();
const createUserSchema = z.object({ name: z.string(), email: z.string().email() });

router.post("/v1/users", (req, res) => {
  const result = createUserSchema.safeParse(req.body);
  if (!result.success) return res.status(400).json({ error: result.error });
  // handle creation (idempotency, validation, enqueue background work...)
  res.status(201).json({ id: "user_123" });
});

export default router;

API design checklist

• Document the contract (OpenAPI / GraphQL schema).
• Design for idempotency and retries.
• Expose clear error structures and codes.
• Plan caching and pagination up front.
• Automate tests against the contract (integration, contract tests).
• Instrument and alert on performance and errors.