Building Scalable APIs: RESTful Design Patterns and Best Practices

Your API is the contract between your backend and the outside world. Whether it's a mobile app, web frontend, or third-party integrations—good API design determines the success of your entire application.

A poorly designed API leads to:

• Confused developers
• Difficult maintenance
• Performance bottlenecks
• Breaking changes that hurt users

Let's explore how to build APIs that scale, perform well, and stand the test of time.

Think in terms of resources, not actions. URLs should represent things, not verbs.

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// ❌ Bad - Action-based URLs
POST / api / createUser;
GET / api / getUserById / 123;
POST / api / deleteUser;

// ✅ Good - Resource-based URLs
POST / api / users;
GET / api / users / 123;
DELETE / api / users / 123;

Use the right HTTP method for the right operation:

Don't just return 200 OK for everything. Use appropriate status codes:

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// Success responses
200 OK           // Successful GET, PUT, PATCH, DELETE
201 Created      // Successful POST
204 No Content   // Successful DELETE with no response body

// Client errors
400 Bad Request      // Invalid input
401 Unauthorized     // Not authenticated
403 Forbidden        // Authenticated but not allowed
404 Not Found        // Resource doesn't exist
422 Unprocessable    // Validation errors

// Server errors
500 Internal Server Error
503 Service Unavailable

Never return unbounded lists. Always paginate:

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// Cursor-based pagination (recommended for large datasets)
GET /api/posts?limit=20&cursor=eyJpZCI6MTIzfQ

// Offset-based pagination (simpler, but slower at high offsets)
GET /api/posts?limit=20&offset=40

Response structure:

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{
  "data": [...],
  "pagination": {
    "next": "eyJpZCI6MTQzfQ",
    "hasMore": true
  }
}

Allow clients to request only what they need:

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// Filtering
GET /api/users?role=admin&status=active

// Sorting
GET /api/posts?sort=-createdAt,title

// Field selection (sparse fieldsets)
GET /api/users/123?fields=id,name,email

Protect your API from abuse:

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// Express rate limiting example
import rateLimit from "express-rate-limit";

const limiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100, // Limit each IP to 100 requests per windowMs
  standardHeaders: true,
  legacyHeaders: false,
});

app.use("/api/", limiter);

Response headers:

X-RateLimit-Limit: 100
X-RateLimit-Remaining: 73
X-RateLimit-Reset: 1694789200

Your API will evolve. Plan for it:

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GET / api / v1 / users;
GET / api / v2 / users;

Pros: Clear, easy to route, cache-friendly
Cons: Can lead to code duplication

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GET / api / users;
Accept: application / vnd.myapi.v2 + json;

Pros: Cleaner URLs
Cons: Harder to test, less discoverable

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GET /api/users?version=2

Pros: Simple
Cons: Can be ignored, messy URLs

My recommendation: Start with URL versioning. It's explicit and widely understood.

Consistent error responses are crucial:

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{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid input data",
    "details": [
      {
        "field": "email",
        "message": "Must be a valid email address"
      },
      {
        "field": "password",
        "message": "Must be at least 8 characters"
      }
    ]
  }
}

Key principles:

• Machine-readable error codes
• Human-readable messages
• Field-level validation details
• Consistent structure across all errors

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// Login endpoint
POST /api/auth/login
{
  "email": "user@example.com",
  "password": "secret"
}

// Response
{
  "token": "eyJhbGciOiJIUzI1NiIs...",
  "refreshToken": "eyJhbGciOiJIUzI1NiIs...",
  "expiresIn": 3600
}

// Subsequent requests
GET /api/users/me
Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

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GET /api/data
X-API-Key: sk_live_1234567890abcdef

Your API is only as good as its documentation. Use tools like:

• OpenAPI/Swagger: Industry standard, interactive docs
• Postman Collections: Share and test endpoints
• API Blueprint: Markdown-based documentation

Example OpenAPI snippet:

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paths:
  /users/{id}:
    get:
      summary: Get user by ID
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: integer
      responses:
        "200":
          description: Successful response
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/User"
        "404":
          description: User not found

Speed up your API with smart caching:

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// Cache for 1 hour
Cache-Control: public, max-age=3600

// Conditional requests
ETag: "686897696a7c876b7e"
Last-Modified: Wed, 15 Sep 2024 12:00:00 GMT

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import Redis from "ioredis";
const redis = new Redis();

app.get("/api/users/:id", async (req, res) => {
  const cached = await redis.get(`user:${req.params.id}`);

  if (cached) {
    return res.json(JSON.parse(cached));
  }

  const user = await db.users.findById(req.params.id);
  await redis.set(`user:${req.params.id}`, JSON.stringify(user), "EX", 300);

  res.json(user);
});

1. Always use HTTPS in production
2. Validate and sanitize all input
3. Implement CORS properly
4. Use parameterized queries to prevent SQL injection
5. Rate limit authentication endpoints
6. Hash passwords with bcrypt (never store plain text)
7. Set security headers (Helmet.js for Express)

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import helmet from "helmet";
app.use(helmet());

Don't ship untested code:

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// Jest + Supertest example
describe("GET /api/users/:id", () => {
  it("should return user when ID exists", async () => {
    const response = await request(app).get("/api/users/123").expect(200);

    expect(response.body).toHaveProperty("id", 123);
    expect(response.body).toHaveProperty("email");
  });

  it("should return 404 when user not found", async () => {
    await request(app).get("/api/users/999999").expect(404);
  });
});

Putting it all together:

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import express from "express";
import { z } from "zod";

const app = express();

// Validation schema
const createUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(2),
  role: z.enum(["user", "admin"]).default("user"),
});

// Create user
app.post("/api/v1/users", async (req, res) => {
  try {
    const data = createUserSchema.parse(req.body);
    const user = await db.users.create(data);

    res.status(201).json({
      data: user,
      message: "User created successfully",
    });
  } catch (error) {
    if (error instanceof z.ZodError) {
      return res.status(422).json({
        error: {
          code: "VALIDATION_ERROR",
          details: error.errors,
        },
      });
    }

    res.status(500).json({
      error: {
        code: "INTERNAL_ERROR",
        message: "Something went wrong",
      },
    });
  }
});

// List users with pagination
app.get("/api/v1/users", async (req, res) => {
  const { limit = 20, cursor } = req.query;

  const users = await db.users.findMany({
    take: parseInt(limit) + 1,
    cursor: cursor ? { id: cursor } : undefined,
    orderBy: { createdAt: "desc" },
  });

  const hasMore = users.length > limit;
  const data = hasMore ? users.slice(0, -1) : users;

  res.json({
    data,
    pagination: {
      next: hasMore ? data[data.length - 1].id : null,
      hasMore,
    },
  });
});

Building scalable APIs is about thinking ahead:

✅ Design for growth (pagination, caching, rate limiting)
✅ Be consistent (naming, error handling, response structure)
✅ Document everything (your future self will thank you)
✅ Security first (HTTPS, validation, authentication)
✅ Version early (breaking changes are inevitable)
✅ Test thoroughly (unit tests, integration tests, load tests)

Your API is a product. Treat it like one.

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