API Design Guide

Best practices for designing developer-friendly, maintainable APIs.

REST API Principles

Resource Naming

Use nouns, not verbs:

✓ GET /users
✓ GET /users/123
✓ GET /users/123/orders

✗ GET /getUsers
✗ GET /fetchUserById
✗ POST /createNewOrder

Use plural nouns:

✓ /users
✓ /orders
✓ /products

✗ /user
✗ /order

Hierarchical relationships:

/users/{userId}/orders           # User's orders
/users/{userId}/orders/{orderId} # Specific order

---

HTTP Methods

Method	Purpose	Idempotent	Request Body
GET	Retrieve resource(s)	Yes	No
POST	Create resource	No	Yes
PUT	Replace resource entirely	Yes	Yes
PATCH	Update resource partially	No	Yes
DELETE	Remove resource	Yes	No

---

Status Codes

Success (2xx):

• 200 OK - Request succeeded
• 201 Created - Resource created (return Location header)
• 204 No Content - Success with no response body

Client Error (4xx):

• 400 Bad Request - Malformed request
• 401 Unauthorized - Authentication required
• 403 Forbidden - No permission
• 404 Not Found - Resource doesn't exist
• 409 Conflict - State conflict
• 422 Unprocessable Entity - Validation failed
• 429 Too Many Requests - Rate limited

Server Error (5xx):

• 500 Internal Server Error - Unexpected error
• 503 Service Unavailable - Temporary outage

---

Response Format

Successful response:

{
  "data": {
    "id": "123",
    "name": "John Doe",
    "email": "john@example.com"
  }
}

Collection response:

{
  "data": [
    { "id": "1", "name": "Item 1" },
    { "id": "2", "name": "Item 2" }
  ],
  "meta": {
    "page": 1,
    "perPage": 20,
    "total": 100,
    "totalPages": 5
  },
  "links": {
    "self": "/items?page=1",
    "next": "/items?page=2",
    "prev": null
  }
}

Error response:

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Validation failed",
    "details": [
      {
        "field": "email",
        "code": "INVALID_FORMAT",
        "message": "Must be a valid email address"
      }
    ]
  }
}

---

Pagination

Offset-based (simple, but slow on large datasets):

GET /users?page=2&perPage=20
GET /users?offset=40&limit=20

Cursor-based (efficient, recommended):

GET /users?cursor=eyJpZCI6MTIzfQ&limit=20

Response includes next cursor:

{
  "data": [...],
  "meta": {
    "nextCursor": "eyJpZCI6MTQzfQ",
    "hasMore": true
  }
}

---

Filtering, Sorting, Fields

Filtering:

GET /users?status=active
GET /users?created_after=2024-01-01
GET /users?role=admin,moderator

Sorting:

GET /users?sort=name
GET /users?sort=-created_at         # Descending
GET /users?sort=status,-created_at  # Multiple fields

Field selection:

GET /users?fields=id,name,email
GET /users?include=orders,profile

---

Versioning

URL path (recommended):

/api/v1/users
/api/v2/users

Header:

Accept: application/vnd.api+json;version=2

---

OpenAPI Specification

openapi: 3.0.3
info:
  title: My API
  version: 1.0.0
  description: API for managing users

servers:
  - url: https://api.example.com/v1

paths:
  /users:
    get:
      summary: List users
      tags: [Users]
      parameters:
        - name: page
          in: query
          schema:
            type: integer
            default: 1
      responses:
        "200":
          description: List of users
          content:
            application/json:
              schema:
                type: object
                properties:
                  data:
                    type: array
                    items:
                      $ref: "#/components/schemas/User"

components:
  schemas:
    User:
      type: object
      required: [id, email]
      properties:
        id:
          type: string
        email:
          type: string
          format: email
        name:
          type: string

---

Authentication

API Keys (simple, for server-to-server):

Authorization: Api-Key YOUR_API_KEY

Bearer Tokens (JWT, OAuth):

Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

Include in OpenAPI:

components:
  securitySchemes:
    bearerAuth:
      type: http
      scheme: bearer
      bearerFormat: JWT

security:
  - bearerAuth: []

---

Rate Limiting

Include headers in responses:

X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 999
X-RateLimit-Reset: 1640995200

Return 429 Too Many Requests when exceeded.

---

Security Checklist

• HTTPS only
• Authentication on protected routes
• Input validation
• Output encoding
• Rate limiting
• CORS configuration
• No sensitive data in URLs
• Audit logging

---

gRPC and Protocol Buffers

Proto Definition

syntax = "proto3";
package user.v1;

service UserService {
  rpc GetUser(GetUserRequest) returns (User);
  rpc ListUsers(ListUsersRequest) returns (ListUsersResponse);
  rpc CreateUser(CreateUserRequest) returns (User);
  rpc StreamUpdates(StreamRequest) returns (stream UserUpdate);
}

message User {
  string id = 1;
  string name = 2;
  string email = 3;
  google.protobuf.Timestamp created_at = 4;
}

message GetUserRequest {
  string id = 1;
}

message ListUsersRequest {
  int32 page_size = 1;
  string page_token = 2;
}

message ListUsersResponse {
  repeated User users = 1;
  string next_page_token = 2;
}

When to Use gRPC vs REST

Factor	gRPC	REST
Performance	Binary, fast	JSON, human-readable
Streaming	Bidirectional	SSE/WebSocket workaround
Type safety	Proto generates types	OpenAPI + codegen
Browser	Needs gRPC-Web proxy	Native
Tooling	Protoc, Buf	Swagger, Postman
Best for	Service-to-service, streaming	Public APIs, web clients

---

tRPC for TypeScript

// server/router.ts
import { router, publicProcedure, protectedProcedure } from './trpc';
import { z } from 'zod';

export const appRouter = router({
  user: router({
    get: publicProcedure
      .input(z.object({ id: z.string() }))
      .query(async ({ input }) => {
        return db.user.findUnique({ where: { id: input.id } });
      }),
    create: protectedProcedure
      .input(z.object({
        name: z.string().min(1),
        email: z.string().email(),
      }))
      .mutation(async ({ input, ctx }) => {
        return db.user.create({ data: { ...input, createdBy: ctx.userId } });
      }),
  }),
});

export type AppRouter = typeof appRouter;

// client.ts - Full type inference, no codegen
const user = trpc.user.get.useQuery({ id: '123' });
const createUser = trpc.user.create.useMutation();

tRPC is ideal for monorepo full-stack TypeScript apps where client and server share the same codebase.

---

Webhook Design Patterns

Webhook Payload

{
  "id": "evt_abc123",
  "type": "order.completed",
  "created_at": "2025-01-15T10:30:00Z",
  "data": {
    "order_id": "ord_456",
    "total": 99.99,
    "currency": "USD"
  }
}

Signature Verification

// Sign webhooks with HMAC-SHA256
import crypto from 'crypto';

function signWebhook(payload: string, secret: string): string {
  return crypto
    .createHmac('sha256', secret)
    .update(payload)
    .digest('hex');
}

// Verify on receiving end
function verifyWebhook(payload: string, signature: string, secret: string): boolean {
  const expected = signWebhook(payload, secret);
  return crypto.timingSafeEqual(
    Buffer.from(signature),
    Buffer.from(expected),
  );
}

Retry Strategy

Attempt 1: Immediately
Attempt 2: After 1 minute
Attempt 3: After 5 minutes
Attempt 4: After 30 minutes
Attempt 5: After 2 hours
Attempt 6: After 24 hours (final)

Failed webhooks: log, alert, manual retry UI

Idempotency

// Include idempotency key in webhook
// Receivers should deduplicate based on event ID
async function handleWebhook(event: WebhookEvent) {
  // Check if already processed
  const existing = await db.processedEvents.findUnique({
    where: { eventId: event.id },
  });
  if (existing) return { status: 'already_processed' };

  // Process and record
  await db.$transaction([
    processEvent(event),
    db.processedEvents.create({ data: { eventId: event.id } }),
  ]);
}

---

API Versioning Strategies

Strategy	Example	Pros	Cons
URL path	/api/v1/users	Explicit, easy to route	URL pollution
Query parameter	/api/users?version=1	Optional parameter	Easy to miss
Header	Accept: application/vnd.api.v1	Clean URLs	Hidden, harder to test
Content negotiation	Accept: application/json;v=2	Standards-based	Complex to implement

Recommendation: URL path versioning for simplicity. Only bump major versions for breaking changes. Use additive, non-breaking changes within a version.