API Design Patterns

Conventions and best practices for designing consistent, developer-friendly REST APIs and GraphQL APIs.

When to Activate

• Designing new API endpoints
• Reviewing existing API contracts
• Adding pagination, filtering, or sorting
• Implementing error handling for APIs
• Planning API versioning strategy
• Building public or partner-facing APIs

Resource Design

#### URL Structure

# Resources are nouns, plural, lowercase, kebab-case
GET    /api/v1/users
GET    /api/v1/users/:id
POST   /api/v1/users
PUT    /api/v1/users/:id
PATCH  /api/v1/users/:id
DELETE /api/v1/users/:id

# Sub-resources for relationships
GET    /api/v1/users/:id/orders
POST   /api/v1/users/:id/orders

# Actions that don't map to CRUD (use verbs sparingly)
POST   /api/v1/orders/:id/cancel
POST   /api/v1/auth/login
POST   /api/v1/auth/refresh

#### Naming Rules

# GOOD
/api/v1/team-members          # kebab-case for multi-word resources
/api/v1/orders?status=active  # query params for filtering
/api/v1/users/123/orders      # nested resources for ownership

# BAD
/api/v1/getUsers              # verb in URL
/api/v1/user                  # singular (use plural)
/api/v1/team_members          # snake_case in URLs
/api/v1/users/123/getOrders   # verb in nested resource

HTTP Methods and Status Codes

#### Method Semantics

*PATCH can be made idempotent with proper implementation

#### Status Code Reference

# Success
200 OK                    — GET, PUT, PATCH (with response body)
201 Created               — POST (include Location header)
204 No Content            — DELETE, PUT (no response body)
207 Multi-Status          — Bulk operations with per-item results
304 Not Modified          — Conditional GET, resource unchanged

# Client Errors
400 Bad Request           — Validation failure, malformed JSON
401 Unauthorized          — Missing or invalid authentication
403 Forbidden             — Authenticated but not authorized
404 Not Found             — Resource doesn't exist
409 Conflict              — Duplicate entry, state conflict
422 Unprocessable Entity  — Semantically invalid (valid JSON, bad data)
429 Too Many Requests     — Rate limit exceeded

# Server Errors
500 Internal Server Error — Unexpected failure (never expose details)
502 Bad Gateway           — Upstream service failed
503 Service Unavailable   — Temporary overload, include Retry-After

#### Common Mistakes

# BAD: 200 for everything
{ "status": 200, "success": false, "error": "Not found" }

# GOOD: Use HTTP status codes semantically
HTTP/1.1 404 Not Found
Content-Type: application/problem+json
{ "type": "https://example.com/errors/not-found", "title": "Not Found", "status": 404, "detail": "User not found" }

# BAD: 500 for validation errors
# GOOD: 400 or 422 with field-level details

# BAD: 200 for created resources
# GOOD: 201 with Location header
HTTP/1.1 201 Created
Location: /api/v1/users/abc-123

Response Format

#### Success Response

{
  "data": {
    "id": "abc-123",
    "email": "[email protected]",
    "name": "Alice",
    "created_at": "2025-01-15T10:30:00Z"
  }
}

#### Collection Response (with Pagination)

{
  "data": [
    { "id": "abc-123", "name": "Alice" },
    { "id": "def-456", "name": "Bob" }
  ],
  "meta": {
    "total": 142,
    "page": 1,
    "per_page": 20,
    "total_pages": 8
  },
  "links": {
    "self": "/api/v1/users?page=1&per_page=20",
    "next": "/api/v1/users?page=2&per_page=20",
    "last": "/api/v1/users?page=8&per_page=20"
  }
}

#### Error Response (RFC 7807 Problem Details)

Use Content-Type: application/problem+json for all error responses.

{
  "type": "https://example.com/errors/validation",
  "title": "Validation Failed",
  "status": 422,
  "detail": "3 fields failed validation",
  "instance": "/orders/123",
  "errors": [
    { "field": "email", "message": "Invalid format", "code": "INVALID_EMAIL" }
  ]
}

#### Response Envelope Variants

// Option A: Envelope with data wrapper (recommended for public APIs)
interface ApiResponse<T> {
  data: T;
  meta?: PaginationMeta;
  links?: PaginationLinks;
}

// Error: RFC 7807 Problem Details (Content-Type: application/problem+json)
interface ProblemDetails {
  type: string;
  title: string;
  status: number;
  detail?: string;
  instance?: string;
  errors?: FieldError[];
}

// Option B: Flat response (simpler, common for internal APIs)
// Success: just return the resource directly
// Error: RFC 7807 Problem Details object
// Distinguish by HTTP status code

Pagination

#### Offset-Based (Simple)

GET /api/v1/users?page=2&per_page=20

# Implementation
SELECT * FROM users
ORDER BY created_at DESC
LIMIT 20 OFFSET 20;

Pros: Easy to implement, supports "jump to page N" Cons: Slow on large offsets (OFFSET 100000), inconsistent with concurrent inserts

#### Cursor-Based (Default for Unbounded Data)

GET /api/v1/users?cursor=eyJpZCI6MTIzfQ&limit=20

# Implementation
SELECT * FROM users
WHERE id > :cursor_id
ORDER BY id ASC
LIMIT 21;  -- fetch one extra to determine has_next

{
  "data": [...],
  "meta": {
    "has_next": true,
    "next_cursor": "eyJpZCI6MTQzfQ"
  }
}

Pros: Consistent performance regardless of position, stable with concurrent inserts Cons: Cannot jump to arbitrary page, cursor is opaque

#### When to Use Which

Cursor pagination is the default for datasets that grow unboundedly; use offset pagination only for fixed/small datasets.

Filtering, Sorting, and Search

#### Filtering

# Simple equality (flat notation)
GET /api/v1/orders?status=active&customer_id=abc-123

# Comparison operators (bracket notation for ranges only)
GET /api/v1/products?price[gte]=10&price[lte]=100
GET /api/v1/orders?created_at[after]=2025-01-01

# Multiple values (comma-separated)
GET /api/v1/products?category=electronics,clothing

# Nested fields (dot notation)
GET /api/v1/orders?customer.country=US

#### Sorting

# Single field (prefix - for descending)
GET /api/v1/products?sort=-created_at

# Multiple fields (comma-separated)
GET /api/v1/products?sort=-featured,price,-created_at

#### Full-Text Search

# Search query parameter
GET /api/v1/products?q=wireless+headphones

# Field-specific search
GET /api/v1/users?email=alice

#### Sparse Fieldsets

# Return only specified fields (reduces payload)
GET /api/v1/users?fields=id,name,email
GET /api/v1/orders?fields=id,total,status&include=customer.name

Authentication and Authorization

#### Token-Based Auth

# Bearer token in Authorization header
GET /api/v1/users
Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

# API key (for server-to-server)
GET /api/v1/data
X-API-Key: sk_live_abc123

#### Authorization Patterns

// Resource-level: check ownership
app.get("/api/v1/orders/:id", async (req, res) => {
  const order = await Order.findById(req.params.id);
  if (!order) return res.status(404).set("Content-Type", "application/problem+json").json({ type: "https://example.com/errors/not-found", title: "Not Found", status: 404 });
  if (order.userId !== req.user.id) return res.status(403).set("Content-Type", "application/problem+json").json({ type: "https://example.com/errors/forbidden", title: "Forbidden", status: 403 });
  return res.json({ data: order });
});

// Role-based: check permissions
app.delete("/api/v1/users/:id", requireRole("admin"), async (req, res) => {
  await User.delete(req.params.id);
  return res.status(204).send();
});

Idempotency

For all non-idempotent POST/PATCH operations, require Idempotency-Key: <UUID v4> header.

• Cache the response for 24 hours keyed by Idempotency-Key
• Reusing the same key with the same request body within the cache window replays the stored response without

re-executing the operation

• Return 422 if the same key is reused with a different request body

POST /api/v1/orders
Idempotency-Key: 550e8400-e29b-41d4-a716-446655440000

Conditional Requests (Caching)

For cacheable GETs, return ETag and Last-Modified headers. Clients use If-None-Match / If-Modified-Since to skip re-fetching unchanged resources. Return 304 Not Modified when the resource is unchanged.

# Server response
HTTP/1.1 200 OK
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
Last-Modified: Tue, 15 Jan 2025 10:30:00 GMT

# Client conditional request
GET /api/v1/users/123
If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"

# Server response when unchanged
HTTP/1.1 304 Not Modified

Bulk Operations

Use POST /resources/batch for bulk create/update. Return 207 Multi-Status with per-item results:

{
  "results": [
    { "id": "1", "status": 201, "data": {} },
    { "id": "2", "status": 422, "error": "Invalid email" }
  ]
}

• Limit bulk request body size; return 413 Payload Too Large if exceeded
• Process atomically where possible; if partial success is allowed, always return 207

Health Endpoints

Two distinct endpoints, do NOT merge them:

• GET /health: liveness: always returns 200 if the process is alive; never checks dependencies
• GET /ready: readiness: checks DB, cache, external deps; returns 503 if any are degraded

These endpoints are outside the versioned API path (no /v1/ prefix).

// GET /ready — degraded example
{ "status": "degraded", "checks": { "db": "ok", "redis": "timeout" } }

Rate Limiting

#### Rate Limiting Headers

Include on all rate-limited responses:

• X-RateLimit-Limit: 100
• X-RateLimit-Remaining: 42
• X-RateLimit-Reset: 1714000000 (UTC epoch seconds)

Return 429 when limit exceeded. Include Retry-After header.

HTTP/1.1 200 OK
X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1714000000

# When exceeded
HTTP/1.1 429 Too Many Requests
X-RateLimit-Limit: 100
X-RateLimit-Remaining: 0
X-RateLimit-Reset: 1714000000
Retry-After: 60
Content-Type: application/problem+json

{
  "type": "https://example.com/errors/rate-limit-exceeded",
  "title": "Too Many Requests",
  "status": 429,
  "detail": "Rate limit exceeded. Try again in 60 seconds."
}

#### Rate Limit Tiers

Versioning

#### URL Path Versioning (Recommended)

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

Pros: Explicit, easy to route, cacheable Cons: URL changes between versions

#### Header Versioning

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

Pros: Clean URLs Cons: Harder to test, easy to forget

#### Version Lifecycle

• Maintain the deprecated version in production for a minimum of 6 months after a new version is released
• Maximum 2 active API versions at any time
• Announce deprecation via Deprecation and Sunset response headers:

Deprecation: true
Sunset: Sat, 01 Jan 2026 00:00:00 GMT

• Non-breaking changes don't need a new version:
• Adding new fields to responses
• Adding new optional query parameters
• Adding new endpoints
• Breaking changes require a new version:
• Removing or renaming fields
• Changing field types
• Changing URL structure
• Changing authentication method
• Return 410 Gone after the sunset date

Implementation Patterns

#### TypeScript (Next.js API Route)

import { z } from "zod";
import { NextRequest, NextResponse } from "next/server";

const createUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1).max(100),
});

export async function POST(req: NextRequest) {
  // Idempotency-Key required for non-idempotent POST
  const idempotencyKey = req.headers.get("Idempotency-Key");
  const body = await req.json();
  const parsed = createUserSchema.safeParse(body);

  if (!parsed.success) {
    return NextResponse.json({
      type: "https://example.com/errors/validation",
      title: "Validation Failed",
      status: 422,
      detail: "Request validation failed",
      errors: parsed.error.issues.map(i => ({
        field: i.path.join("."),
        message: i.message,
        code: i.code,
      })),
    }, {
      status: 422,
      headers: { "Content-Type": "application/problem+json" },
    });
  }

  const user = await createUser(parsed.data);

  return NextResponse.json(
    { data: user },
    {
      status: 201,
      headers: { Location: `/api/v1/users/${user.id}` },
    },
  );
}

#### Python (Django REST Framework)

from rest_framework import serializers, viewsets, status
from rest_framework.response import Response

class CreateUserSerializer(serializers.Serializer):
    email = serializers.EmailField()
    name = serializers.CharField(max_length=100)

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ["id", "email", "name", "created_at"]

class UserViewSet(viewsets.ModelViewSet):
    serializer_class = UserSerializer
    permission_classes = [IsAuthenticated]

    def get_serializer_class(self):
        if self.action == "create":
            return CreateUserSerializer
        return UserSerializer

    def create(self, request):
        serializer = CreateUserSerializer(data=request.data)
        serializer.is_valid(raise_exception=True)
        user = UserService.create(**serializer.validated_data)
        return Response(
            {"data": UserSerializer(user).data},
            status=status.HTTP_201_CREATED,
            headers={"Location": f"/api/v1/users/{user.id}"},
        )

#### Go (net/http)

func (h *UserHandler) CreateUser(w http.ResponseWriter, r *http.Request) {
    var req CreateUserRequest
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        // writeProblem sets Content-Type: application/problem+json
        writeProblem(w, http.StatusBadRequest, "https://example.com/errors/invalid-json", "Invalid Request Body")
        return
    }

    if err := req.Validate(); err != nil {
        writeProblem(w, http.StatusUnprocessableEntity, "https://example.com/errors/validation", "Validation Failed")
        return
    }

    user, err := h.service.Create(r.Context(), req)
    if err != nil {
        switch {
        case errors.Is(err, domain.ErrEmailTaken):
            writeProblem(w, http.StatusConflict, "https://example.com/errors/email-taken", "Email Already Registered")
        default:
            writeProblem(w, http.StatusInternalServerError, "https://example.com/errors/internal", "Internal Server Error")
        }
        return
    }

    w.Header().Set("Location", fmt.Sprintf("/api/v1/users/%s", user.ID))
    writeJSON(w, http.StatusCreated, map[string]any{"data": user})
}

API Design Checklist

Before shipping a new endpoint:

• [ ] Resource URL follows naming conventions (plural, kebab-case, no verbs)
• [ ] Correct HTTP method used (GET for reads, POST for creates, etc.)
• [ ] Appropriate status codes returned (not 200 for everything)
• [ ] Input validated with schema (Zod, Pydantic, Bean Validation)
• [ ] Error responses use RFC 7807 Problem Details (application/problem+json)
• [ ] Pagination implemented for list endpoints (cursor default for unbounded data)
• [ ] Idempotency-Key header required for non-idempotent POST/PATCH
• [ ] ETag / Last-Modified returned for cacheable GETs
• [ ] Authentication required (or explicitly marked as public)
• [ ] Authorization checked (user can only access their own resources)
• [ ] Rate limiting configured with X-RateLimit-* headers
• [ ] Deprecation / Sunset headers set on deprecated endpoints
• [ ] /health (liveness) and /ready (readiness) endpoints present and separated
• [ ] Response does not leak internal details (stack traces, SQL errors)
• [ ] Consistent naming with existing endpoints (camelCase vs snake_case)
• [ ] Documented (OpenAPI/Swagger spec updated)

GraphQL API Standards

#### Schema Design

• Types: PascalCase: UserProfile, OrderItem
• Fields: camelCase: createdAt, totalAmount
• Enum values: UPPER_SNAKE_CASE: ORDER_STATUS_PENDING
• Every field must have a description: enforced by schema linting
• Input types suffixed Input: CreateOrderInput
• Mutation payload types suffixed Payload: CreateOrderPayload

#### Mutation Payload Structure

All mutations return a consistent payload:

type CreateOrderPayload {
  success: Boolean!
  errors: [UserError!]!
  order: Order
}

type UserError {
  field: String
  message: String!
  code: String!
}

#### N+1 Prevention, DataLoader

Always use DataLoader for batching related-object fetches:

const userLoader = new DataLoader(async (ids: readonly string[]) => {
  const users = await db.users.findMany({ where: { id: { in: [...ids] } } })
  return ids.map(id => users.find(u => u.id === id) ?? null)
})

#### Authorization

• Implement field-level authorization: return explicit errors, never silent null
• Use a shield or directive pattern; do not inline auth logic in resolvers

#### Production Security

• Disable introspection in production
• Enforce query depth limit (max 10 levels)
• Enforce query complexity limits (assign cost per field; reject above threshold)
• Use Automatic Persisted Queries (APQ) to reduce payload size and allow CDN caching

#### Subscriptions

Use graphql-ws protocol (not the deprecated subscriptions-transport-ws).

#### Schema Evolution

• Additive only: never remove or rename fields/types without a deprecation cycle
• Mark deprecated fields with @deprecated(reason: "Use X instead")
• Run schema diffing in CI with graphql-inspector; block breaking changes

#### Pagination

Follow the Relay Cursor Connections Specification for all paginated fields:

type UserConnection {
  edges: [UserEdge!]!
  pageInfo: PageInfo!
  totalCount: Int!
}
type UserEdge {
  node: User!
  cursor: String!
}
type PageInfo {
  hasNextPage: Boolean!
  hasPreviousPage: Boolean!
  startCursor: String
  endCursor: String
}