Api Design Patterns

API Design Patterns

Design robust, scalable APIs using proven patterns for REST, GraphQL, and gRPC with proper versioning, authentication, and error handling.

Quick Reference

API Style Selection:

• REST: Resource-based CRUD, simple clients, HTTP-native caching
• GraphQL: Client-driven queries, complex data graphs, real-time subscriptions
• gRPC: High-performance RPC, microservices, strong typing, streaming

Critical Patterns:

• Versioning: URI (/v1/users), header (Accept: application/vnd.api+json;version=1), content negotiation
• Pagination: Offset (simple), cursor (stable), keyset (performant)
• Auth: OAuth2 (delegated), JWT (stateless), API keys (service-to-service)
• Rate limiting: Token bucket, fixed window, sliding window
• Idempotency: Idempotency keys, conditional requests, safe retry

See references/ for deep dives: rest-patterns.md, graphql-patterns.md, grpc-patterns.md, versioning-strategies.md, authentication.md

Core Principles

Universal API Design Standards

Apply these principles across all API styles:

1. Consistency Over Cleverness

• Follow established conventions for your API style
• Use predictable naming patterns (snake_case or camelCase, pick one)
• Maintain consistent error response formats
• Version breaking changes, never surprise clients

2. Design for Evolution

• Plan for versioning from day one
• Use optional fields with sensible defaults
• Deprecate gracefully with sunset dates
• Document breaking vs non-breaking changes

3. Security by Default

• Require authentication unless explicitly public
• Use HTTPS/TLS for all production endpoints
• Implement rate limiting and throttling
• Validate and sanitize all inputs
• Return minimal error details to clients

4. Developer Experience First

• Provide comprehensive documentation (OpenAPI, GraphQL schema)
• Return meaningful error messages with actionable guidance
• Use standard HTTP status codes correctly
• Include request IDs for debugging
• Offer SDKs and code generators

API Style Decision Tree

When to Choose REST

✅ Use REST when:

• Building CRUD-focused resource APIs
• Clients need HTTP caching (ETags, Cache-Control)
• Wide platform compatibility required (browsers, mobile, IoT)
• Simple, stateless client-server model fits
• Team familiar with HTTP/REST conventions

❌ Avoid REST when:

• Complex data fetching with nested relationships (N+1 queries)
• Real-time updates are primary use case
• Need strong typing and code generation
• High-performance RPC between microservices

Example Use Cases: Public APIs, mobile backends, traditional web services

When to Choose GraphQL

✅ Use GraphQL when:

• Clients need flexible, client-driven queries
• Complex data graphs with nested relationships
• Multiple client types with different data needs
• Real-time subscriptions required
• Strong typing and schema validation needed

❌ Avoid GraphQL when:

• Simple CRUD operations dominate
• HTTP caching is critical (GraphQL uses POST)
• File uploads are primary feature (requires extensions)
• Team lacks GraphQL expertise
• Performance optimization is complex (N+1 problem)

Example Use Cases: Client-facing APIs, dashboards, mobile apps with varied UIs

When to Choose gRPC

✅ Use gRPC when:

• Microservice-to-microservice communication
• High performance and low latency critical
• Bidirectional streaming needed
• Strong typing with Protocol Buffers
• Polyglot environments (language interop)

❌ Avoid gRPC when:

• Browser clients (limited support, needs grpc-web)
• HTTP/JSON required for compatibility
• Human-readable payloads preferred
• Simple request/response patterns

Example Use Cases: Internal microservices, streaming data, service mesh

REST API Patterns

Resource Naming

✅ Good: Plural nouns, hierarchical

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GET    /users              # List users
GET    /users/123          # Get user
POST   /users              # Create user
PUT    /users/123          # Update user (full)
PATCH  /users/123          # Update user (partial)
DELETE /users/123          # Delete user
GET    /users/123/orders   # User's orders (sub-resource)

❌ Bad: Verbs, mixed conventions

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GET    /getUsers           # Don't use verbs
POST   /user/create        # Don't use verbs
GET    /Users/123          # Don't capitalize
GET    /user/123           # Don't mix singular/plural

HTTP Status Codes

Success Codes:

• 200 OK: Successful GET, PUT, PATCH, DELETE with body
• 201 Created: Successful POST, return Location header
• 202 Accepted: Async operation started
• 204 No Content: Successful DELETE, no body

Client Error Codes:

• 400 Bad Request: Invalid input, validation error
• 401 Unauthorized: Missing or invalid authentication
• 403 Forbidden: Authenticated but insufficient permissions
• 404 Not Found: Resource doesn't exist
• 409 Conflict: State conflict (duplicate, version mismatch)
• 422 Unprocessable Entity: Semantic validation error
• 429 Too Many Requests: Rate limit exceeded

Server Error Codes:

• 500 Internal Server Error: Unexpected error
• 502 Bad Gateway: Upstream service error
• 503 Service Unavailable: Temporary outage
• 504 Gateway Timeout: Upstream timeout

Error Response Format

✅ Consistent error structure

jsonCopy code
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid request parameters",
    "details": [
      {
        "field": "email",
        "message": "Invalid email format",
        "code": "INVALID_FORMAT"
      }
    ],
    "request_id": "req_abc123",
    "documentation_url": "https://api.example.com/docs/errors/validation"
  }
}

Pagination Patterns

Offset Pagination (simple, familiar):

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GET /users?limit=20&offset=40

✅ Use for: Small datasets, admin interfaces ❌ Avoid for: Large datasets (skips become expensive), real-time data

Cursor Pagination (stable, efficient):

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GET /users?limit=20&cursor=eyJpZCI6MTIzfQ
Response: { "data": [...], "next_cursor": "eyJpZCI6MTQzfQ" }

✅ Use for: Infinite scroll, real-time feeds, large datasets ❌ Avoid for: Random access, page numbers

Keyset Pagination (performant):

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GET /users?limit=20&after_id=123

✅ Use for: Ordered data, database index friendly ❌ Avoid for: Complex sorting, multiple sort keys

See references/rest-patterns.md for filtering, sorting, field selection, HATEOAS

GraphQL Patterns

Schema Design

✅ Good: Clear types, nullable by default

graphqlCopy code
type User {
  id: ID!                    # Non-null ID
  email: String!             # Required field
  name: String               # Optional (nullable by default)
  createdAt: DateTime!
  orders: [Order!]!          # Non-null array of non-null orders
}

type Query {
  user(id: ID!): User
  users(first: Int, after: String): UserConnection!
}

type Mutation {
  createUser(input: CreateUserInput!): CreateUserPayload!
}

input CreateUserInput {
  email: String!
  name: String
}

type CreateUserPayload {
  user: User
  userEdge: UserEdge
  errors: [UserError!]
}

Resolver Patterns

Avoid N+1 Queries with DataLoader:

typescriptCopy code
import DataLoader from 'dataloader';

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

// Resolver batches queries automatically
const resolvers = {
  Order: {
    user: (order) => userLoader.load(order.userId)
  }
};

Query Complexity Analysis

Prevent expensive queries:

typescriptCopy code
import { createComplexityLimitRule } from 'graphql-validation-complexity';

const server = new ApolloServer({
  schema,
  validationRules: [
    createComplexityLimitRule(1000, {
      onCost: (cost) => console.log('Query cost:', cost),
    }),
  ],
});

See references/graphql-patterns.md for subscriptions, relay cursor connections, error handling

gRPC Patterns

Service Definition

protobufCopy code
syntax = "proto3";

package users.v1;

service UserService {
  rpc GetUser (GetUserRequest) returns (User) {}
  rpc ListUsers (ListUsersRequest) returns (ListUsersResponse) {}
  rpc CreateUser (CreateUserRequest) returns (User) {}
  rpc StreamUsers (StreamUsersRequest) returns (stream User) {}
  rpc BidiChat (stream ChatMessage) returns (stream ChatMessage) {}
}

message User {
  string id = 1;
  string email = 2;
  string name = 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;
}

Error Handling

goCopy code
import (
    "google.golang.org/grpc/codes"
    "google.golang.org/grpc/status"
)

func (s *server) GetUser(ctx context.Context, req *pb.GetUserRequest) (*pb.User, error) {
    if req.Id == "" {
        return nil, status.Error(codes.InvalidArgument, "user ID is required")
    }

    user, err := s.db.GetUser(ctx, req.Id)
    if err != nil {
        if errors.Is(err, sql.ErrNoRows) {
            return nil, status.Error(codes.NotFound, "user not found")
        }
        return nil, status.Error(codes.Internal, "database error")
    }

    return user, nil
}

See references/grpc-patterns.md for streaming, interceptors, metadata, health checks

Versioning Strategies

URI Versioning (Simple, Explicit)

✅ Most common, easy to understand

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GET /v1/users/123
GET /v2/users/123

Pros: Clear, easy to route, browser-friendly Cons: Couples version to URL, duplicates routes

Header Versioning (Clean URLs)

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GET /users/123
Accept: application/vnd.myapi.v2+json

Pros: Clean URLs, version separate from resource Cons: Less visible, harder to test manually

Content Negotiation (Granular)

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GET /users/123
Accept: application/vnd.myapi.user.v2+json

Pros: Resource-level versioning, backward compatible Cons: Complex, harder to implement

Version Deprecation Process

jsonCopy code
{
  "version": "1.0",
  "deprecated": true,
  "sunset_date": "2025-12-31",
  "migration_guide": "https://docs.api.com/v1-to-v2",
  "replacement_version": "2.0"
}

Include deprecation warnings:

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HTTP/1.1 200 OK
Deprecation: true
Sunset: Sat, 31 Dec 2025 23:59:59 GMT
Link: <https://docs.api.com/v1-to-v2>; rel="deprecation"

See references/versioning-strategies.md for detailed migration patterns

Authentication & Authorization

OAuth 2.0 (Delegated Access)

Use for: Third-party access, user consent, token refresh

Authorization Code Flow (most secure for web/mobile):

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1. Client redirects to /authorize
2. User authenticates, grants permissions
3. Auth server redirects to callback with code
4. Client exchanges code for access token
5. Client uses access token for API requests

httpCopy code
# Request token
POST /oauth/token
Content-Type: application/x-www-form-urlencoded

grant_type=authorization_code
&code=AUTH_CODE
&redirect_uri=https://client.com/callback
&client_id=CLIENT_ID
&client_secret=CLIENT_SECRET

# Response
{
  "access_token": "eyJhbGc...",
  "token_type": "Bearer",
  "expires_in": 3600,
  "refresh_token": "tGzv3JOkF0XG5Qx2TlKWIA",
  "scope": "read write"
}

# Use token
GET /v1/users/me
Authorization: Bearer eyJhbGc...

JWT (Stateless Auth)

Use for: Microservices, stateless API auth, short-lived tokens

✅ Good: Minimal claims, short expiry

jsonCopy code
{
  "sub": "user_123",
  "iat": 1516239022,
  "exp": 1516242622,
  "scope": "read:users write:orders"
}

Validation:

typescriptCopy code
import jwt from 'jsonwebtoken';

const token = req.headers.authorization?.split(' ')[1];
const payload = jwt.verify(token, process.env.JWT_SECRET);
req.userId = payload.sub;

API Keys (Service-to-Service)

Use for: Server-to-server, CLI tools, webhooks

httpCopy code
GET /v1/users
X-API-Key: sk_live_abc123...

# Or query parameter (less secure)
GET /v1/users?api_key=sk_live_abc123

Key Practices:

• Prefix keys with environment (sk_live_, sk_test_)
• Hash keys before storage (bcrypt, scrypt)
• Allow key rotation without downtime
• Support multiple keys per user
• Rate limit per key

See references/authentication.md for API key rotation, scopes, RBAC

Rate Limiting

Token Bucket (Burst-Friendly)

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Bucket: 100 tokens, refill 10/second
Request costs 1 token
Allows bursts up to bucket size

Headers:

httpCopy code
HTTP/1.1 200 OK
X-RateLimit-Limit: 100
X-RateLimit-Remaining: 73
X-RateLimit-Reset: 1640995200

429 Response:

httpCopy code
HTTP/1.1 429 Too Many Requests
Retry-After: 60
X-RateLimit-Limit: 100
X-RateLimit-Remaining: 0
X-RateLimit-Reset: 1640995200

{
  "error": {
    "code": "RATE_LIMIT_EXCEEDED",
    "message": "Rate limit exceeded. Try again in 60 seconds.",
    "limit": 100,
    "reset_at": "2025-01-01T00:00:00Z"
  }
}

Sliding Window (Fair Distribution)

Counts requests in rolling time window. More accurate than fixed window.

Per-User vs Per-IP

• Per-User: Authenticated requests, fair quotas
• Per-IP: Unauthenticated requests, prevent abuse
• Combined: Both limits, take stricter

Idempotency

Idempotent Methods (HTTP Spec)

Naturally Idempotent: GET, PUT, DELETE, HEAD, OPTIONS Not Idempotent: POST, PATCH

Idempotency Keys

Make POST requests idempotent:

httpCopy code
POST /v1/payments
Idempotency-Key: uuid-or-client-generated-key
Content-Type: application/json

{
  "amount": 1000,
  "currency": "USD",
  "customer": "cust_123"
}

Server behavior:

1. First request: Process and store result with key
2. Duplicate request (same key): Return stored result (200 or 201)
3. Different request (same key): Return 409 Conflict

Implementation:

typescriptCopy code
const idempotencyKey = req.headers['idempotency-key'];
if (idempotencyKey) {
  const cached = await redis.get(`idempotency:${idempotencyKey}`);
  if (cached) {
    return res.status(cached.status).json(cached.body);
  }
}

const result = await processPayment(req.body);
await redis.setex(`idempotency:${idempotencyKey}`, 86400, {
  status: 201,
  body: result
});

Conditional Requests

Use ETags for safe updates:

httpCopy code
# Get resource with ETag
GET /v1/users/123
Response: ETag: "abc123"

# Update only if unchanged
PUT /v1/users/123
If-Match: "abc123"

# 412 Precondition Failed if ETag changed

Caching Strategies

HTTP Caching Headers

httpCopy code
# Public, cacheable for 1 hour
Cache-Control: public, max-age=3600

# Private (user-specific), revalidate
Cache-Control: private, must-revalidate, max-age=0

# No caching
Cache-Control: no-store, no-cache, must-revalidate

ETag Validation

httpCopy code
# Server returns ETag
GET /v1/users/123
Response:
  ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
  Cache-Control: max-age=3600

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

# 304 Not Modified if unchanged (saves bandwidth)
HTTP/1.1 304 Not Modified

Last-Modified

httpCopy code
GET /v1/users/123
Response:
  Last-Modified: Wed, 21 Oct 2025 07:28:00 GMT

# Conditional request
GET /v1/users/123
If-Modified-Since: Wed, 21 Oct 2025 07:28:00 GMT

# 304 Not Modified if not modified

Webhooks

Event Delivery

httpCopy code
POST https://client.com/webhooks/payments
Content-Type: application/json
X-Webhook-Signature: sha256=abc123...
X-Webhook-Id: evt_abc123
X-Webhook-Timestamp: 1640995200

{
  "id": "evt_abc123",
  "type": "payment.succeeded",
  "created": 1640995200,
  "data": {
    "object": {
      "id": "pay_123",
      "amount": 1000,
      "status": "succeeded"
    }
  }
}

Signature Verification

typescriptCopy code
import crypto from 'crypto';

function verifyWebhookSignature(
  payload: string,
  signature: string,
  secret: string
): boolean {
  const expectedSignature = crypto
    .createHmac('sha256', secret)
    .update(payload)
    .digest('hex');
  return crypto.timingSafeEqual(
    Buffer.from(signature),
    Buffer.from(`sha256=${expectedSignature}`)
  );
}

Retry Strategy

• Exponential backoff: 1s, 2s, 4s, 8s, 16s, 32s, 64s
• Timeout: 5-30 seconds per attempt
• Max attempts: 3-7 attempts
• Dead letter queue: Store failed events
• Manual retry: UI for re-sending failed events

API Documentation

OpenAPI/Swagger (REST)

yamlCopy code
openapi: 3.0.0
info:
  title: User API
  version: 1.0.0
paths:
  /users/{id}:
    get:
      summary: Get user by ID
      parameters:
        - name: id
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: Successful response
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'
        '404':
          description: User not found
components:
  schemas:
    User:
      type: object
      required: [id, email]
      properties:
        id:
          type: string
        email:
          type: string
          format: email
        name:
          type: string

GraphQL Schema (Self-Documenting)

GraphQL introspection provides automatic documentation. Use descriptions:

graphqlCopy code
"""
Represents a user account in the system.
Created via the createUser mutation.
"""
type User {
  """Unique identifier for the user"""
  id: ID!

  """Email address, must be unique"""
  email: String!

  """Optional display name"""
  name: String
}

API Documentation Best Practices

1. Interactive examples: Provide working code samples
2. Authentication guide: Step-by-step auth setup
3. Error catalog: Document all error codes with examples
4. Rate limits: Clearly state limits and headers
5. Changelog: Track breaking and non-breaking changes
6. Migration guides: Version upgrade instructions
7. SDKs: Provide client libraries for popular languages

Anti-Patterns

❌ Over-fetching (REST): Returning entire objects when fields are unused ✅ Solution: Support field selection (?fields=id,name,email)

❌ Under-fetching (REST): Requiring multiple requests for related data ✅ Solution: Support expansion (?expand=orders,profile) or use GraphQL

❌ Chatty APIs: Too many round-trips for common operations ✅ Solution: Batch endpoints, compound documents, or GraphQL

❌ Ignoring HTTP semantics: Using GET for mutations, wrong status codes ✅ Solution: Follow HTTP spec, use correct methods and status codes

❌ Exposing internal structure: URLs/schemas mirror database ✅ Solution: Design resource-oriented APIs independent of storage

❌ Missing versioning: Breaking changes without version increments ✅ Solution: Version from day one, never break existing versions

❌ Poor error messages: Generic "An error occurred" ✅ Solution: Specific, actionable error messages with codes

❌ No rate limiting: APIs vulnerable to abuse ✅ Solution: Implement rate limiting from the start

Testing Strategies

Contract Testing

typescriptCopy code
// Pact contract test
import { PactV3 } from '@pact-foundation/pact';

const provider = new PactV3({
  consumer: 'FrontendApp',
  provider: 'UserAPI'
});

it('gets a user by ID', () => {
  provider
    .given('user 123 exists')
    .uponReceiving('a request for user 123')
    .withRequest({
      method: 'GET',
      path: '/users/123'
    })
    .willRespondWith({
      status: 200,
      body: { id: '123', email: 'user@example.com' }
    });
});

Load Testing

javascriptCopy code
// k6 load test
import http from 'k6/http';
import { check } from 'k6';

export const options = {
  stages: [
    { duration: '30s', target: 20 },
    { duration: '1m', target: 20 },
    { duration: '10s', target: 0 }
  ],
  thresholds: {
    http_req_duration: ['p(95)<500'], // 95% under 500ms
    http_req_failed: ['rate<0.01']    // <1% errors
  }
};

export default function () {
  const res = http.get('https://api.example.com/users');
  check(res, {
    'status is 200': (r) => r.status === 200,
    'response time < 500ms': (r) => r.timings.duration < 500
  });
}

Related Skills

• graphql: Deep GraphQL schema design, resolvers, Apollo Server
• typescript: Type-safe API clients and servers
• nodejs-backend: Express/Fastify REST API implementation
• django: Django REST Framework patterns
• fastapi: FastAPI Python REST/GraphQL APIs
• flask: Flask-RESTful patterns

References

• rest-patterns.md: Deep REST coverage (HATEOAS, filtering, field selection)
• graphql-patterns.md: GraphQL subscriptions, relay cursor connections, federation
• grpc-patterns.md: Streaming patterns, interceptors, service mesh integration
• versioning-strategies.md: Detailed versioning approaches and migration patterns
• authentication.md: OAuth flows, JWT best practices, API key rotation, RBAC

Additional Resources

• REST API Design Rulebook - O'Reilly REST guide
• GraphQL Best Practices - Official GraphQL guide
• gRPC Best Practices - Official gRPC guide
• RFC 7807: Problem Details for HTTP APIs - Standard error format
• OpenAPI Specification - REST documentation standard