API Design Principles

API design is grounded in clarity, consistency, and usability. The following principles guide our decisions:

RESTful Design

1. Client-Server Architecture

Separation of Concerns: Clear separation between client and server
Stateless Communication: Each request contains all necessary information
Cacheable: Responses define cacheability
Uniform Interface: Standardized way of interacting with resources
Layered System: Client can't tell if connected to end server or intermediary

2. Resource Identification

Use nouns (not verbs) to identify resources
Use plural nouns for collections (/users not /user)
Use forward slashes (/) for hierarchy (/users/123/orders)
Use hyphens (-) for multi-word path segments (/product-categories)
Use lowercase letters in paths and headers
Never use file extensions in URIs

Resource-Oriented

1. Resource Naming

Pluralization: Use plural for collections (/users)
Consistency: Stick to either plural or singular (prefer plural)
Concreteness: Use concrete rather than abstract names
Avoid Verbs: Use HTTP methods to indicate actions

2. Resource Relationships

Hierarchy: Parent/child relationships via path segments (/users/123/orders)
References: Use IDs to reference related resources
Embedding: Include related resources when it improves efficiency
Pagination: Always paginate collections

Statelessness

1. Stateless Servers

No client context stored on server between requests
Each request contains all necessary authentication/authorization
Session state held entirely on the client
Improves reliability and scalability

2. Authentication & Authorization

Use stateless authentication (JWT, OAuth2)
Include auth tokens in headers
Implement proper token expiration and refresh
Follow principle of least privilege

Consistency

1. Naming Conventions

camelCase for JSON properties
kebab-case for URLs and headers
UPPER_SNAKE_CASE for constants and enums
PascalCase for type names in documentation

2. Response Format

{
  "data": {
    "id": "123",
    "type": "users",
    "attributes": {
      "name": "John Doe",
      "email": "john@example.com"
    },
    "relationships": {
      "organization": {
        "data": { "id": "456", "type": "organizations" }
      }
    }
  },
  "included": [
    {
      "id": "456",
      "type": "organizations",
      "attributes": {
        "name": "Acme Inc."
      }
    }
  ],
  "meta": {
    "total": 100,
    "page": 1,
    "limit": 10
  },
  "links": {
    "self": "/users?page=1",
    "next": "/users?page=2",
    "prev": null,
    "first": "/users?page=1",
    "last": "/users?page=10"
  }
}

Idempotency

1. Safe Methods

GET, HEAD, OPTIONS, and TRACE are naturally idempotent
Multiple identical requests have the same effect as a single request

2. Making Mutations Idempotent

PUT: Replace entire resource (idempotent)
PATCH: Partial updates (make idempotent with versioning)
DELETE: Idempotent by nature
POST: Not idempotent (use idempotency keys)

3. Idempotency Keys

POST /payments
Idempotency-Key: 550e8400-e29b-41d4-a716-446655440000
Content-Type: application/json

{
  "amount": 1000,
  "currency": "USD",
  "description": "Subscription payment"
}

Versioning

1. Versioning Strategies

URI Path: /v1/users
Header: Accept: application/vnd.dexaminds.v1+json
Query Param: /users?version=1 (not recommended)
Media Type: application/vnd.dexaminds+json;version=1

2. Versioning Best Practices

Version early, version often
Support multiple versions in parallel
Provide clear deprecation policies
Document version changes thoroughly

Error Handling

1. Error Response Format

{
  "error": {
    "code": "validation_error",
    "message": "One or more validation errors occurred",
    "target": "user_creation",
    "details": [
      {
        "code": "required",
        "target": "email",
        "message": "Email is required"
      },
      {
        "code": "min_length",
        "target": "password",
        "message": "Password must be at least 8 characters"
      }
    ],
    "innerError": {
      "code": "validation_failed",
      "requestId": "a1b2c3d4",
      "timestamp": "2025-06-30T06:27:45Z"
    }
  }
}

2. HTTP Status Codes

2xx: Success
4xx: Client errors
5xx: Server errors
Use specific status codes (e.g., 429 for rate limiting)

Performance

1. Caching

Client Caching: Cache-Control, ETag, Last-Modified
Server Caching: Redis, Memcached
CDN Caching: For static assets

2. Pagination

Offset-based: ?page=2&limit=25
Cursor-based: ?after=cursor&limit=25
Keyset Pagination: ?since_id=123&limit=25

3. Filtering & Sorting

Filtering: ?status=active&role=admin
Sorting: ?sort=-created_at,name
Field Selection: ?fields=id,name,email

Security

1. Authentication

OAuth 2.0 with OpenID Connect
JWT for stateless authentication
API keys for service-to-service
MFA for sensitive operations

2. Authorization

Role-Based Access Control (RBAC)
Attribute-Based Access Control (ABAC)
Fine-grained permissions
Policy-based authorization

3. Input Validation

Validate all input data
Use allow-listing
Sanitize output
Prevent injection attacks

Documentation

1. API Reference

Endpoint descriptions
Request/response examples
Error codes
Authentication requirements

2. Interactive Documentation

Swagger/OpenAPI
Postman collections
API explorers

3. Guides & Tutorials

Getting started
Authentication
Common use cases
Migration guides

Evolution

1. Backward Compatibility

Add new fields as optional
Don't remove or rename fields
Use feature flags
Version your API

2. Deprecation Policy

Announce deprecations in advance
Provide migration guides
Maintain deprecated versions for a period
Monitor usage of deprecated endpoints

Best Practices

1. Design First

Design before implementation
Use API-first approach
Get feedback early
Iterate on design

2. Keep It Simple (KISS)

Favor simplicity over complexity
Follow the principle of least surprise
Avoid over-engineering
Make common tasks easy

3. Don't Repeat Yourself (DRY)

Reuse schemas and components
Centralize common functionality
Avoid code duplication
Use references in OpenAPI/Swagger

4. You Aren't Gonna Need It (YAGNI)

Implement features when needed
Avoid speculative generality
Keep the API surface small
Focus on current requirements

5. Principle of Least Privilege

Grant minimum necessary permissions
Use scopes for OAuth
Implement proper access controls
Audit permissions regularly

6. Fail Fast, Fail Loudly

Validate early
Provide clear error messages
Don't hide errors
Log failures appropriately

Anti-Patterns to Avoid

1. Over-fetching/Under-fetching

Problem: Getting too much or too little data
Solution: Use field selection and sparse fieldsets

2. Chatty APIs

Problem: Too many round-trips
Solution: Batch requests or use GraphQL

3. Ignoring Caching

Problem: Unnecessary load on servers
Solution: Implement proper caching headers

4. Ignoring Rate Limiting

Problem: API abuse and DDoS attacks
Solution: Implement rate limiting and quotas

5. Poor Error Handling

Problem: Unhelpful error messages
Solution: Standardized error responses with codes

API Maturity Model

Level 0: HTTP as a Tunnel

Single POST endpoint
All actions as parameters
No REST principles

Level 1: Resources

Multiple URIs
Resources as nouns
Still mostly RPC-style

Level 2: HTTP Verbs

Proper use of HTTP methods
Status codes
Still some RPC endpoints

Level 3: Hypermedia Controls

HATEOAS
Discoverable APIs
Self-documenting

Measuring API Quality

1. Performance Metrics

Response time (p50, p95, p99)
Throughput (requests/second)
Error rate
Uptime/availability

2. Developer Experience

Time to first successful call
Documentation quality
SDK availability
Community support

3. Business Metrics

API adoption rate
Active developers
Revenue impact
Cost per API call

Conclusion

Designing great APIs requires careful consideration of many factors. By following these principles, you can create APIs that are:

Intuitive: Easy to understand and use
Efficient: Fast and resource-friendly
Reliable: Consistent and available
Secure: Protected against threats
Maintainable: Easy to evolve over time

Remember that API design is an iterative process. Continuously gather feedback from your consumers and be prepared to make improvements based on real-world usage patterns.