# API Design Guidelines and Standards

## Overview

This document establishes comprehensive guidelines for designing, implementing, and documenting APIs within our organization. These standards ensure consistency, usability, and maintainability across all API implementations.

## RESTful API Design Principles

### Resource-Oriented Design

**Resource Identification:**
- Use nouns for resource names, not verbs
- Use plural nouns for collections (`/users`, `/orders`)
- Use hierarchical structure for relationships (`/users/{id}/orders`)
- Avoid deep nesting (maximum 3 levels)

**URL Structure:**
```
# Good Examples
GET /api/v1/users
GET /api/v1/users/123
GET /api/v1/users/123/orders
POST /api/v1/users
PUT /api/v1/users/123
DELETE /api/v1/users/123

# Bad Examples
GET /api/v1/getUsers
POST /api/v1/createUser
GET /api/v1/users/123/orders/456/items/789/details
```

### HTTP Methods and Status Codes

**HTTP Method Usage:**
- **GET**: Retrieve resources (idempotent, safe)
- **POST**: Create new resources or non-idempotent operations
- **PUT**: Update entire resource (idempotent)
- **PATCH**: Partial resource updates
- **DELETE**: Remove resources (idempotent)

**Standard Status Codes:**
```
Success Responses:
200 OK - Successful GET, PUT, PATCH
201 Created - Successful POST
204 No Content - Successful DELETE

Client Error Responses:
400 Bad Request - Invalid request syntax
401 Unauthorized - Authentication required
403 Forbidden - Insufficient permissions
404 Not Found - Resource doesn't exist
409 Conflict - Resource conflict
422 Unprocessable Entity - Validation errors

Server Error Responses:
500 Internal Server Error - Generic server error
502 Bad Gateway - Upstream server error
503 Service Unavailable - Temporary unavailability
```

## API Versioning Strategy

### Versioning Approach

**URL Path Versioning (Recommended):**
```
/api/v1/users
/api/v2/users
```

**Header Versioning (Alternative):**
```
Accept: application/vnd.api+json;version=1
API-Version: 2
```

**Version Lifecycle:**
- Support minimum 2 major versions simultaneously
- 6-month deprecation notice for version retirement
- Clear migration guides between versions
- Backward compatibility within major versions

### Breaking Changes

**Breaking Change Examples:**
- Removing or renaming fields
- Changing field types
- Modifying URL structure
- Changing authentication methods

**Non-Breaking Change Examples:**
- Adding new optional fields
- Adding new endpoints
- Expanding enum values
- Performance improvements

## Request and Response Design

### Request Format

**JSON Request Structure:**
```json
{
  "data": {
    "type": "user",
    "attributes": {
      "name": "John Doe",
      "email": "john@example.com"
    },
    "relationships": {
      "organization": {
        "data": {
          "type": "organization",
          "id": "123"
        }
      }
    }
  }
}
```

**Query Parameters:**
```
# Filtering
GET /api/v1/users?status=active&role=admin

# Sorting
GET /api/v1/users?sort=name,-created_at

# Pagination
GET /api/v1/users?page=2&limit=20

# Field Selection
GET /api/v1/users?fields=id,name,email
```

### Response Format

**Standard Response Structure:**
```json
{
  "data": {
    "type": "user",
    "id": "123",
    "attributes": {
      "name": "John Doe",
      "email": "john@example.com",
      "created_at": "2024-01-15T10:30:00Z"
    },
    "relationships": {
      "organization": {
        "data": {
          "type": "organization",
          "id": "456"
        },
        "links": {
          "related": "/api/v1/organizations/456"
        }
      }
    }
  },
  "meta": {
    "timestamp": "2024-01-15T10:30:00Z",
    "request_id": "req_123456789"
  }
}
```

**Collection Response:**
```json
{
  "data": [
    {
      "type": "user",
      "id": "123",
      "attributes": {...}
    }
  ],
  "meta": {
    "total_count": 150,
    "page": 1,
    "per_page": 20,
    "total_pages": 8
  },
  "links": {
    "self": "/api/v1/users?page=1",
    "next": "/api/v1/users?page=2",
    "last": "/api/v1/users?page=8"
  }
}
```

### Error Response Format

**Standard Error Structure:**
```json
{
  "errors": [
    {
      "id": "error_123",
      "status": "422",
      "code": "VALIDATION_ERROR",
      "title": "Validation Failed",
      "detail": "Email address is required",
      "source": {
        "pointer": "/data/attributes/email"
      },
      "meta": {
        "timestamp": "2024-01-15T10:30:00Z",
        "request_id": "req_123456789"
      }
    }
  ]
}
```

## Authentication and Authorization

### Authentication Methods

**JWT Bearer Tokens (Recommended):**
```
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
```

**API Key Authentication:**
```
X-API-Key: your-api-key-here
```

**OAuth 2.0 Integration:**
- Authorization Code flow for web applications
- Client Credentials flow for service-to-service
- PKCE for mobile and SPA applications

### Authorization Patterns

**Role-Based Access Control (RBAC):**
```json
{
  "user": {
    "id": "123",
    "roles": ["user", "admin"],
    "permissions": [
      "users:read",
      "users:write",
      "orders:read"
    ]
  }
}
```

**Resource-Level Permissions:**
```
GET /api/v1/users/123/orders
# User can only access their own orders
# Admin can access any user's orders
```

## Data Validation and Constraints

### Input Validation

**Validation Rules:**
- Required field validation
- Data type validation
- Format validation (email, phone, etc.)
- Range and length constraints
- Business rule validation

**Validation Response:**
```json
{
  "errors": [
    {
      "status": "422",
      "code": "INVALID_EMAIL",
      "title": "Invalid email format",
      "detail": "Email must be a valid email address",
      "source": {
        "pointer": "/data/attributes/email"
      }
    }
  ]
}
```

### Data Sanitization

**Input Sanitization:**
- HTML entity encoding
- SQL injection prevention
- XSS protection
- Input length limits

## Pagination and Filtering

### Pagination Standards

**Offset-Based Pagination:**
```
GET /api/v1/users?page=2&limit=20
```

**Cursor-Based Pagination (for large datasets):**
```
GET /api/v1/users?cursor=eyJpZCI6MTIzfQ&limit=20
```

**Pagination Metadata:**
```json
{
  "meta": {
    "pagination": {
      "current_page": 2,
      "per_page": 20,
      "total_pages": 10,
      "total_count": 200,
      "has_next": true,
      "has_previous": true
    }
  }
}
```

### Filtering and Searching

**Query Parameter Filtering:**
```
# Simple filtering
GET /api/v1/users?status=active

# Multiple values
GET /api/v1/users?status=active,pending

# Range filtering
GET /api/v1/users?created_after=2024-01-01&created_before=2024-12-31

# Full-text search
GET /api/v1/users?search=john+doe
```

**Advanced Filtering:**
```
# Complex queries using query language
GET /api/v1/users?filter=status eq 'active' and created_at gt '2024-01-01'
```

## Performance and Caching

### Response Optimization

**Field Selection:**
```
GET /api/v1/users?fields=id,name,email
```

**Resource Expansion:**
```
GET /api/v1/users/123?include=organization,roles
```

**Compression:**
- Enable gzip compression for responses
- Use appropriate content encoding headers

### Caching Strategy

**HTTP Caching Headers:**
```
Cache-Control: public, max-age=3600
ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
Last-Modified: Wed, 15 Jan 2024 10:30:00 GMT
```

**Conditional Requests:**
```
# Client sends
If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"

# Server responds with 304 Not Modified if unchanged
```

## Rate Limiting and Throttling

### Rate Limiting Implementation

**Rate Limit Headers:**
```
X-RateLimit-Limit: 1000
X-RateLimit-Remaining: 999
X-RateLimit-Reset: 1609459200
Retry-After: 3600
```

**Rate Limiting Strategies:**
- Fixed window rate limiting
- Sliding window rate limiting
- Token bucket algorithm
- Different limits for different endpoints

### Throttling Policies

**Tier-Based Limits:**
```
Free Tier: 100 requests/hour
Premium Tier: 1000 requests/hour
Enterprise Tier: 10000 requests/hour
```

## API Documentation Standards

### OpenAPI Specification

**Required Documentation Elements:**
- Complete endpoint documentation
- Request/response schemas
- Authentication requirements
- Error response examples
- Rate limiting information

**OpenAPI Example:**
```yaml
openapi: 3.0.3
info:
  title: User Management API
  version: 1.0.0
  description: API for managing users and organizations

paths:
  /users:
    get:
      summary: List users
      parameters:
        - name: page
          in: query
          schema:
            type: integer
            default: 1
      responses:
        '200':
          description: Successful response
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/UserList'
```

### Interactive Documentation

**Required Features:**
- Interactive API explorer
- Code examples in multiple languages
- Authentication testing capability
- Response examples and schemas

**Tools:**
- Swagger UI for OpenAPI specs
- Postman collections
- Insomnia workspaces

## Security Best Practices

### Input Security

**Security Measures:**
- Input validation and sanitization
- SQL injection prevention
- XSS protection
- CSRF protection for state-changing operations

### Transport Security

**HTTPS Requirements:**
- TLS 1.2 minimum (TLS 1.3 preferred)
- Valid SSL certificates
- HSTS headers
- Secure cookie attributes

### API Security Headers

**Required Security Headers:**
```
Strict-Transport-Security: max-age=31536000; includeSubDomains
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
X-XSS-Protection: 1; mode=block
Content-Security-Policy: default-src 'self'
```

## Testing Standards

### API Testing Requirements

**Test Categories:**
- Unit tests for business logic
- Integration tests for API endpoints
- Contract tests for API compatibility
- Performance tests for load handling
- Security tests for vulnerability scanning

**Test Coverage:**
- All endpoints tested
- All error conditions covered
- Authentication and authorization tested
- Input validation tested

### Testing Tools

**Recommended Tools:**
- **Postman/Newman**: API testing and automation
- **Jest/Mocha**: Unit and integration testing
- **Pact**: Contract testing
- **Artillery/k6**: Performance testing

## Monitoring and Observability

### API Metrics

**Required Metrics:**
- Request rate and response time
- Error rates by endpoint
- Authentication success/failure rates
- Rate limiting violations
- Resource utilization

### Logging Standards

**Log Format:**
```json
{
  "timestamp": "2024-01-15T10:30:00Z",
  "level": "INFO",
  "request_id": "req_123456789",
  "method": "GET",
  "path": "/api/v1/users/123",
  "status_code": 200,
  "response_time_ms": 45,
  "user_id": "user_456",
  "ip_address": "192.168.1.1"
}
```

### Health Checks

**Health Endpoint:**
```
GET /health
{
  "status": "healthy",
  "timestamp": "2024-01-15T10:30:00Z",
  "version": "1.2.3",
  "dependencies": {
    "database": "healthy",
    "cache": "healthy",
    "external_api": "degraded"
  }
}
```

## Deprecation and Migration

### Deprecation Process

**Deprecation Timeline:**
1. **Announcement**: 6 months before removal
2. **Warning Headers**: Add deprecation warnings
3. **Documentation**: Update docs with migration guide
4. **Support**: Provide migration assistance
5. **Removal**: Remove deprecated endpoints

**Deprecation Headers:**
```
Deprecation: true
Sunset: Wed, 15 Jul 2024 10:30:00 GMT
Link: </api/v2/users>; rel="successor-version"
```

### Migration Support

**Migration Tools:**
- Automated migration scripts
- Compatibility layers
- Side-by-side version support
- Data migration utilities

## Governance and Compliance

### API Governance

**Review Process:**
- Design review before implementation
- Security review for all APIs
- Performance review for critical APIs
- Documentation review before release

**Compliance Requirements:**
- Data privacy regulations (GDPR, CCPA)
- Industry standards (PCI DSS, HIPAA)
- Internal security policies
- Audit trail requirements

### Change Management

**Change Categories:**
- **Breaking Changes**: Require major version bump
- **Non-Breaking Changes**: Can be added to existing version
- **Bug Fixes**: Patch version updates
- **Security Updates**: Immediate deployment required
