api-design-principles

# API Design Principles

Master REST and GraphQL API design principles to build intuitive, scalable, and maintainable APIs that delight developers and stand the test of time.

## When to Use This Skill

- Designing new REST or GraphQL APIs
- Refactoring existing APIs for better usability
- Establishing API design standards for your team
- Reviewing API specifications before implementation
- Migrating between API paradigms (REST to GraphQL, etc.)
- Creating developer-friendly API documentation
- Optimizing APIs for specific use cases (mobile, third-party integrations)

## Core Concepts

### 1. RESTful Design Principles

**Resource-Oriented Architecture**
- Resources are nouns (users, orders, products), not verbs
- Use HTTP methods for actions (GET, POST, PUT, PATCH, DELETE)
- URLs represent resource hierarchies
- Consistent naming conventions

**HTTP Methods Semantics:**
- `GET`: Retrieve resources (idempotent, safe)
- `POST`: Create new resources
- `PUT`: Replace entire resource (idempotent)
- `PATCH`: Partial resource updates
- `DELETE`: Remove resources (idempotent)

### 2. GraphQL Design Principles

**Schema-First Development**
- Types define your domain model
- Queries for reading data
- Mutations for modifying data
- Subscriptions for real-time updates

**Query Structure:**
- Clients request exactly what they need
- Single endpoint, multiple operations
- Strongly typed schema
- Introspection built-in

### 3. API Versioning Strategies

**URL Versioning:**
```
/api/v1/users
/api/v2/users
```

**Header Versioning:**
```
Accept: application/vnd.api+json; version=1
```

**Query Parameter Versioning:**
```
/api/users?version=1
```

## REST API Design Patterns

### Pattern 1: Resource Collection Design

```python
# Good: Resource-oriented endpoints
GET    /api/users              # List users (with pagination)
POST   /api/users              # Create user
GET    /api/users/{id}         # Get specific user
PUT    /api/users/{id}         # Replace user
PATCH  /api/users/{id}         # Update user fields
DELETE /api/users/{id}         # Delete user

# Nested resources
GET    /api/users/{id}/orders  # Get user's orders
POST   /api/users/{id}/orders  # Create order for user

# Bad: Action-oriented endpoints (avoid)
POST   /api/createUser
POST   /api/getUserById
POST   /api/deleteUser
```

### Pattern 2: Pagination and Filtering

```python
from typing import List, Optional
from pydantic import BaseModel, Field

class PaginationParams(BaseModel):
    page: int = Field(1, ge=1, description="Page number")
    page_size: int = Field(20, ge=1, le=100, description="Items per page")

class FilterParams(BaseModel):
    status: Optional[str] = None
    created_after: Optional[str] = None
    search: Optional[str] = None

class PaginatedResponse(BaseModel):
    items: List[dict]
    total: int
    page: int
    page_size: int
    pages: int

    @property
    def has_next(self) -> bool:
        return self.page < self.pages

    @property
    def has_prev(self) -> bool:
        return self.page > 1

# FastAPI endpoint example
from fastapi import FastAPI, Query, Depends

app = FastAPI()

@app.get("/api/users", response_model=PaginatedResponse)
async def list_users(
    page: int = Query(1, ge=1),
    page_size: int = Query(20, ge=1, le=100),
    status: Optional[str] = Query(None),
    search: Optional[str] = Query(None)
):
    # Apply filters
    query = build_query(status=status, search=search)

    # Count total
    total = await count_users(query)

    # Fetch page
    offset = (page - 1) * page_size
    users = await fetch_users(query, limit=page_size, offset=offset)

    return PaginatedResponse(
        items=users,
        total=total,
        page=page,
        page_size=page_size,
        pages=(total + page_size - 1) // page_size
    )
```

### Pattern 3: Error Handling and Status Codes

```python
from fastapi import HTTPException, status
from pydantic import BaseModel

class ErrorResponse(BaseModel):
    error: str
    message: str
    details: Optional[dict] = None
    timestamp: str
    path: str

class ValidationErrorDetail(BaseModel):
    field: str
    message: str
    value: Any

# Consistent error responses
STATUS_CODES = {
    "success": 200,
    "created": 201,
    "no_content": 204,
    "bad_request": 400,
    "unauthorized": 401,
    "forbidden": 403,
    "not_found": 404,
    "conflict": 409,
    "unprocessable": 422,
    "internal_error": 500
}

def raise_not_found(resource: str, id: str):
    raise HTTPException(
        status_code=status.HTTP_404_NOT_FOUND,
        detail={
            "error": "NotFound",
            "message": f"{resource} not found",
            "details": {"id": id}
        }
    )

def raise_validation_error(errors: List[ValidationErrorDetail]):
    raise HTTPException(
        status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
        detail={
            "error": "ValidationError",
            "message": "Request validation failed",
            "details": {"errors": [e.dict() for e in errors]}
        }
    )

# Example usage
@app.get("/api/users/{user_id}")
async def get_user(user_id: str):
    user = await fetch_user(user_id)
    if not user:
        raise_not_found("User", user_id)
    return user
```

### Pattern 4: HATEOAS (Hypermedia as the Engine of Application State)

```python
class UserResponse(BaseModel):
    id: str
    name: str
    email: str
    _links: dict

    @classmethod
    def from_user(cls, user: User, base_url: str):
        return cls(
            id=user.id,
            name=user.name,
            email=user.email,
            _links={
                "self": {"href": f"{base_url}/api/users/{user.id}"},
                "orders": {"href": f"{base_url}/api/users/{user.id}/orders"},
                "update": {
                    "href": f"{base_url}/api/users/{user.id}",
                    "method": "PATCH"
                },
                "delete": {
                    "href": f"{base_url}/api/users/{user.id}",