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api-design-principles
Master REST and GraphQL API design principles to build intuitive,
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# 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}",
"method": "DELETE"
}
}
)
```
## GraphQL Design Patterns
### Pattern 1: Schema Design
```graphql
# schema.graphql
# Clear type definitions
type User {
id: ID!
email: String!
name: String!
createdAt: DateTime!
# Relationships
orders(first: Int = 20, after: String, status: OrderStatus): OrderConnection!
profile: UserProfile
}
type Order {
id: ID!
status: OrderStatus!
total: Money!
items: [OrderItem!]!
createdAt: DateTime!
# Back-reference
user: User!
}
# Pagination pattern (Relay-style)
type OrderConnection {
edges: [OrderEdge!]!
pageInfo: PageInfo!
totalCount: Int!
}
type OrderEdge {
node: Order!
cursor: String!
}
type PageInfo {
hasNextPage: Boolean!
hasPreviousPage: Boolean!
startCursor: String
endCursor: String
}
# Enums for type safety
enum OrderStatus {
PENDING
CONFIRMED
SHIPPED
DELIVERED
CANCELLED
}
# Custom scalars
scalar DateTime
scalar Money
# Query root
type Query {
user(id: ID!): User
users(first: Int = 20, after: String, search: String): UserConnection!
order(id: ID!): Order
}
# Mutation root
type Mutation {
createUser(input: CreateUserInput!): CreateUserPayload!
updateUser(input: UpdateUserInput!): UpdateUserPayload!
deleteUser(id: ID!): DeleteUserPayload!
createOrder(input: CreateOrderInput!): CreateOrderPayload!
}
# Input types for mutations
input CreateUserInput {
email: String!
name: String!
password: String!
}
# Payload types for mutations
type CreateUserPayload {
user: User
errors: [Error!]
}
type Error {
field: String
message: String!
}
```
### Pattern 2: Resolver Design
```python
from typing import Optional, List
from ariadne import QueryType, MutationType, ObjectType
from dataclasses import dataclass
query = QueryType()
mutation = MutationType()
user_type = ObjectType("User")
@query.field("user")
async def resolve_user(obj, info, id: str) -> Optional[dict]:
"""Resolve single user by ID."""
return await fetch_user_by_id(id)
@query.field("users")
async def resolve_users(
obj,
info,
first: int = 20,
after: Optional[str] = None,
search: Optional[str] = None
) -> dict:
"""Resolve paginated user list."""
# Decode cursor
offset = decode_cursor(after) if after else 0
# Fetch users
users = await fetch_users(
limit=first + 1, # Fetch one extra to check hasNextPage
offset=offset,
search=search
)
# Pagination
has_next = len(users) > first
if has_next:
users = users[:first]
edges = [
{
"node": user,
"cursor": encode_cursor(offset + i)
}
for i, user in enumerate(users)
]
return {
"edges": edges,
"pageInfo": {
"hasNextPage": has_next,
"hasPreviousPage": offset > 0,
"startCursor": edges[0]["cursor"] if edges else None,
"endCursor": edges[-1]["cursor"] if edges else None
},
"totalCount": await count_users(search=search)
}
@user_type.field("orders")
async def resolve_user_orders(user: dict, info, first: int = 20) -> dict:
"""Resolve user's orders (N+1 prevention with DataLoader)."""
# Use DataLoader to batch requests
loader = info.context["loaders"]["orders_by_user"]
orders = await loader.load(user["id"])
return paginate_orders(orders, first)
@mutation.field("createUser")
async def resolve_create_user(obj, info, input: dict) -> dict:
"""Create new user."""
try:
# Validate input
validate_user_input(input)
# Create user
user = await create_user(
email=input["email"],
name=input["name"],
password=hash_password(input["password"])
)
return {
"user": user,
"errors": []
}
except ValidationError as e:
return {
"user": None,
"errors": [{"field": e.field, "message": e.message}]
}
```
### Pattern 3: DataLoader (N+1 Problem Prevention)
```python
from aiodataloader import DataLoader
from typing import List, Optional
class UserLoader(DataLoader):
"""Batch load users by ID."""
async def batch_load_fn(self, user_ids: List[str]) -> List[Optional[dict]]:
"""Load multiple users in single query."""
users = await fetch_users_by_ids(user_ids)
# Map results back to input order
user_map = {user["id"]: user for user in users}
return [user_map.get(user_id) for user_id in user_ids]
class OrdersByUserLoader(DataLoader):
"""Batch load orders by user ID."""
async def batch_load_fn(self, user_ids: List[str]) -> List[List[dict]]:
"""Load orders for multiple users in single query."""
orders = await fetch_orders_by_user_ids(user_ids)
# Group orders by user_id
orders_by_user = {}
for order in orders:
user_id = order["user_id"]
if user_id not in orders_by_user:
orders_by_user[user_id] = []
orders_by_user[user_id].append(order)
# Return in input order
return [orders_by_user.get(user_id, []) for user_id in user_ids]
# Context setup
def create_context():
return {
"loaders": {
"user": UserLoader(),
"orders_by_user": OrdersByUserLoader()
}
}
```
## Best Practices
### REST APIs
1. **Consistent Naming**: Use plural nouns for collections (`/users`, not `/user`)
2. **Stateless**: Each request contains all necessary information
3. **Use HTTP Status Codes Correctly**: 2xx success, 4xx client errors, 5xx server errors
4. **Version Your API**: Plan for breaking changes from day one
5. **Pagination**: Always paginate large collections
6. **Rate Limiting**: Protect your API with rate limits
7. **Documentation**: Use OpenAPI/Swagger for interactive docs
### GraphQL APIs
1. **Schema First**: Design schema before writing resolvers
2. **Avoid N+1**: Use DataLoaders for efficient data fetching
3. **Input Validation**: Validate at schema and resolver levels
4. **Error Handling**: Return structured errors in mutation payloads
5. **Pagination**: Use cursor-based pagination (Relay spec)
6. **Deprecation**: Use `@deprecated` directive for gradual migration
7. **Monitoring**: Track query complexity and execution time
## Common Pitfalls
- **Over-fetching/Under-fetching (REST)**: Fixed in GraphQL but requires DataLoaders
- **Breaking Changes**: Version APIs or use deprecation strategies
- **Inconsistent Error Formats**: Standardize error responses
- **Missing Rate Limits**: APIs without limits are vulnerable to abuse
- **Poor Documentation**: Undocumented APIs frustrate developers
- **Ignoring HTTP Semantics**: POST for idempotent operations breaks expectations
- **Tight Coupling**: API structure shouldn't mirror database schema
## Resources
- **references/rest-best-practices.md**: Comprehensive REST API design guide
- **references/graphql-schema-design.md**: GraphQL schema patterns and anti-patterns
- **references/api-versioning-strategies.md**: Versioning approaches and migration paths
- **assets/rest-api-template.py**: FastAPI REST API template
- **assets/graphql-schema-template.graphql**: Complete GraphQL schema example
- **assets/api-design-checklist.md**: Pre-implementation review checklist
- **scripts/openapi-generator.py**: Generate OpenAPI specs from code
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