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skills/api-design-principles/SKILL.md

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+---
+name: api-design-principles
+description: Master REST and GraphQL API design principles to build intuitive, scalable, and maintainable APIs that delight developers. Use when designing new APIs, reviewing API specifications, or establishing API design standards.
+---
+
+# 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

skills/api-design-principles/assets/api-design-checklist.md

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+# API Design Checklist
+
+## Pre-Implementation Review
+
+### Resource Design
+- [ ] Resources are nouns, not verbs
+- [ ] Plural names for collections
+- [ ] Consistent naming across all endpoints
+- [ ] Clear resource hierarchy (avoid deep nesting >2 levels)
+- [ ] All CRUD operations properly mapped to HTTP methods
+
+### HTTP Methods
+- [ ] GET for retrieval (safe, idempotent)
+- [ ] POST for creation
+- [ ] PUT for full replacement (idempotent)
+- [ ] PATCH for partial updates
+- [ ] DELETE for removal (idempotent)
+
+### Status Codes
+- [ ] 200 OK for successful GET/PATCH/PUT
+- [ ] 201 Created for POST
+- [ ] 204 No Content for DELETE
+- [ ] 400 Bad Request for malformed requests
+- [ ] 401 Unauthorized for missing auth
+- [ ] 403 Forbidden for insufficient permissions
+- [ ] 404 Not Found for missing resources
+- [ ] 422 Unprocessable Entity for validation errors
+- [ ] 429 Too Many Requests for rate limiting
+- [ ] 500 Internal Server Error for server issues
+
+### Pagination
+- [ ] All collection endpoints paginated
+- [ ] Default page size defined (e.g., 20)
+- [ ] Maximum page size enforced (e.g., 100)
+- [ ] Pagination metadata included (total, pages, etc.)
+- [ ] Cursor-based or offset-based pattern chosen
+
+### Filtering & Sorting
+- [ ] Query parameters for filtering
+- [ ] Sort parameter supported
+- [ ] Search parameter for full-text search
+- [ ] Field selection supported (sparse fieldsets)
+
+### Versioning
+- [ ] Versioning strategy defined (URL/header/query)
+- [ ] Version included in all endpoints
+- [ ] Deprecation policy documented
+
+### Error Handling
+- [ ] Consistent error response format
+- [ ] Detailed error messages
+- [ ] Field-level validation errors
+- [ ] Error codes for client handling
+- [ ] Timestamps in error responses
+
+### Authentication & Authorization
+- [ ] Authentication method defined (Bearer token, API key)
+- [ ] Authorization checks on all endpoints
+- [ ] 401 vs 403 used correctly
+- [ ] Token expiration handled
+
+### Rate Limiting
+- [ ] Rate limits defined per endpoint/user
+- [ ] Rate limit headers included
+- [ ] 429 status code for exceeded limits
+- [ ] Retry-After header provided
+
+### Documentation
+- [ ] OpenAPI/Swagger spec generated
+- [ ] All endpoints documented
+- [ ] Request/response examples provided
+- [ ] Error responses documented
+- [ ] Authentication flow documented
+
+### Testing
+- [ ] Unit tests for business logic
+- [ ] Integration tests for endpoints
+- [ ] Error scenarios tested
+- [ ] Edge cases covered
+- [ ] Performance tests for heavy endpoints
+
+### Security
+- [ ] Input validation on all fields
+- [ ] SQL injection prevention
+- [ ] XSS prevention
+- [ ] CORS configured correctly
+- [ ] HTTPS enforced
+- [ ] Sensitive data not in URLs
+- [ ] No secrets in responses
+
+### Performance
+- [ ] Database queries optimized
+- [ ] N+1 queries prevented
+- [ ] Caching strategy defined
+- [ ] Cache headers set appropriately
+- [ ] Large responses paginated
+
+### Monitoring
+- [ ] Logging implemented
+- [ ] Error tracking configured
+- [ ] Performance metrics collected
+- [ ] Health check endpoint available
+- [ ] Alerts configured for errors
+
+## GraphQL-Specific Checks
+
+### Schema Design
+- [ ] Schema-first approach used
+- [ ] Types properly defined
+- [ ] Non-null vs nullable decided
+- [ ] Interfaces/unions used appropriately
+- [ ] Custom scalars defined
+
+### Queries
+- [ ] Query depth limiting
+- [ ] Query complexity analysis
+- [ ] DataLoaders prevent N+1
+- [ ] Pagination pattern chosen (Relay/offset)
+
+### Mutations
+- [ ] Input types defined
+- [ ] Payload types with errors
+- [ ] Optimistic response support
+- [ ] Idempotency considered
+
+### Performance
+- [ ] DataLoader for all relationships
+- [ ] Query batching enabled
+- [ ] Persisted queries considered
+- [ ] Response caching implemented
+
+### Documentation
+- [ ] All fields documented
+- [ ] Deprecations marked
+- [ ] Examples provided
+- [ ] Schema introspection enabled

skills/api-design-principles/assets/rest-api-template.py

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+"""
+Production-ready REST API template using FastAPI.
+Includes pagination, filtering, error handling, and best practices.
+"""
+
+from fastapi import FastAPI, HTTPException, Query, Path, Depends, status
+from fastapi.responses import JSONResponse
+from pydantic import BaseModel, Field, EmailStr
+from typing import Optional, List, Any
+from datetime import datetime
+from enum import Enum
+
+app = FastAPI(
+    title="API Template",
+    version="1.0.0",
+    docs_url="/api/docs"
+)
+
+# Models
+class UserStatus(str, Enum):
+    ACTIVE = "active"
+    INACTIVE = "inactive"
+    SUSPENDED = "suspended"
+
+class UserBase(BaseModel):
+    email: EmailStr
+    name: str = Field(..., min_length=1, max_length=100)
+    status: UserStatus = UserStatus.ACTIVE
+
+class UserCreate(UserBase):
+    password: str = Field(..., min_length=8)
+
+class UserUpdate(BaseModel):
+    email: Optional[EmailStr] = None
+    name: Optional[str] = Field(None, min_length=1, max_length=100)
+    status: Optional[UserStatus] = None
+
+class User(UserBase):
+    id: str
+    created_at: datetime
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True
+
+# Pagination
+class PaginationParams(BaseModel):
+    page: int = Field(1, ge=1)
+    page_size: int = Field(20, ge=1, le=100)
+
+class PaginatedResponse(BaseModel):
+    items: List[Any]
+    total: int
+    page: int
+    page_size: int
+    pages: int
+
+# Error handling
+class ErrorDetail(BaseModel):
+    field: Optional[str] = None
+    message: str
+    code: str
+
+class ErrorResponse(BaseModel):
+    error: str
+    message: str
+    details: Optional[List[ErrorDetail]] = None
+
+@app.exception_handler(HTTPException)
+async def http_exception_handler(request, exc):
+    return JSONResponse(
+        status_code=exc.status_code,
+        content=ErrorResponse(
+            error=exc.__class__.__name__,
+            message=exc.detail if isinstance(exc.detail, str) else exc.detail.get("message", "Error"),
+            details=exc.detail.get("details") if isinstance(exc.detail, dict) else None
+        ).dict()
+    )
+
+# Endpoints
+@app.get("/api/users", response_model=PaginatedResponse, tags=["Users"])
+async def list_users(
+    page: int = Query(1, ge=1),
+    page_size: int = Query(20, ge=1, le=100),
+    status: Optional[UserStatus] = Query(None),
+    search: Optional[str] = Query(None)
+):
+    """List users with pagination and filtering."""
+    # Mock implementation
+    total = 100
+    items = [
+        User(
+            id=str(i),
+            email=f"user{i}@example.com",
+            name=f"User {i}",
+            status=UserStatus.ACTIVE,
+            created_at=datetime.now(),
+            updated_at=datetime.now()
+        ).dict()
+        for i in range((page-1)*page_size, min(page*page_size, total))
+    ]
+
+    return PaginatedResponse(
+        items=items,
+        total=total,
+        page=page,
+        page_size=page_size,
+        pages=(total + page_size - 1) // page_size
+    )
+
+@app.post("/api/users", response_model=User, status_code=status.HTTP_201_CREATED, tags=["Users"])
+async def create_user(user: UserCreate):
+    """Create a new user."""
+    # Mock implementation
+    return User(
+        id="123",
+        email=user.email,
+        name=user.name,
+        status=user.status,
+        created_at=datetime.now(),
+        updated_at=datetime.now()
+    )
+
+@app.get("/api/users/{user_id}", response_model=User, tags=["Users"])
+async def get_user(user_id: str = Path(..., description="User ID")):
+    """Get user by ID."""
+    # Mock: Check if exists
+    if user_id == "999":
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail={"message": "User not found", "details": {"id": user_id}}
+        )
+
+    return User(
+        id=user_id,
+        email="user@example.com",
+        name="User Name",
+        status=UserStatus.ACTIVE,
+        created_at=datetime.now(),
+        updated_at=datetime.now()
+    )
+
+@app.patch("/api/users/{user_id}", response_model=User, tags=["Users"])
+async def update_user(user_id: str, update: UserUpdate):
+    """Partially update user."""
+    # Validate user exists
+    existing = await get_user(user_id)
+
+    # Apply updates
+    update_data = update.dict(exclude_unset=True)
+    for field, value in update_data.items():
+        setattr(existing, field, value)
+
+    existing.updated_at = datetime.now()
+    return existing
+
+@app.delete("/api/users/{user_id}", status_code=status.HTTP_204_NO_CONTENT, tags=["Users"])
+async def delete_user(user_id: str):
+    """Delete user."""
+    await get_user(user_id)  # Verify exists
+    return None
+
+if __name__ == "__main__":
+    import uvicorn
+    uvicorn.run(app, host="0.0.0.0", port=8000)

skills/api-design-principles/references/graphql-schema-design.md

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+# GraphQL Schema Design Patterns
+
+## Schema Organization
+
+### Modular Schema Structure
+```graphql
+# user.graphql
+type User {
+  id: ID!
+  email: String!
+  name: String!
+  posts: [Post!]!
+}
+
+extend type Query {
+  user(id: ID!): User
+  users(first: Int, after: String): UserConnection!
+}
+
+extend type Mutation {
+  createUser(input: CreateUserInput!): CreateUserPayload!
+}
+
+# post.graphql
+type Post {
+  id: ID!
+  title: String!
+  content: String!
+  author: User!
+}
+
+extend type Query {
+  post(id: ID!): Post
+}
+```
+
+## Type Design Patterns
+
+### 1. Non-Null Types
+```graphql
+type User {
+  id: ID!              # Always required
+  email: String!       # Required
+  phone: String        # Optional (nullable)
+  posts: [Post!]!      # Non-null array of non-null posts
+  tags: [String!]      # Nullable array of non-null strings
+}
+```
+
+### 2. Interfaces for Polymorphism
+```graphql
+interface Node {
+  id: ID!
+  createdAt: DateTime!
+}
+
+type User implements Node {
+  id: ID!
+  createdAt: DateTime!
+  email: String!
+}
+
+type Post implements Node {
+  id: ID!
+  createdAt: DateTime!
+  title: String!
+}
+
+type Query {
+  node(id: ID!): Node
+}
+```
+
+### 3. Unions for Heterogeneous Results
+```graphql
+union SearchResult = User | Post | Comment
+
+type Query {
+  search(query: String!): [SearchResult!]!
+}
+
+# Query example
+{
+  search(query: "graphql") {
+    ... on User {
+      name
+      email
+    }
+    ... on Post {
+      title
+      content
+    }
+    ... on Comment {
+      text
+      author { name }
+    }
+  }
+}
+```
+
+### 4. Input Types
+```graphql
+input CreateUserInput {
+  email: String!
+  name: String!
+  password: String!
+  profileInput: ProfileInput
+}
+
+input ProfileInput {
+  bio: String
+  avatar: String
+  website: String
+}
+
+input UpdateUserInput {
+  id: ID!
+  email: String
+  name: String
+  profileInput: ProfileInput
+}
+```
+
+## Pagination Patterns
+
+### Relay Cursor Pagination (Recommended)
+```graphql
+type UserConnection {
+  edges: [UserEdge!]!
+  pageInfo: PageInfo!
+  totalCount: Int!
+}
+
+type UserEdge {
+  node: User!
+  cursor: String!
+}
+
+type PageInfo {
+  hasNextPage: Boolean!
+  hasPreviousPage: Boolean!
+  startCursor: String
+  endCursor: String
+}
+
+type Query {
+  users(
+    first: Int
+    after: String
+    last: Int
+    before: String
+  ): UserConnection!
+}
+
+# Usage
+{
+  users(first: 10, after: "cursor123") {
+    edges {
+      cursor
+      node {
+        id
+        name
+      }
+    }
+    pageInfo {
+      hasNextPage
+      endCursor
+    }
+  }
+}
+```
+
+### Offset Pagination (Simpler)
+```graphql
+type UserList {
+  items: [User!]!
+  total: Int!
+  page: Int!
+  pageSize: Int!
+}
+
+type Query {
+  users(page: Int = 1, pageSize: Int = 20): UserList!
+}
+```
+
+## Mutation Design Patterns
+
+### 1. Input/Payload Pattern
+```graphql
+input CreatePostInput {
+  title: String!
+  content: String!
+  tags: [String!]
+}
+
+type CreatePostPayload {
+  post: Post
+  errors: [Error!]
+  success: Boolean!
+}
+
+type Error {
+  field: String
+  message: String!
+  code: String!
+}
+
+type Mutation {
+  createPost(input: CreatePostInput!): CreatePostPayload!
+}
+```
+
+### 2. Optimistic Response Support
+```graphql
+type UpdateUserPayload {
+  user: User
+  clientMutationId: String
+  errors: [Error!]
+}
+
+input UpdateUserInput {
+  id: ID!
+  name: String
+  clientMutationId: String
+}
+
+type Mutation {
+  updateUser(input: UpdateUserInput!): UpdateUserPayload!
+}
+```
+
+### 3. Batch Mutations
+```graphql
+input BatchCreateUserInput {
+  users: [CreateUserInput!]!
+}
+
+type BatchCreateUserPayload {
+  results: [CreateUserResult!]!
+  successCount: Int!
+  errorCount: Int!
+}
+
+type CreateUserResult {
+  user: User
+  errors: [Error!]
+  index: Int!
+}
+
+type Mutation {
+  batchCreateUsers(input: BatchCreateUserInput!): BatchCreateUserPayload!
+}
+```
+
+## Field Design
+
+### Arguments and Filtering
+```graphql
+type Query {
+  posts(
+    # Pagination
+    first: Int = 20
+    after: String
+
+    # Filtering
+    status: PostStatus
+    authorId: ID
+    tag: String
+
+    # Sorting
+    orderBy: PostOrderBy = CREATED_AT
+    orderDirection: OrderDirection = DESC
+
+    # Searching
+    search: String
+  ): PostConnection!
+}
+
+enum PostStatus {
+  DRAFT
+  PUBLISHED
+  ARCHIVED
+}
+
+enum PostOrderBy {
+  CREATED_AT
+  UPDATED_AT
+  TITLE
+}
+
+enum OrderDirection {
+  ASC
+  DESC
+}
+```
+
+### Computed Fields
+```graphql
+type User {
+  firstName: String!
+  lastName: String!
+  fullName: String!  # Computed in resolver
+
+  posts: [Post!]!
+  postCount: Int!    # Computed, doesn't load all posts
+}
+
+type Post {
+  likeCount: Int!
+  commentCount: Int!
+  isLikedByViewer: Boolean!  # Context-dependent
+}
+```
+
+## Subscriptions
+
+```graphql
+type Subscription {
+  postAdded: Post!
+
+  postUpdated(postId: ID!): Post!
+
+  userStatusChanged(userId: ID!): UserStatus!
+}
+
+type UserStatus {
+  userId: ID!
+  online: Boolean!
+  lastSeen: DateTime!
+}
+
+# Client usage
+subscription {
+  postAdded {
+    id
+    title
+    author {
+      name
+    }
+  }
+}
+```
+
+## Custom Scalars
+
+```graphql
+scalar DateTime
+scalar Email
+scalar URL
+scalar JSON
+scalar Money
+
+type User {
+  email: Email!
+  website: URL
+  createdAt: DateTime!
+  metadata: JSON
+}
+
+type Product {
+  price: Money!
+}
+```
+
+## Directives
+
+### Built-in Directives
+```graphql
+type User {
+  name: String!
+  email: String! @deprecated(reason: "Use emails field instead")
+  emails: [String!]!
+
+  # Conditional inclusion
+  privateData: PrivateData @include(if: $isOwner)
+}
+
+# Query
+query GetUser($isOwner: Boolean!) {
+  user(id: "123") {
+    name
+    privateData @include(if: $isOwner) {
+      ssn
+    }
+  }
+}
+```
+
+### Custom Directives
+```graphql
+directive @auth(requires: Role = USER) on FIELD_DEFINITION
+
+enum Role {
+  USER
+  ADMIN
+  MODERATOR
+}
+
+type Mutation {
+  deleteUser(id: ID!): Boolean! @auth(requires: ADMIN)
+  updateProfile(input: ProfileInput!): User! @auth
+}
+```
+
+## Error Handling
+
+### Union Error Pattern
+```graphql
+type User {
+  id: ID!
+  email: String!
+}
+
+type ValidationError {
+  field: String!
+  message: String!
+}
+
+type NotFoundError {
+  message: String!
+  resourceType: String!
+  resourceId: ID!
+}
+
+type AuthorizationError {
+  message: String!
+}
+
+union UserResult = User | ValidationError | NotFoundError | AuthorizationError
+
+type Query {
+  user(id: ID!): UserResult!
+}
+
+# Usage
+{
+  user(id: "123") {
+    ... on User {
+      id
+      email
+    }
+    ... on NotFoundError {
+      message
+      resourceType
+    }
+    ... on AuthorizationError {
+      message
+    }
+  }
+}
+```
+
+### Errors in Payload
+```graphql
+type CreateUserPayload {
+  user: User
+  errors: [Error!]
+  success: Boolean!
+}
+
+type Error {
+  field: String
+  message: String!
+  code: ErrorCode!
+}
+
+enum ErrorCode {
+  VALIDATION_ERROR
+  UNAUTHORIZED
+  NOT_FOUND
+  INTERNAL_ERROR
+}
+```
+
+## N+1 Query Problem Solutions
+
+### DataLoader Pattern
+```python
+from aiodataloader import DataLoader
+
+class PostLoader(DataLoader):
+    async def batch_load_fn(self, post_ids):
+        posts = await db.posts.find({"id": {"$in": post_ids}})
+        post_map = {post["id"]: post for post in posts}
+        return [post_map.get(pid) for pid in post_ids]
+
+# Resolver
+@user_type.field("posts")
+async def resolve_posts(user, info):
+    loader = info.context["loaders"]["post"]
+    return await loader.load_many(user["post_ids"])
+```
+
+### Query Depth Limiting
+```python
+from graphql import GraphQLError
+
+def depth_limit_validator(max_depth: int):
+    def validate(context, node, ancestors):
+        depth = len(ancestors)
+        if depth > max_depth:
+            raise GraphQLError(
+                f"Query depth {depth} exceeds maximum {max_depth}"
+            )
+    return validate
+```
+
+### Query Complexity Analysis
+```python
+def complexity_limit_validator(max_complexity: int):
+    def calculate_complexity(node):
+        # Each field = 1, lists multiply
+        complexity = 1
+        if is_list_field(node):
+            complexity *= get_list_size_arg(node)
+        return complexity
+
+    return validate_complexity
+```
+
+## Schema Versioning
+
+### Field Deprecation
+```graphql
+type User {
+  name: String! @deprecated(reason: "Use firstName and lastName")
+  firstName: String!
+  lastName: String!
+}
+```
+
+### Schema Evolution
+```graphql
+# v1 - Initial
+type User {
+  name: String!
+}
+
+# v2 - Add optional field (backward compatible)
+type User {
+  name: String!
+  email: String
+}
+
+# v3 - Deprecate and add new field
+type User {
+  name: String! @deprecated(reason: "Use firstName/lastName")
+  firstName: String!
+  lastName: String!
+  email: String
+}
+```
+
+## Best Practices Summary
+
+1. **Nullable vs Non-Null**: Start nullable, make non-null when guaranteed
+2. **Input Types**: Always use input types for mutations
+3. **Payload Pattern**: Return errors in mutation payloads
+4. **Pagination**: Use cursor-based for infinite scroll, offset for simple cases
+5. **Naming**: Use camelCase for fields, PascalCase for types
+6. **Deprecation**: Use `@deprecated` instead of removing fields
+7. **DataLoaders**: Always use for relationships to prevent N+1
+8. **Complexity Limits**: Protect against expensive queries
+9. **Custom Scalars**: Use for domain-specific types (Email, DateTime)
+10. **Documentation**: Document all fields with descriptions

skills/api-design-principles/references/rest-best-practices.md

@@ -0,0 +1,385 @@
+# REST API Best Practices
+
+## URL Structure
+
+### Resource Naming
+```
+# Good - Plural nouns
+GET /api/users
+GET /api/orders
+GET /api/products
+
+# Bad - Verbs or mixed conventions
+GET /api/getUser
+GET /api/user  (inconsistent singular)
+POST /api/createOrder
+```
+
+### Nested Resources
+```
+# Shallow nesting (preferred)
+GET /api/users/{id}/orders
+GET /api/orders/{id}
+
+# Deep nesting (avoid)
+GET /api/users/{id}/orders/{orderId}/items/{itemId}/reviews
+# Better:
+GET /api/order-items/{id}/reviews
+```
+
+## HTTP Methods and Status Codes
+
+### GET - Retrieve Resources
+```
+GET /api/users              → 200 OK (with list)
+GET /api/users/{id}         → 200 OK or 404 Not Found
+GET /api/users?page=2       → 200 OK (paginated)
+```
+
+### POST - Create Resources
+```
+POST /api/users
+  Body: {"name": "John", "email": "john@example.com"}
+  → 201 Created
+  Location: /api/users/123
+  Body: {"id": "123", "name": "John", ...}
+
+POST /api/users (validation error)
+  → 422 Unprocessable Entity
+  Body: {"errors": [...]}
+```
+
+### PUT - Replace Resources
+```
+PUT /api/users/{id}
+  Body: {complete user object}
+  → 200 OK (updated)
+  → 404 Not Found (doesn't exist)
+
+# Must include ALL fields
+```
+
+### PATCH - Partial Update
+```
+PATCH /api/users/{id}
+  Body: {"name": "Jane"}  (only changed fields)
+  → 200 OK
+  → 404 Not Found
+```
+
+### DELETE - Remove Resources
+```
+DELETE /api/users/{id}
+  → 204 No Content (deleted)
+  → 404 Not Found
+  → 409 Conflict (can't delete due to references)
+```
+
+## Filtering, Sorting, and Searching
+
+### Query Parameters
+```
+# Filtering
+GET /api/users?status=active
+GET /api/users?role=admin&status=active
+
+# Sorting
+GET /api/users?sort=created_at
+GET /api/users?sort=-created_at  (descending)
+GET /api/users?sort=name,created_at
+
+# Searching
+GET /api/users?search=john
+GET /api/users?q=john
+
+# Field selection (sparse fieldsets)
+GET /api/users?fields=id,name,email
+```
+
+## Pagination Patterns
+
+### Offset-Based Pagination
+```python
+GET /api/users?page=2&page_size=20
+
+Response:
+{
+  "items": [...],
+  "page": 2,
+  "page_size": 20,
+  "total": 150,
+  "pages": 8
+}
+```
+
+### Cursor-Based Pagination (for large datasets)
+```python
+GET /api/users?limit=20&cursor=eyJpZCI6MTIzfQ
+
+Response:
+{
+  "items": [...],
+  "next_cursor": "eyJpZCI6MTQzfQ",
+  "has_more": true
+}
+```
+
+### Link Header Pagination (RESTful)
+```
+GET /api/users?page=2
+
+Response Headers:
+Link: <https://api.example.com/users?page=3>; rel="next",
+      <https://api.example.com/users?page=1>; rel="prev",
+      <https://api.example.com/users?page=1>; rel="first",
+      <https://api.example.com/users?page=8>; rel="last"
+```
+
+## Versioning Strategies
+
+### URL Versioning (Recommended)
+```
+/api/v1/users
+/api/v2/users
+
+Pros: Clear, easy to route
+Cons: Multiple URLs for same resource
+```
+
+### Header Versioning
+```
+GET /api/users
+Accept: application/vnd.api+json; version=2
+
+Pros: Clean URLs
+Cons: Less visible, harder to test
+```
+
+### Query Parameter
+```
+GET /api/users?version=2
+
+Pros: Easy to test
+Cons: Optional parameter can be forgotten
+```
+
+## Rate Limiting
+
+### Headers
+```
+X-RateLimit-Limit: 1000
+X-RateLimit-Remaining: 742
+X-RateLimit-Reset: 1640000000
+
+Response when limited:
+429 Too Many Requests
+Retry-After: 3600
+```
+
+### Implementation Pattern
+```python
+from fastapi import HTTPException, Request
+from datetime import datetime, timedelta
+
+class RateLimiter:
+    def __init__(self, calls: int, period: int):
+        self.calls = calls
+        self.period = period
+        self.cache = {}
+
+    def check(self, key: str) -> bool:
+        now = datetime.now()
+        if key not in self.cache:
+            self.cache[key] = []
+
+        # Remove old requests
+        self.cache[key] = [
+            ts for ts in self.cache[key]
+            if now - ts < timedelta(seconds=self.period)
+        ]
+
+        if len(self.cache[key]) >= self.calls:
+            return False
+
+        self.cache[key].append(now)
+        return True
+
+limiter = RateLimiter(calls=100, period=60)
+
+@app.get("/api/users")
+async def get_users(request: Request):
+    if not limiter.check(request.client.host):
+        raise HTTPException(
+            status_code=429,
+            headers={"Retry-After": "60"}
+        )
+    return {"users": [...]}
+```
+
+## Authentication and Authorization
+
+### Bearer Token
+```
+Authorization: Bearer eyJhbGciOiJIUzI1NiIs...
+
+401 Unauthorized - Missing/invalid token
+403 Forbidden - Valid token, insufficient permissions
+```
+
+### API Keys
+```
+X-API-Key: your-api-key-here
+```
+
+## Error Response Format
+
+### Consistent Structure
+```json
+{
+  "error": {
+    "code": "VALIDATION_ERROR",
+    "message": "Request validation failed",
+    "details": [
+      {
+        "field": "email",
+        "message": "Invalid email format",
+        "value": "not-an-email"
+      }
+    ],
+    "timestamp": "2025-10-16T12:00:00Z",
+    "path": "/api/users"
+  }
+}
+```
+
+### Status Code Guidelines
+- `200 OK`: Successful GET, PATCH, PUT
+- `201 Created`: Successful POST
+- `204 No Content`: Successful DELETE
+- `400 Bad Request`: Malformed request
+- `401 Unauthorized`: Authentication required
+- `403 Forbidden`: Authenticated but not authorized
+- `404 Not Found`: Resource doesn't exist
+- `409 Conflict`: State conflict (duplicate email, etc.)
+- `422 Unprocessable Entity`: Validation errors
+- `429 Too Many Requests`: Rate limited
+- `500 Internal Server Error`: Server error
+- `503 Service Unavailable`: Temporary downtime
+
+## Caching
+
+### Cache Headers
+```
+# Client caching
+Cache-Control: public, max-age=3600
+
+# No caching
+Cache-Control: no-cache, no-store, must-revalidate
+
+# Conditional requests
+ETag: "33a64df551425fcc55e4d42a148795d9f25f89d4"
+If-None-Match: "33a64df551425fcc55e4d42a148795d9f25f89d4"
+→ 304 Not Modified
+```
+
+## Bulk Operations
+
+### Batch Endpoints
+```python
+POST /api/users/batch
+{
+  "items": [
+    {"name": "User1", "email": "user1@example.com"},
+    {"name": "User2", "email": "user2@example.com"}
+  ]
+}
+
+Response:
+{
+  "results": [
+    {"id": "1", "status": "created"},
+    {"id": null, "status": "failed", "error": "Email already exists"}
+  ]
+}
+```
+
+## Idempotency
+
+### Idempotency Keys
+```
+POST /api/orders
+Idempotency-Key: unique-key-123
+
+If duplicate request:
+→ 200 OK (return cached response)
+```
+
+## CORS Configuration
+
+```python
+from fastapi.middleware.cors import CORSMiddleware
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["https://example.com"],
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+```
+
+## Documentation with OpenAPI
+
+```python
+from fastapi import FastAPI
+
+app = FastAPI(
+    title="My API",
+    description="API for managing users",
+    version="1.0.0",
+    docs_url="/docs",
+    redoc_url="/redoc"
+)
+
+@app.get(
+    "/api/users/{user_id}",
+    summary="Get user by ID",
+    response_description="User details",
+    tags=["Users"]
+)
+async def get_user(
+    user_id: str = Path(..., description="The user ID")
+):
+    """
+    Retrieve user by ID.
+
+    Returns full user profile including:
+    - Basic information
+    - Contact details
+    - Account status
+    """
+    pass
+```
+
+## Health and Monitoring Endpoints
+
+```python
+@app.get("/health")
+async def health_check():
+    return {
+        "status": "healthy",
+        "version": "1.0.0",
+        "timestamp": datetime.now().isoformat()
+    }
+
+@app.get("/health/detailed")
+async def detailed_health():
+    return {
+        "status": "healthy",
+        "checks": {
+            "database": await check_database(),
+            "redis": await check_redis(),
+            "external_api": await check_external_api()
+        }
+    }
+```