fastapi-advanced

---
name: fastapi-advanced
description: FastAPI  advanced patterns including lifespan, dependencies, middleware, and Pydantic settings. Use when configuring FastAPI lifespan events, creating dependency injection, building Starlette middleware, or managing async Python services with uvicorn.
license: MIT
---

# FastAPI Advanced Patterns ()

Production-ready FastAPI patterns for modern Python backends.

## Lifespan Management ()

### Modern Lifespan Context Manager

```python
from contextlib import asynccontextmanager
from fastapi import FastAPI
from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
import redis.asyncio as redis

@asynccontextmanager
async def lifespan(app: FastAPI):
    """Application lifespan with resource management."""
    # Startup
    app.state.db_engine = create_async_engine(
        settings.database_url,
        pool_size=5,
        max_overflow=10,
    )
    app.state.redis = redis.from_url(settings.redis_url)

    # Health check connections
    async with app.state.db_engine.connect() as conn:
        await conn.execute(text("SELECT 1"))
    await app.state.redis.ping()

    yield  # Application runs

    # Shutdown
    await app.state.db_engine.dispose()
    await app.state.redis.close()

app = FastAPI(lifespan=lifespan)
```

### Lifespan with Services

```python
from app.services import EmbeddingsService, LLMService

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Initialize services
    app.state.embeddings = EmbeddingsService(
        model=settings.embedding_model,
        batch_size=100,
    )
    app.state.llm = LLMService(
        providers=["openai", "anthropic"],
        default="anthropic",
    )

    # Warm up models (optional)
    await app.state.embeddings.warmup()

    yield

    # Cleanup
    await app.state.embeddings.close()
    await app.state.llm.close()
```

## Dependency Injection Patterns

### Database Session

```python
from typing import AsyncGenerator
from sqlalchemy.ext.asyncio import AsyncSession
from fastapi import Depends, Request

async def get_db(request: Request) -> AsyncGenerator[AsyncSession, None]:
    """Yield database session from app state."""
    async with AsyncSession(
        request.app.state.db_engine,
        expire_on_commit=False,
    ) as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise
```

### Service Dependencies

```python
from functools import lru_cache

class AnalysisService:
    def __init__(
        self,
        db: AsyncSession,
        embeddings: EmbeddingsService,
        llm: LLMService,
    ):
        self.db = db
        self.embeddings = embeddings
        self.llm = llm

def get_analysis_service(
    db: AsyncSession = Depends(get_db),
    request: Request = None,
) -> AnalysisService:
    return AnalysisService(
        db=db,
        embeddings=request.app.state.embeddings,
        llm=request.app.state.llm,
    )

@router.post("/analyses")
async def create_analysis(
    data: AnalysisCreate,
    service: AnalysisService = Depends(get_analysis_service),
):
    return await service.create(data)
```

### Cached Dependencies

```python
from functools import lru_cache
from pydantic_settings import BaseSettings

class Settings(BaseSettings):
    database_url: str
    redis_url: str
    api_key: str

    model_config = {"env_file": ".env"}

@lru_cache
def get_settings() -> Settings:
    return Settings()

# Usage in dependencies
def get_db_url(settings: Settings = Depends(get_settings)) -> str:
    return settings.database_url
```

### Authenticated User

```python
from fastapi import Depends, HTTPException, Security
from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials

security = HTTPBearer()

async def get_current_user(
    credentials: HTTPAuthorizationCredentials = Security(security),
    db: AsyncSession = Depends(get_db),
) -> User:
    token = credentials.credentials
    payload = decode_jwt(token)

    user = await db.get(User, payload["sub"])
    if not user:
        raise HTTPException(401, "Invalid credentials")
    return user

async def get_admin_user(
    user: User = Depends(get_current_user),
) -> User:
    if not user.is_admin:
        raise HTTPException(403, "Admin access required")
    return user
```

## Middleware Patterns

### Request ID Middleware

```python
import uuid
from starlette.middleware.base import BaseHTTPMiddleware
from starlette.requests import Request

class RequestIDMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        request_id = request.headers.get("X-Request-ID", str(uuid.uuid4()))
        request.state.request_id = request_id

        response = await call_next(request)
        response.headers["X-Request-ID"] = request_id
        return response

app.add_middleware(RequestIDMiddleware)
```

### Timing Middleware

```python
import time
from starlette.middleware.base import BaseHTTPMiddleware

class TimingMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        start = time.perf_counter()
        response = await call_next(request)
        duration = time.perf_counter() - start

        response.headers["X-Response-Time"] = f"{duration:.3f}s"
        return response
```

### Structured Logging Middleware

```python
import structlog
from starlette.middleware.base import BaseHTTPMiddleware

logger = structlog.get_logger()

class LoggingMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        log = logger.bind(
            request_id=getattr(request.state, "request_id", None),
            method=request.method,
            path=request.url.path,
        )

        try:
            response = await call_next(request)
            log.info(
                "request_completed",
                status_code=response.status_code,
            )
            return response
        except Exception as exc:
            log.exception("request_failed", error=str(exc))
            raise
```

### CORS Configuration

```python
from fastapi.middleware.cors import CORSMiddleware

app.add_middleware(
    CORSMiddleware,
    allow_origins=settings.cors_origins,
    allow_credentials=True,
    allow_methods=["GET", "POST", "PUT", "DELETE", "PATCH"],
    allow_headers=["*"],
    expose_headers=["X-Request-ID", "X-Response-Time"],
)
```

## Settings with Pydantic

```python
from pydantic import Field, field_validator, PostgresDsn
from pydantic_settings import BaseSettings, SettingsConfigDict

class Settings(BaseSettings):
    model_config = SettingsConfigDict(
        env_file=".env",
        env_file_encoding="utf-8",
        case_sensitive=False,
    )

    # Database
    database_url: PostgresDsn
    db_pool_size: int = Field(default=5, ge=1, le=20)
    db_max_overflow: int = Field(default=10, ge=0, le=50)

    # Redis
    redis_url: str = "redis://localhost:6379"

    # API
    api_key: str = Field(min_length=32)
    debug: bool = False

    # LLM
    openai_api_key: str | None = None
    anthropic_api_key: str | None = None

    @field_validator("database_url", mode="before")
    @classmethod
    def validate_database_url(cls, v: str) -> str:
        if v and "+asyncpg" not in v:
            return v.replace("postgresql://", "postgresql+asyncpg://")
        return v

    @property
    def async_database_url(self) -> str:
        return str(self.database_url)
```

## Exception Handlers

```python
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse
from sqlalchemy.exc import IntegrityError
from app.core.exceptions import ProblemException

@app.exception_handler(ProblemException)
async def problem_exception_handler(request: Request, exc: ProblemException):
    return JSONResponse(
        status_code=exc.status_code,
        content=exc.to_problem_detail(),
        media_type="application/problem+json",
    )

@app.exception_handler(IntegrityError)
async def integrity_error_handler(request: Request, exc: IntegrityError):
    return JSONResponse(
        status_code=409,
        content={
            "type": "https://api.example.com/problems/conflict",
            "title": "Conflict",
            "status": 409,
            "detail": "Resource already exists or constraint violated",
        },
        media_type="application/problem+json",
    )
```

## Response Optimization

```python
from fastapi.responses import ORJSONResponse

# Use orjson for faster JSON serialization
app = FastAPI(default_response_class=ORJSONResponse)

# Streaming response
from fastapi.responses import StreamingResponse

@router.get("/export")
async def export_data():
    async def generate():
        async for chunk in fetch_large_dataset():
            yield json.dumps(chunk) + "\n"

    return StreamingResponse(
        generate(),
        media_type="application/x-ndjson",
    )
```

## Health Checks

```python
from fastapi import APIRouter

health_router = APIRouter(tags=["health"])

@health_router.get("/health")
async def health_check(request: Request):
    checks = {}

    # Database
    try:
        async with request.app.state.db_engine.connect() as conn:
            await conn.execute(text("SELECT 1"))
        checks["database"] = "healthy"
    except Exception as e:
        checks["database"] = f"unhealthy: {e}"

    # Redis
    try:
        await request.app.state.redis.ping()
        checks["redis"] = "healthy"
    except Exception as e:
        checks["redis"] = f"unhealthy: {e}"

    status = "healthy" if all(v == "healthy" for v in checks.values()) else "unhealthy"
    return {"status": status, "checks": checks}
```

## Anti-Patterns (FORBIDDEN)

```python
# NEVER use global state
db_session = None  # Global mutable state!

# NEVER block the event loop
def sync_db_query():  # Blocking in async context!
    return session.query(User).all()

# NEVER skip dependency injection
@router.get("/users")
async def get_users():
    db = create_session()  # Creating session in route!
    return db.query(User).all()

# NEVER ignore lifespan cleanup
@asynccontextmanager
async def lifespan(app: FastAPI):
    app.state.pool = create_pool()
    yield
    # Missing cleanup! Pool never closed
```

## Key Decisions

| Decision | Recommendation |
|----------|----------------|
| Lifespan | Use `asynccontextmanager` (not events) |
| Dependencies | Class-based services with DI |
| Settings | Pydantic Settings with `.env` |
| Response | ORJSONResponse for performance |
| Middleware | Order: CORS → RequestID → Timing → Logging |
| Health | Check all critical dependencies |

## Available Scripts

- **`scripts/create-fastapi-app.md`** - Context-aware FastAPI application generator
  - Auto-detects: Python version, database type, Redis usage, project structure
  - Usage: `/create-fastapi-app [app-name]`
  - Uses `$ARGUMENTS` and `!command` for project-specific configuration
  - Generates production-ready app with detected dependencies
  
- **`assets/fastapi-app-template.py`** - Static FastAPI application template

## Related Skills

- `clean-architecture` - Service layer patterns
- `database-schema-designer` - SQLAlchemy models
- `observability-monitoring` - Logging and metrics

## Capability Details

### lifespan
**Keywords:** lifespan, startup, shutdown, asynccontextmanager
**Solves:**
- FastAPI startup/shutdown
- Resource management in FastAPI

### dependencies
**Keywords:** dependency injection, Depends, get_db, service dependency
**Solves:**
- FastAPI dependency injection patterns
- Reusable dependencies

### middleware
**Keywords:** middleware, request id, timing, cors, logging middleware
**Solves:**
- Custom FastAPI middleware
- Request/response interceptors

### settings
**Keywords:** settings, pydantic settings, env, configuration
**Solves:**
- FastAPI configuration management
- Environment variables

### health-checks
**Keywords:** health check, readiness, liveness, health endpoint
**Solves:**
- Kubernetes health checks
- Service health monitoring