FastAPI Production Patterns

Critical rules (non-obvious)

• `async def` endpoint blocking sync DB call → blocks entire event loop. Either use async DB driver (asyncpg/aiomysql) throughout OR switch endpoint to plain def (FastAPI runs it in threadpool).
• Pydantic V2 `model_config = ConfigDict(...)` replaces V1 `class Config`. Forgetting this silently loses settings like from_attributes=True needed for ORM → DTO conversion.
• `Depends()` caches per-request: same dependency called twice in one request returns same instance. Don't rely on this for cross-request state — use app state / Redis instead.
• SQLAlchemy 2.0 async session must not leak across requests: always scope via Depends with async with AsyncSession(...) — raw module-level session causes GreenletError under load.
• `BackgroundTasks` runs AFTER response sent in the same worker process: if worker dies mid-task the work is lost. For durable background jobs use Celery / Dramatiq / ARQ.
• Uvicorn `--workers N` forks processes — can't share in-memory state. Use Redis or DB for any shared state (rate-limit counters, cache).

Project layout

app/
├── main.py               # FastAPI() instance + lifespan
├── api/
│   ├── deps.py           # shared Depends (get_db, get_current_user)
│   └── v1/
│       ├── users.py      # APIRouter
│       └── products.py
├── core/
│   ├── config.py         # Pydantic Settings
│   ├── security.py       # JWT encode/decode, password hashing
│   └── db.py             # engine + AsyncSession factory
├── models/               # SQLAlchemy ORM models
├── schemas/              # Pydantic DTOs (Request/Response)
├── services/             # business logic (no framework coupling)
└── tests/

Pydantic V2 settings + config

# app/core/config.py
from pydantic import Field
from pydantic_settings import BaseSettings, SettingsConfigDict

class Settings(BaseSettings):
    model_config = SettingsConfigDict(env_file=".env", env_prefix="APP_")

    database_url: str = Field(..., description="postgresql+asyncpg://...")
    jwt_secret: str = Field(..., min_length=32)
    jwt_algorithm: str = "HS256"
    jwt_exp_minutes: int = 30
    cors_origins: list[str] = Field(default_factory=list)

settings = Settings()  # fails fast at import if required vars missing

SQLAlchemy 2.0 async session (per-request)

# app/core/db.py
from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine, async_sessionmaker

engine = create_async_engine(
    settings.database_url,
    pool_size=20,
    max_overflow=10,
    pool_pre_ping=True,          # reconnect on stale conns (LB idle timeout)
    echo=False,
)
SessionLocal = async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)

# app/api/deps.py
from typing import AsyncIterator
from fastapi import Depends

async def get_db() -> AsyncIterator[AsyncSession]:
    async with SessionLocal() as session:
        try:
            yield session
            await session.commit()
        except Exception:
            await session.rollback()
            raise
        # close is automatic via `async with`

Lifespan + startup/shutdown

# app/main.py
from contextlib import asynccontextmanager
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup: warm caches, test DB
    async with engine.begin() as conn:
        await conn.execute(text("SELECT 1"))
    yield
    # Shutdown: drain connections
    await engine.dispose()

app = FastAPI(title="My API", lifespan=lifespan)

JWT auth with OAuth2PasswordBearer

# app/core/security.py
from datetime import datetime, timedelta, timezone
from jose import jwt, JWTError
from passlib.context import CryptContext

pwd = CryptContext(schemes=["bcrypt"], deprecated="auto")

def hash_password(p: str) -> str: return pwd.hash(p)
def verify_password(p: str, h: str) -> bool: return pwd.verify(p, h)

def create_access_token(sub: str) -> str:
    exp = datetime.now(timezone.utc) + timedelta(minutes=settings.jwt_exp_minutes)
    return jwt.encode({"sub": sub, "exp": exp}, settings.jwt_secret, settings.jwt_algorithm)

# app/api/deps.py
from fastapi import HTTPException, status
from fastapi.security import OAuth2PasswordBearer

oauth2_scheme = OAuth2PasswordBearer(tokenUrl="/api/v1/auth/login")

async def get_current_user(
    token: str = Depends(oauth2_scheme),
    db: AsyncSession = Depends(get_db),
) -> User:
    try:
        payload = jwt.decode(token, settings.jwt_secret, algorithms=[settings.jwt_algorithm])
        user_id: str = payload.get("sub")
    except JWTError:
        raise HTTPException(status.HTTP_401_UNAUTHORIZED, "Invalid token")
    user = await db.get(User, user_id)
    if not user:
        raise HTTPException(status.HTTP_401_UNAUTHORIZED, "User not found")
    return user

Endpoint pattern (thin controller, service below)

# app/api/v1/users.py
from fastapi import APIRouter, Depends, HTTPException, status

router = APIRouter(prefix="/users", tags=["users"])

@router.post("", response_model=UserOut, status_code=status.HTTP_201_CREATED)
async def create_user(
    payload: UserCreate,
    db: AsyncSession = Depends(get_db),
) -> UserOut:
    try:
        user = await user_service.create(db, payload)
    except DuplicateEmailError:
        raise HTTPException(status.HTTP_409_CONFLICT, "Email taken")
    return UserOut.model_validate(user)   # V2 from-attributes

@router.get("/{user_id}", response_model=UserOut)
async def get_user(
    user_id: int,
    db: AsyncSession = Depends(get_db),
    current: User = Depends(get_current_user),
) -> UserOut:
    user = await db.get(User, user_id)
    if not user:
        raise HTTPException(status.HTTP_404_NOT_FOUND)
    if user.id != current.id and not current.is_admin:
        raise HTTPException(status.HTTP_403_FORBIDDEN)
    return UserOut.model_validate(user)

Pydantic V2 schemas with from_attributes

from pydantic import BaseModel, ConfigDict, EmailStr, Field

class UserBase(BaseModel):
    model_config = ConfigDict(from_attributes=True)
    email: EmailStr
    full_name: str = Field(min_length=1, max_length=100)

class UserCreate(UserBase):
    password: str = Field(min_length=8, max_length=128)

class UserOut(UserBase):
    id: int
    created_at: datetime

Global exception handler

# app/main.py
from fastapi import Request
from fastapi.responses import JSONResponse

class AppError(Exception):
    def __init__(self, msg: str, status_code: int = 400):
        self.msg, self.status_code = msg, status_code

@app.exception_handler(AppError)
async def app_error_handler(req: Request, exc: AppError):
    return JSONResponse(status_code=exc.status_code, content={"error": exc.msg})

@app.exception_handler(Exception)
async def unhandled(req: Request, exc: Exception):
    logger.exception("Unhandled error", extra={"path": req.url.path})
    return JSONResponse(status_code=500, content={"error": "Internal server error"})

Testing with pytest-asyncio

# tests/conftest.py
import pytest_asyncio
from httpx import AsyncClient, ASGITransport

@pytest_asyncio.fixture
async def client():
    async with AsyncClient(transport=ASGITransport(app=app), base_url="http://test") as c:
        yield c

@pytest_asyncio.fixture
async def db_session():
    async with SessionLocal() as s:
        yield s
        await s.rollback()  # isolate test

# tests/test_users.py
@pytest.mark.asyncio
async def test_create_user(client: AsyncClient):
    r = await client.post("/api/v1/users", json={"email": "[email protected]", "full_name": "A", "password": "pw12345678"})
    assert r.status_code == 201
    assert r.json()["email"] == "[email protected]"

Production deployment (Uvicorn + Gunicorn)

# Dockerfile CMD — recommended for production
gunicorn app.main:app \
  --workers 4 \
  --worker-class uvicorn.workers.UvicornWorker \
  --bind 0.0.0.0:8000 \
  --timeout 60 \
  --keep-alive 5 \
  --access-logfile -

Workers = (2 × CPU) + 1 for CPU-bound; lower for IO-heavy async (async workers share event loop already).

Common pitfalls

Observability hooks

# Structured logging
import structlog
logger = structlog.get_logger()

@app.middleware("http")
async def log_requests(request: Request, call_next):
    start = time.monotonic()
    response = await call_next(request)
    logger.info("http_request",
        method=request.method, path=request.url.path,
        status=response.status_code,
        duration_ms=(time.monotonic() - start) * 1000,
    )
    return response

• Add prometheus-fastapi-instrumentator for /metrics.
• Health check: GET /healthz returns {"status": "ok"} + DB SELECT 1.
• Request ID: X-Request-ID header middleware → bind to contextvars for logging.