fastapi-pro

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