Fastapi App Factory

FastAPI Application Factory

Overview

This skill covers creating the FastAPI application factory pattern with proper lifespan management, middleware registration, pagination setup, and router configuration.

Create main.py

Create src/app/main.py:

import logging
from contextlib import asynccontextmanager
from collections.abc import AsyncGenerator

from fastapi import FastAPI
from fastapi_pagination import add_pagination

from app.api import api_router
from app.config import settings
from app.database import engine
from app.exception_handlers import register_exception_handlers
from app.logging import setup_logging
from app.middleware import CorrelationIdMiddleware

# Setup logging before anything else
setup_logging()

logger = logging.getLogger(__name__)


@asynccontextmanager
async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
    """
    Application lifespan context manager.
    
    Handles startup and shutdown events:
    - Startup: Log application start, initialize resources
    - Shutdown: Close database connections, cleanup resources
    
    This replaces the deprecated @app.on_event decorators.
    """
    # Startup
    logger.info(
        "Starting application",
        extra={
            "app_name": settings.app_name,
            "debug": settings.debug,
        },
    )

    yield

    # Shutdown
    logger.info("Shutting down application")
    await engine.dispose()
    logger.info("Database connections closed")


def create_app() -> FastAPI:
    """
    Application factory function.
    
    Creates and configures the FastAPI application with:
    - Lifespan management
    - Exception handlers
    - Middleware (correlation ID)
    - Pagination support
    - API routers
    
    Returns:
        Configured FastAPI application instance
    """
    app = FastAPI(
        title=settings.app_name,
        debug=settings.debug,
        lifespan=lifespan,
        # OpenAPI configuration
        openapi_url="/api/openapi.json" if settings.debug else None,
        docs_url="/api/docs" if settings.debug else None,
        redoc_url="/api/redoc" if settings.debug else None,
    )

    # Register exception handlers
    register_exception_handlers(app)

    # Add middleware (order matters - first added = outermost)
    app.add_middleware(CorrelationIdMiddleware)

    # Add pagination support
    add_pagination(app)

    # Include routers
    app.include_router(api_router, prefix="/api")

    return app


# Create the application instance
app = create_app()

Create api/init.py

Create src/app/api/__init__.py:

from fastapi import APIRouter

from app.api.v1 import router as v1_router

api_router = APIRouter()

# Include versioned routers
api_router.include_router(v1_router, prefix="/v1")


@api_router.get("/health", tags=["health"])
async def health_check() -> dict[str, str]:
    """
    Health check endpoint.
    
    Returns a simple status indicating the API is running.
    Use this for load balancer health checks.
    """
    return {"status": "healthy"}

Create api/v1/init.py

Create src/app/api/v1/__init__.py:

from fastapi import APIRouter

# Import entity routers here
# from app.items.router import router as items_router
# from app.users.router import router as users_router

router = APIRouter()

# Include entity routers
# router.include_router(items_router)
# router.include_router(users_router)

Create api/v1/router.py (Alternative)

If you prefer a separate router file:

# src/app/api/v1/router.py
from fastapi import APIRouter

# Import entity routers
# from app.items.router import router as items_router

router = APIRouter()

# Include entity routers
# router.include_router(items_router)

Running the Application

Development

# Using uvicorn directly
uv run uvicorn app.main:app --reload --host 0.0.0.0 --port 8000

# Or with more options
uv run uvicorn app.main:app \
    --reload \
    --host 0.0.0.0 \
    --port 8000 \
    --log-level info

Production

# Using uvicorn with workers
uv run uvicorn app.main:app \
    --host 0.0.0.0 \
    --port 8000 \
    --workers 4 \
    --log-level warning

# Or with gunicorn + uvicorn workers
uv run gunicorn app.main:app \
    --worker-class uvicorn.workers.UvicornWorker \
    --workers 4 \
    --bind 0.0.0.0:8000

Application Configuration Options

FastAPI Constructor Options

app = FastAPI(
    # Basic
    title="My API",
    description="API description",
    version="1.0.0",
    debug=settings.debug,
    
    # Lifespan
    lifespan=lifespan,
    
    # OpenAPI/Docs
    openapi_url="/api/openapi.json",
    docs_url="/api/docs",
    redoc_url="/api/redoc",
    openapi_tags=[
        {"name": "items", "description": "Item operations"},
        {"name": "users", "description": "User operations"},
    ],
    
    # Response configuration
    default_response_class=JSONResponse,
    
    # Root path for reverse proxy
    root_path="/api",
)

Conditional OpenAPI

# Disable OpenAPI in production
openapi_url="/api/openapi.json" if settings.debug else None,
docs_url="/api/docs" if settings.debug else None,
redoc_url=None,  # Disable ReDoc entirely

Middleware Order

Middleware is executed in reverse order of addition (first added = outermost):

# Request flow: CorrelationID -> RequestLogging -> ... -> Router
# Response flow: Router -> ... -> RequestLogging -> CorrelationID

app.add_middleware(CorrelationIdMiddleware)  # Outermost
app.add_middleware(RequestLoggingMiddleware)  # Inner

Adding CORS (if needed)

from fastapi.middleware.cors import CORSMiddleware

def create_app() -> FastAPI:
    app = FastAPI(...)
    
    # Add CORS before other middleware
    app.add_middleware(
        CORSMiddleware,
        allow_origins=["http://localhost:3000"],  # Or ["*"] for all
        allow_credentials=True,
        allow_methods=["*"],
        allow_headers=["*"],
    )
    
    # Then other middleware
    app.add_middleware(CorrelationIdMiddleware)
    
    return app

Adding Custom Request State

from starlette.middleware.base import BaseHTTPMiddleware

class RequestStateMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        # Add custom state to request
        request.state.start_time = time.time()
        request.state.request_id = str(uuid4())
        
        response = await call_next(request)
        return response

Application Structure Summary

main.py (create_app)
    │
    ├── Lifespan (startup/shutdown)
    │
    ├── Exception Handlers
    │   └── register_exception_handlers(app)
    │
    ├── Middleware
    │   └── CorrelationIdMiddleware
    │
    ├── Pagination
    │   └── add_pagination(app)
    │
    └── Routers
        └── api_router (/api)
            ├── /health
            └── v1_router (/api/v1)
                ├── items_router (/api/v1/items)
                └── users_router (/api/v1/users)

Health Check Patterns

Simple Health Check

@api_router.get("/health")
async def health_check():
    return {"status": "healthy"}

Detailed Health Check

from sqlalchemy import text
from app.database import async_session_factory

@api_router.get("/health/detailed")
async def detailed_health_check():
    health = {
        "status": "healthy",
        "checks": {}
    }
    
    # Database check
    try:
        async with async_session_factory() as session:
            await session.execute(text("SELECT 1"))
        health["checks"]["database"] = "healthy"
    except Exception as e:
        health["status"] = "unhealthy"
        health["checks"]["database"] = f"unhealthy: {str(e)}"
    
    return health

Environment-Specific Configuration

def create_app() -> FastAPI:
    app = FastAPI(
        title=settings.app_name,
        debug=settings.debug,
        lifespan=lifespan,
    )
    
    # Development-only features
    if settings.debug:
        # Enable Swagger UI
        app.openapi_url = "/api/openapi.json"
        app.docs_url = "/api/docs"
        
        # Add debug middleware
        from app.middleware.debug import DebugMiddleware
        app.add_middleware(DebugMiddleware)
    
    # ... rest of configuration
    
    return app

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