第十七章：后台任务 —— 窗口先把回执给你，慢活去后台工单中心做

from fastapi import BackgroundTasks, FastAPI

app = FastAPI()

def write_audit_log(message: str):
    with open("audit.log", "a", encoding="utf-8") as f:
        f.write(message + "\n")

@app.post("/permits/{permit_id}/approve")
async def approve_permit(permit_id: int, background_tasks: BackgroundTasks):
    background_tasks.add_task(write_audit_log, f"permit {permit_id} approved")
    return {"message": "已批准，审计日志稍后写入"}