Errors in Middleware Due to Startup Race Condition

[RakeshReddyKondeti]

First Check

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Example Code

import asyncio
from nicegui import ui, app
from fastapi import Request
from starlette.middleware.base import BaseHTTPMiddleware

# --- A generic singleton service that must be initialized ---
class ServiceSingleton:
    _instance = None

    @classmethod
    def initialize(cls):
        print("Initializing Service...")
        cls._instance = ServiceSingleton()
        print("Service initialized ✅")

    @classmethod
    async def get_instance(cls):
        if cls._instance is None:
            raise ValueError("Service not initialized ❌")
        return cls._instance


class SessionMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request: Request, call_next):
        try:
            await ServiceSingleton.get_instance()
        except Exception as e:
            print("ERROR in middleware:", e)
            raise ValueError("Service not initialized. Cannot proceed.")
        return await call_next(request)


async def on_startup():
    print("Starting app async initialization...")
    await asyncio.sleep(10)   # pretend heavy init
    ServiceSingleton.initialize()
    print("Startup completed!")


@ui.page("/")
async def index():
    ui.label("Hello, World!")

def main():
    app.on_startup(on_startup)

    app.add_middleware(UserSessionMiddleware)
    ui.run()

if __name__ in {"__main__", "__mp_main__"}:
    main()

Description

There is a race condition between our NiceGUI on_startup async initialization and the first user requests handled by SessionMiddleware

• During startup, we do some slow async initialization (e.g., vector DB setup, workflow instantiation, model loading).
• Middleware relies on these services being globally initialized.
• If user requests arrive before startup finishes, middleware attempts to use uninitialized services and crashes with errors like:

NiceGUI Version

2.23.3

Python Version

3.12.9

Browser

Chrome

Operating System

Windows

Additional Context

No response

[RakeshReddyKondeti]

@falkoschindler Could you please take a look at this ?

[evnchn]

Thank you @RakeshReddyKondeti for the report.

The on_startup is meant to be run in the background in async manner after the server has started. It is intentional by design. So I don't think it's necessarily a "race condition".

I think you have two choices:

• Run the initialization before ui.run is called
• Rewrite your middleware to check, gracefully, whether the initialization is done, and deny all requests until it is done.

What do you think? Do try it out!