Support mounting Mesop with other Python servers

[wwwillchen]

Motivation

This could satisfy a few advanced use cases:

• Allows client-side code (web components) to directly call long-running APIs which could support: 1) not blocking state updates and 2) cancelling an LLM call (see Better concurrent UI event handling #623)
• Support serving custom files (e.g. download links how to respond to client/browser with a file download #453)
• Allow forwarding requests which can be needed in certain auth integrations (see: Firebase Auth "redirect" broken #713)

Considerations

• We'd like to eventually migrate from Flask to an async-friendly framework like Quart or Starlette so we shouldn't directly expose the framework so we have flexibility to do this migration. However, we should try to provide an API that's similar so the learning curve is reasonably small.

API proposal

Simple request

@me.route(methods=["GET", "POST"], path="/foo")
def route():
  return output

Async

@me.route(path="/books/{book_id}")
async def route(book_id: int):
  output = await call_api()
  return output

Path parameters

@me.route(methods=["GET", "POST"], path="/books/{book_id}")
def route(book_id: int):
  return output

Query params

@me.route(methods=["GET", "POST"], path="/books")
def route():
  qp = me.query_params()
  return do_work(qp["foo"])

output could be:

• dict
• json
• binary file

Questions

• Should we try to expose Fast API directly? Or, should we implement this ourselves?
• Should we allow path-dependent ordering? See FastAPI
• Should we support multiple HTTP verbs in a single HTTP handler?
• Should you be able to access me.state(State)? It would be nice to do this, otherwise it creates an island but you could update the state indirectly from the client-side code.
• https://fastapi.tiangolo.com/tutorial/body/#request-body-path-query-parameters:~:text=The%20function%20parameters%20will%20be%20recognized%20as%20follows%3A

[wwwillchen]

Perhaps we can just offer a way to mount apps, e.g.:

# app is any WSGI app
me.mount_app(app)

https://flask.palletsprojects.com/en/2.3.x/patterns/appdispatch/
https://github.com/django/asgiref/tree/main

[wwwillchen]

Turns out you can actually mount Mesop because me actually satisfies the WSGI protocol. Here's an example of combining a Mesop app and a FastAPI app together:
https://github.com/wwwillchen/mesop-fastapi

It works!

Next steps:

• Expose me.wsgi_app which is just an alias of me
• Create an actual guide - tell users about --timeout-graceful-shutdown for https://www.uvicorn.org/deployment/
• Be able to configure Mesop dev mode as an environmental variable? (seems generally useful for debugging) - pretty simple - modify

  mesop/mesop/server/wsgi_app.py

  Line 78 in 85bfac6

  _app = create_app(prod_mode=True)

  .
• JS file changes doesn't trigger uvicorn reload - fixable by following: https://www.uvicorn.org/settings/#reloading-with-watchfiles
• Hot reloading doesn't work - this isn't a showstopper, but not nice. You can still run mesop foo.py on the part of the app that doesn't require the other Python server and get hot reloading.

[richard-to]

Nice, this sounds pretty awesome. This definitely could potentially solve quite a few issues, such as #564 and #472