How to / doc: async configuration / awaitable loop.start()

[dimaqq]

User guide advocates following to start a web server:

application = ...
application.listen(...)
IOLoop.current().start()

Now let's bring in some async library and use that to get application config:

config = IOLoop.current().run_sync(get_config_from_async_db)
application = SomeApp(config, ...)
application.listen(config["port"])
IOLoop.current().start()

Then comes aio-libs/aiohttp#3331 where iohttp (kinda rightfully) decided that asyncio.get_event_loop().is_running() should be True.

Now SomeApp constructor is going to create a aiohttp.TCPConnector(...) (a transitive dependency), which makes aiohttp unhappy (warning only, nothing is awaited at this point, but iohttp takes a reference to current event loop).

I wish I could express application setup and start as:

async def main():
    config = await get_config_from_async_db()
    application = SomeApp(config, ...)
    application.listen(config["port"])
    await application.serve_forever()

asyncio.run(main())  # Or IOLoop.current().run(main())

I this something like this would also be useful in tests, where a server may be started and stopped for each test.

P.S. I apologise if this was covered somewhere and I couldn't find a doc or past issue.

[ploxiln]

You can pretty much do that:

async def main():
    config = await get_config()
    application = SomeApp(config)
    application.listen(config["port"])

mainloop = IOLoop.current()
mainloop.spawn_callback(main)
mainloop.start()

You then probably want to add a signal handler which calls IOLoop.stop() ... here's one way which may be sufficient: #1791 (comment)

[bdarnell]

Yeah, this is probably worth documenting as an example; I use async def main with run_sync a lot of the time now. My version of await serve_forever() is await tornado.locks.Event().wait() (and then when/if I add graceful shutdown I set that event instead of just stopping the event loop; the rest of my cleanup happens in main()).

[dimaqq]

You can pretty much do that:

async def main():
    config = await get_config()
    application = SomeApp(config)
    application.listen(config["port"])

mainloop = IOLoop.current()
mainloop.spawn_callback(main)
mainloop.start()

You then probably want to add a signal handler which calls IOLoop.stop() ... here's one way which may be sufficient: #1791 (comment)

This is cool, but hangs if startup code raises an exception:

async def foo(): 1/0
IOLoop.current().spawn_callback(foo)
IOLoop.current().start()
ERROR:tornado.application:Exception ...
Traceback ...
<hangs>

Yeah, this is probably worth documenting as an example; I use async def main with run_sync a lot of the time now. My version of await serve_forever() is await tornado.locks.Event().wait() (and then when/if I add graceful shutdown I set that event instead of just stopping the event loop; the rest of my cleanup happens in main()).

This is better, as run_sync propagates exceptions. And there's a chance to cleanup in main().

I was kinda hoping to use try: finally: for cleanup... I'll try to bolt this on top of Event().wait().

[bdarnell]

Deprecation changes in python 3.10 forced us to update all the examples to use asyncio.run() and an async main as discussed here.