Ability to serve an ASGI app object directly, rather than passing a module string

[simonw]

This is a very exciting project - thanks for releasing this!

I tried to get it working with my https://datasette.io/ ASGI app and ran into this error:

  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/multiprocessing/process.py", line 121, in start
    self._popen = self._Popen(self)
                  ^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/multiprocessing/context.py", line 288, in _Popen
    return Popen(process_obj)
           ^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/multiprocessing/popen_spawn_posix.py", line 32, in __init__
    super().__init__(process_obj)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/multiprocessing/popen_fork.py", line 19, in __init__
    self._launch(process_obj)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
AttributeError: Can't pickle local object 'asgi_csrf_decorator.<locals>._asgi_csrf_decorator.<locals>.app_wrapped_with_csrf'

Here's the script I wrote to replicate the problem, saved as serve_datasette_with_granian.py:

from granian.server import Granian
from granian.constants import HTTPModes, Interfaces, Loops, ThreadModes
from granian.log import LogLevels
from datasette.app import Datasette


def serve_with_granian():
    ds = Datasette(memory=True)
    app = ds.app()
    Granian(
        app,
        address="127.0.0.1",
        port=8002,
        interface=Interfaces.ASGI,
        workers=1,
        threads=1,
        pthreads=1,
        threading_mode=ThreadModes.workers.value,
        loop=Loops.auto.value,
        http=HTTPModes.auto.value,
        websockets=True,
        backlog=1024,
        log_level=LogLevels.info.value,
        ssl_cert=None,
        ssl_key=None,
    ).serve()


if __name__ == "__main__":
    serve_with_granian()

Run it like this to see the error (run pip install datasette first):

python serve_datasette_with_granian.py

Are there changes I can make to Datasette to get this to work, or is this something that illustrates a bug in Granian?

Relevant Datasette code is here: https://github.com/simonw/datasette/blob/6a352e99ab988dbf8fd22a100049caa6ad33f1ec/datasette/app.py#L1429-L1454

It's applying my asgi-csrf ASGI middleware from https://github.com/simonw/asgi-csrf

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[gi0baro]

Hi @simonw,
the app argument should be the import location of your application, not the actual object. The actual load of the app will be handled by Granian itself.

You should be able to serve your application just writing a main.py file with these contents:

from datasette.app import Datasette

ds = Datasette(memory=True)
app = ds.app()

and run Granian from cli with: granian --interface asgi main:app

[simonw]

Thanks - that recipe worked for me right now, and didn't return any errors.

However... I really want the ability to start Granian running from my own Python scripts, in the manner shown above.

The reason is that Datasette is configured by the command line. The usual way of starting it looks like this:

datasette serve mydb.db -p 8003 --setting sql_time_limit_ms 10000 --crossdb

There are a whole ton of options like those: https://docs.datasette.io/en/stable/cli-reference.html#datasette-serve

datasette serve starts a server running using Uvicorn - but I also have a plugin for using gunicorn instead which works like this:

# Install the plugin
datasette install datasette-gunicorn
datasette gunicorn mydb.db -p 8003 --setting sql_time_limit_ms 10000 --crossdb

I ran into the "can't pickle local object" error while trying to build a new plugin, datasette-granian, which worked in a similar way to datasette-gunicorn.

The implementation of datasette-gunicorn is here: https://github.com/simonw/datasette-gunicorn/blob/0.1.1/datasette_gunicorn/__init__.py

So I guess this is a feature request: I would like a documented way to programatically start a Granian server without having to use the granian --interface asgi CLI tool.

[gi0baro]

@simonw using Granian interface directly is absolutely possible, but as I said, the app argument shouldn't be your application instance, but a string with the importable location of your application object. It depends on how you named your module/package, but generally speaking should be something like yourmodule:app.

Benchmarks apps in Granian repo can show you an example:

granian/benchmarks/app/asgi.py

Lines 76 to 82 in e5139e2

	def granian(wrk, thr):
	from granian import Granian
	Granian("asgi:app", workers=int(wrk), threads=int(thr), interface="asgi").serve()
	if __name__ == "__main__":
	granian(sys.argv[1], sys.argv[2])

[simonw]

The challenge with that is that my application object needs to be instantiated with additional arguments that have been provided by the user.

One thing that might work is that I could code-generate a Python script file that instantiates an app object with all of the user's command line options, save that script to /tmp and then pass it to Granian(...) to start running - but that feels pretty messy! I'd much rather be able to serve an ASGI app directly, like I can with Uvicorn and Gunicorn:

• https://github.com/simonw/datasette/blob/6a352e99ab988dbf8fd22a100049caa6ad33f1ec/datasette/cli.py#L660
• https://github.com/simonw/datasette-gunicorn/blob/5b2ca643eb174f19d664c5bf97dc839da64188ec/datasette_gunicorn/__init__.py#L49-L56

[gi0baro]

@simonw considering how granian is designed, is not possible to directly pass an application instance to the server, as it would require that every loaded object should be pickable due to multiprocessing module usage.

Probably we can solve this with a --factory option in Granian, so instead of pointing to the application, you can point to a function that does everything you need and return the application instance, WDYT?

[simonw]

That could work. I'm not sure how I'd pass in the additional arguments though.

The thing I want to build is effectively a CLI script that looks something like this:

python datasette_granian.py -h 127.0.0.1 -p 8005 --setting page_size 5

(Plus a whole bunch more options).

Running this script would instantiate my existing Datasette() class with various options, then use ds.app() to get an ASGI app and start serving that using Granian.

[gi0baro]

@simonw wait, I got an idea to get this working with the current implementation.

The serve method of Granian class actually accepts a target_loader parameter:

granian/granian/server.py

Line 303 in 9265a03

def serve(self, spawn_target = None, target_loader = None):

You can use this to override the default behaviour to loading the application, and thus you can write something like this in a run.py file:

from datasette.app import Datasette
from granian import Granian

ds_kwargs = {}

def load_app(target):
    if target != "dyn":
        raise RuntimeError("Should never get there")
    ds = Datasette(**ds_kwargs)
    return ds.app()

def main():
    global ds_kwargs

    ds_kwargs.update(some_util_to_convert_cli_param_to_dict())
    srv = Granian("dyn", address="127.0.0.1", port=8002, interface="asgi")
    srv.serve(target_loader=load_app)

if __name__ == "__main__":
    main()

then running python run.py --whatever param --you-need-for ds should work :)

[simonw]

That didn't quite work - the subprocess couldn't see the ds_kwargs updated dictionary - but thanks to this I did figure out a pattern that works: https://github.com/simonw/datasette-granian/blob/0.1a0/datasette_granian/__init__.py

I passed the arguments as serialized JSON:

    srv = Granian(
        # Pass kwars as serialized JSON to the subprocess
        json.dumps(kwargs),
        address=host,

Then in load_app:

def load_app(target):
    from datasette import cli

    ds_kwargs = json.loads(target)
    ds = cli.serve.callback(**ds_kwargs)
    return ds.app()

[simonw]

You can try the above out like this:

pip install datasette datasette-granian
wget https://datasette.io/content.db
datasette granian content.db -p 8000

This will start a server on port 8000 serving the Datasette interface.

[gi0baro]

@simonw probably wrapping works and is a cleaner solution. Eg:

from datasette.app import Datasette
from granian import Granian

def app_loader(kwargs):
    def load_app(target):
        if target != "dyn":
            raise RuntimeError("Should never get there")
        ds = Datasette(**kwargs)
        return ds.app()
    return load_app

def main():
    ds_kwargs = some_util_to_convert_cli_param_to_dict()
    srv = Granian("dyn", address="127.0.0.1", port=8002, interface="asgi")
    srv.serve(target_loader=app_loader(ds_kwargs))

if __name__ == "__main__":
    main()

[simonw]

Tried that just now but it didn't work - I got this error:

  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/multiprocessing/popen_spawn_posix.py", line 47, in _launch
    reduction.dump(process_obj, fp)
  File "/Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/multiprocessing/reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
AttributeError: Can't pickle local object 'app_loader.<locals>.load_app'

[simonw]

How about if Granian had some kind of mechanism where you could specify a pickle-able object which should be passed to each of the workers, specifically designed for this kind of use-case?

[gi0baro]

How about if Granian had some kind of mechanism where you could specify a pickle-able object which should be passed to each of the workers, specifically designed for this kind of use-case?

Gonna think about it. Probably the theme here is making the target_loader argument more customisable.

[novichikhin]

@gi0baro Is there a solution to the problem? I want to run application without global variables.

def app_loader(settings: AppSettings):

    def load_app(_) -> FastAPI:
        return register_app(settings=settings)

    return load_app


def run_application() -> None:
    settings = get_app_settings(
        app_env=AppEnvTypes.prod if os.getenv("IS_PRODUCTION") else AppEnvTypes.dev
    )
    server = Granian(
        "app",
        address=settings.server_host,
        port=settings.server_port,
        interface=Interfaces.ASGI
    )

    server.serve(target_loader=app_loader(settings=settings))


if __name__ == "__main__":
    run_application()

not work

[gi0baro]

@novichikhin what's the error? Do you have a stack trace?

[novichikhin]

@novichikhin what's the error? Do you have a stack trace?

[INFO] Starting granian
[INFO] Listening at: 0.0.0.0:8002
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "C:\dev\python\backend\users-service\users_service\api\__main__.py", line 37, in <module>
    run_application()
  File "C:\dev\python\backend\users-service\users_service\api\__main__.py", line 33, in run_application
    server.serve(target_loader=app_loader(settings=settings))
  File "C:\Users\novichikhin\AppData\Local\pypoetry\Cache\virtualenvs\users-service-auTBzo84-py3.11\Lib\site-packages\granian\server.py", line 390, in serve
    serve_method(spawn_target, target_loader)
  File "C:\Users\novichikhin\AppData\Local\pypoetry\Cache\virtualenvs\users-service-auTBzo84-py3.11\Lib\site-packages\granian\server.py", line 342, in _serve
    self.startup(spawn_target, target_loader)
  File "C:\Users\novichikhin\AppData\Local\pypoetry\Cache\virtualenvs\users-service-auTBzo84-py3.11\Lib\site-packages\granian\server.py", line 334, in startup
    self._spawn_workers(sock, spawn_target, target_loader)
  File "C:\Users\novichikhin\AppData\Local\pypoetry\Cache\virtualenvs\users-service-auTBzo84-py3.11\Lib\site-packages\granian\server.py", line 313, in _spawn_workers
    proc.start()
  File "C:\Users\novichikhin\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\process.py", line 121, in start
    self._popen = self._Popen(self)
                  ^^^^^^^^^^^^^^^^^
  File "C:\Users\novichikhin\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\context.py", line 336, in _Popen
    return Popen(process_obj)
           ^^^^^^^^^^^^^^^^^^
  File "C:\Users\novichikhin\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\popen_spawn_win32.py", line 94, in __init__
    reduction.dump(process_obj, to_child)
  File "C:\Users\novichikhin\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\reduction.py", line 60, in dump
    ForkingPickler(file, protocol).dump(obj)
AttributeError: Can't pickle local object 'app_loader.<locals>.load_app'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\novichikhin\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\spawn.py", line 122, in spawn_main
    exitcode = _main(fd, parent_sentinel)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\novichikhin\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\spawn.py", line 132, in _main
    self = reduction.pickle.load(from_parent)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
EOFError: Ran out of input

[gi0baro]

@novichikhin can you try with wrap_loader=False in the serve method and defining/importing everything into a single method? Multiprocessing requires objects to be pickable. Something like:

def app_loader():
    from whatever import get_app_settings, register_app
    settings = ...
    return register_app(...)

[novichikhin]

@gi0baro

def load_app() -> FastAPI:
    from users_service.api.setup import register_app
    from users_service.settings.main import get_app_settings

    settings = get_app_settings(
        app_env=AppEnvTypes.prod if os.getenv("IS_PRODUCTION") else AppEnvTypes.dev
    )

    return register_app(settings=settings)


def run_application() -> None:
    server = Granian(
        "app",
        address="127.0.0.1",
        port=8002,
        interface=Interfaces.ASGI
    )

    server.serve(target_loader=load_app, wrap_loader=False)


if __name__ == "__main__":
    run_application()

[INFO] Starting granian
[INFO] Listening at: 127.0.0.1:8002
[INFO] Spawning worker-1 with pid: 12772
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\novichikhin\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\spawn.py", line 122, in spawn_main
    exitcode = _main(fd, parent_sentinel)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\novichikhin\AppData\Local\Programs\Python\Python311\Lib\multiprocessing\spawn.py", line 132, in _main
    self = reduction.pickle.load(from_parent)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: Can't get attribute 'load_app' on <module '__main__' (built-in)>

[gi0baro]

@novichikhin I don't really know how to solve the pickling issues.

Maybe adding support for factories should solve your need?

[fjarri]

Subscribing, this issue is unfortunately a blocker for me. I currently use hypercorn, and a thing I rely on in testing is that I can launch several instances of it as trio tasks (to simulate several nodes in a network), and they even support reporting back when the startup is completed via a trio event (hypercorn has a trio compatibility submodule). I don't mind if it's not as efficient as launching from CLI, but it's incredibly convenient for tests.

[alwavshumilin]

@novichikhin pickle cannot serialise closures for obvious reasons, they are not compiled into byte-code until called at least once. If you want to use settings as an argument you can either:

• use functools.partial to define your loader. Signature would mostly look something like this (target: str, *, settings: AppSettings) -> App
• make a small custom class that'll cache your settings and define __call__ to return your app from the factory

Both variants are applicable only for global module scope, i. e. no closures :)
Also, also - if you are using __main__.py file to start your server, you should consider moving loader somewhere else, because it's not a valid target for pickle unless you're willing to do some sketchy path hacking

# runner.py
def app_loader(_: str, *, settings: AppSettings):
    return register_app(settings=settings)

# __main__.py
import functools
from .runner import app_loader
settings = ...
loader = functools.partial(app_loader, settings=settings)
Granian(...).serve(target_loader=loader)