How can I avoid django channels runtime errors when using signals?

Question

I have a django service (say 'brisbane') that is pushing updates to client browsers when database models are saved using channels and signals thus:

def invalidated_serialized_model_receiver(self, sender, **kwargs):
    ...
    async_to_sync(get_channel_layer().group_send)(name, update)

This works and gives nice real time updates.

'brisbane' now needs to interact with another service (same code) 'sydney' so that it can be similarly updated in real time with respect to changes in sydney's data. This uses a consumer running in another process looking something like this:

async def remote_site_consume(site):
    socket_url = site.urn(protocol='ws', resource_name='/ws/watch/')
    async with websockets.connect(socket_url) as websocket:
        await websocket.send(json.dumps({'type': 'watch_messages'}))
        ...
        async for event in websocket:
            event = json.loads(event)
            await get_event_handler(event)(site, websocket, event)

The signal may get sent legitimately from within the event handler which is where the problem occurs. When this happens, it throws a RuntimeError

'You cannot use AsyncToSync in the same thread as an async event loop - just await the async function directly.'

I can't just use await though because the signal is sent from threads with no event loop as well.

Paul Whipp · Accepted Answer · 2018-12-13 01:44:05Z

1

I'm trying with this replacement for async_to_sync which seems to work in local tests at least:

def invalidated_serialized_model_receiver(self, sender, **kwargs):
    ...
    create_or_use_loop(self.channel_layer.group_send(name, update))


def create_or_use_loop(awaitable):
    try:
        event_loop = asyncio.get_event_loop()
    except RuntimeError:
        event_loop = None

    if event_loop and event_loop.is_running():
        event_loop.call_soon_threadsafe(event_loop.create_task, awaitable)
    else:
        loop = asyncio.new_event_loop()
        asyncio.set_event_loop(loop)
        try:
            loop.run_until_complete(awaitable)
        finally:
            loop.close()
            asyncio.set_event_loop(event_loop)

This feels very clunky though.

edited Dec 13, 2018 at 1:44

answered Dec 12, 2018 at 4:39

Paul Whipp

16.7k6 gold badges49 silver badges57 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How can I avoid django channels runtime errors when using signals?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related