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Database Access¶
The Django ORM is a synchronous piece of code, and so if you want to access it from asynchronous code you need to do special handling to make sure its connections are closed properly.
If you’re using SyncConsumer, or anything based on it - like
JsonWebsocketConsumer - you don’t need to do anything special, as all your
code is already run in a synchronous mode and Channels will do the cleanup
for you as part of the SyncConsumer code.
If you are writing asynchronous code, however, you will need to call
database methods in a safe, synchronous context, using database_sync_to_async.
Database Connections¶
Channels can potentially open a lot more database connections than you may be used to if you are using threaded consumers (synchronous ones) - it can open up to one connection per thread.
If you wish to control the maximum number of threads used, set the
ASGI_THREADS environment variable to the maximum number you wish to allow.
By default, the number of threads is set to “the number of CPUs * 5” for
Python 3.7 and below, and min(32, os.cpu_count() + 4) for Python 3.8+.
To avoid having too many threads idling in connections, you can instead rewrite your code to use async consumers and only dip into threads when you need to use Django’s ORM (using database_sync_to_async).
database_sync_to_async¶
channels.db.database_sync_to_async is a version of asgiref.sync.sync_to_async
that also cleans up database connections on exit.
To use it, write your ORM queries in a separate function or method, and then
call it with database_sync_to_async like so:
from channels.db import database_sync_to_async
async def connect(self):
self.username = await database_sync_to_async(self.get_name)()
def get_name(self):
return User.objects.all()[0].name
You can also use it as a decorator:
from channels.db import database_sync_to_async
async def connect(self):
self.username = await self.get_name()
@database_sync_to_async
def get_name(self):
return User.objects.all()[0].name