Django ASGI and Synchronous View Timeout Issues

Based on the information available, I can provide some insights into your issue with Daphne and synchronous views.

Understanding the Problem

When migrating from WSGI to ASGI (using Daphne 4.1.2 with Django 4.2.16), you're experiencing deadlocks in Daphne's main thread when synchronous views time out. This is likely related to how ASGI servers handle synchronous code.

Why Async Views Work Better

Asynchronous views work better in this scenario because:

1. In an ASGI environment, synchronous views are run in a thread pool to prevent blocking the main event loop
2. When a synchronous view times out, the thread handling it might not be properly released, causing resource exhaustion

Regarding Your Questions

Does Daphne not support timeout handling for synchronous view functions?

Daphne does support synchronous views, but timeout handling for synchronous code is more complex in an asynchronous environment. When Django runs synchronous views under ASGI, it uses asgiref.sync.sync_to_async internally, which runs the synchronous code in a thread pool. If a timeout occurs, proper cleanup of these threads depends on how the timeout is implemented.

Alternative Solutions

If you want to keep your synchronous views:

1. Implement middleware timeout handling: Create custom middleware that can monitor and terminate long-running requests before they cause deadlocks

2. Use sync_to_async with cancellation support: If you must keep synchronous views, you could wrap critical sections with properly configured sync_to_async:

   PYTHONCopied
   from asgiref.sync import sync_to_async
   from django.http import HttpResponse
   
   async def view_wrapper(request):
       # Use sync_to_async with cancellation support
       result = await sync_to_async(
           your_sync_function, 
           thread_sensitive=True,
           cancellable=True
       )(request)
       return result

3. Gradually migrate to async views: Since you've observed that async views resolve the issue, consider gradually converting your most problematic views to async first:

   PYTHONCopied
   async def my_view(request):
       # Async implementation
       return HttpResponse("Hello, async world!")

4. Configure Daphne's timeout settings: Ensure Daphne is configured with appropriate timeout settings for your application needs

Important Note

As mentioned in the Django documentation, you'll only get the full benefits of asynchronous processing if you have no synchronous middleware in your stack. If you have synchronous middleware, Django must use a thread per request to safely emulate a synchronous environment, which could contribute to your deadlock issues.

If you decide to keep synchronous views, you should carefully monitor thread usage and implement proper timeout handling at the application level to prevent resource exhaustion.