Django Channels doesn’t conform to Django middleware specifications

[carl-phili...@solvdapp.io]

After some digging, I was able to find the reason for why adding the Whitenoise middleware to my Channels-enabled project was causing a massive performance drop. 

It appears that for every inbound HTTP request, Channels initializes a new AsgiHandler. In turn, this means that the entire middleware stack is being loaded for every HTTP request. See below:

https://github.com/django/channels/blob/master/channels/routing.py#L45-L62

https://github.com/django/channels/blob/master/channels/http.py#L157-L183

This has two consequences:

1. Channels is causing middleware to fall out of conformity with Django documentation, which states that middleware’s __init__() is only called once, on server startup. 
2. Any middleware which performs more-than-trivial work in its __init__() causes massive latency on any incoming request.

The issue was diagnosed in:

https://github.com/django/channels/issues/1099

https://github.com/django/channels/issues/1121

In response to the diagnosis in django/channels #1121, Andrew Godwin says:

Hmm, ASGI middleware is loaded every request (#1099) so it's quite possible I made the same mistake with the Django middleware as the handler is loaded every request too. If so, someone should be able to patch that quite easily.

However, I feel that a patch to this issue would be non-trivial to create. 

If the AsgiHandler is refactored such that it receives scope upon __call__() rather than on __init__() (and is only initialized once), it no longer conforms to the tests defined in https://github.com/django/asgiref/blob/master/asgiref/testing.py#L17, as they require passing scope for initialization. Thus, the AsgiHandler would no longer fit the shape of an Asgi Application. 

I would attempt a fix for this, but I am unsure of how to approach it given the constraints of the problem. 

Perhaps someone more knowledgeable about Channels could chime in with their $0.02? I would love to devote some time to fixing this, but need some input before I do.

[John Obelenus]

Can I ask a dumb question? Why did you only discover this with adding Whitenoise, and not the default/common middleware stack that comes in django? Asking that question in another way: What is specific to whitenoise that this constant loading of middleware causes such a perf hit? This question in no way implies this should not be fixed, its just my curiosity (I'm in the exact same situation of running channels and whitenoise)

[John Obelenus]

Just finished reading both bug reports and I see the file loader lookup in whitenoise. This explains a lot actually. We are still on Channels 1, and when we tried an upgrade to Channels 2, but encountered terrible performance at load and had to undo it.

[Andrew Godwin]

Just to say that I've reopened the original issue in question as it's clear now this needs to be fixed. I won't be able to get to writing a fix for a bit, though, so if someone else wants to they should.

Andrew

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