Can someone help me understand the concept of websocket “backpressure” in a 
Django Channels project? What is it? How do diagnose it? At what level of 
the stack does it occur? How do I cure it? The docs are a little hazy on 
this.


I wired up a quick Channels project for my mid-sized website. Before 
deploying the project, I load-tested it with thor 
<https://github.com/observing/thor> and started scaling up. When I reached 
two Daphne processes and four worker processes, it seemed like I had enough 
power behind the project to handle the load on my site. It was able to 
handle 2000 simultaneous websocket connections without errors, according to 
thor. That should have been more than enough.


I deployed, and everything went fine for a while. After a bit, though, the 
websockets got slow and the server started to drop connections. Eventually 
the whole project stalled out. I looked through the Daphne logs and found a 
flurry of lines like this:


2016-12-01 14:35:14,513 WARNING WebSocket force closed for 
> websocket.send!QbxCqPhvyxVt due to receive backpressure


I restarted all the server and worker processes to no effect. I was able to 
put the project back online by manually deleting all the “asgi:*” keys in 
Redis. But then, after a while, the backpressure built up and everything 
crashed again.


The problem, I suppose, has something to do with the high frequency of 
messages that were to be passed via websocket in this particular project. A 
click triggers a message in each direction, and people were encouraged to 
click rapidly. So I probably have to throttle this, or else launch more 
workers and/or servers.


But I'd like to know what, specifically, triggers these “backpressure” 
disconnections, and where I might look to monitor them /before/ errors 
start to occur. In one of the Redis queues, I suppose? If so, which one(s) 
– inbound or outbound? I suppose my idea, here, is that I might be able to 
automatically scale up if the queues start to fill up.


Thank you in advance. Fun project!

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