Nice I accidentally deleted my reply and forum says “You’ve performed this action too many times. Please wait 23 hours before trying again.” when I try to undelete. I’m pretty sure I haven’t undeleted anything before and post will deleted 24 hours 
Here are those links again
https://github.com/elixir-plug/plug_cowboy/issues/10
https://github.com/ninenines/cowboy/issues/1376
I think that’s temporary solution because if you have multiple streaming connections that mailbox might be filling from all of them and checking mailbox size might not be a very good solution. Hopefully this gets fixed in Cowboy 2.7. I read that Cowboy 1.x isn’t effected because chunk is synchronous function in 1.x. Maybe you could also try downgrading to 1.x in the meantime, if that’s possible with Phoenix.
Once people start with streams it’s common to think they need to keep using a stream. The code should be:
url
|> HttpStream.stream()
|> Enum.reduce(conn, fn n, conn -.
chunk(conn, n)
end)
This ensures that the controller action returns a conn, and it returns specifically the conn that has sent all the chunks, which is what you want. It also avoids accumulating all the data in memory.
HttpStream still needs to be implemented in a way that doesn’t overload the process with messages though.
It seems that you didn’t read this thread before replying. Problem doesn’t have anything to do with that code, there is a bug in Cowboy 2.x that is causing this. Look at my earlier post with links. Calling chunk pushed messages to cowboy_clear:connection_process’s mailbox and returns. That means there is no back pressure like in Cowboy 1.x where call to chunk was a synchronous. Mailbox keeps filling up because file chunks are coming in fast cowboy_clear:connection_process process gets slow and huge amount memory is allocated.
If you want to try it yourself @no_one provided code in Github in some posts back.
Also are you sure it’s ok to use
Enum.reducewith streams? I’m pretty sure that will load whole file in to memory because it will create intermediate list as you can see in examples here Stream — Elixir v1.20.2
Edited: I think I was wrong and using Enum.reduce doesn’t cause any intermediate list to be created because it returns a single value so it’s probably OK in that code.
Tried your code and downgraded {:plug_cowboy, "~> 2.0"} to {:plug_cowboy, "~> 1.0"} in deps and it seems to fix this problem and memory doesn’t increase anymore.