Is a client uploading a file a slow client from unicorn's point of view?

From: Jimmy Soho <jimmy.soho@gmail.com>
To: mongrel-unicorn@rubyforge.org
Subject: Is a client uploading a file a slow client from unicorn's point of view?
Date: Tue, 9 Oct 2012 11:39:45 +1100	[thread overview]
Message-ID: <CAHStS5iqBqJxoPZGEUaSHL2qSVf_tfZfbdedc_bRcjcTbcKwAQ@mail.gmail.com> (raw)

Hi All,

I was wondering what would happen when large files were uploaded to
our system in parallel to endpoints that don't process file uploads.
In particular I was wondering if we're vulnerable to a simple DoS
attack.

The setup I tested with was nginx v1.2.4 with upload module (v2.2.0)
configured only for location /uploads with 2 unicorn (v4.3.1) workers
with timeout 30 secs, all running on 1 small unix box.

In a few terminals I started this command 3 times in parallel:

   $ curl -i -F importer_input=@/Users/admin/largefile.tar.gz
https://mywebserver.com/doesnotexist

In a browser I then tried to go a page that would be served by a unicorn worker.

My expectation was that I would not get to see the web page as all
unicorn workers would be busy with receiving / saving the upload. As
discussed in for example this article:
http://stackoverflow.com/questions/9592664/unicorn-rails-large-uploads.
Or as https://github.com/dwilkie/carrierwave_direct describes it:

  "Processing and saving file uploads are typically long running tasks
and should be done in a background process."

But I don't see this. The page is served just fine in my setup. The
requests for the file uploads appear in the nginx access log at the
same time the curl upload command eventually finishes minutes later
client side, and then it's handed off to a unicorn/rack worker
process, which quickly returns a 404 page not found. Response times of
less than 50ms.

What am I missing here? I'm starting to wonder what's the use of the
nginx upload module? My understanding was that it's use was to keep
unicorn workers available as long as a file upload was in progress,
but it seems that without that module it does the same thing.

Another question (more an nginx question though I guess): is there a
way to kill an upload request as early as possible if the request is
not made against known / accepted URI locations, instead of waiting
for it to be completely uploaded to our system and/or waiting for it
to reach the unicorn workers?

Cheers,
Jim
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