Unicorn hangs on POST request

Tom Pesman tom at tnux.net
Tue Apr 2 11:55:02 UTC 2013

> The request body doesn't seem to be there, presumably because Heroku
> isn't sending it.
> Doe heroku fully buffer the request body before sending it to unicorn?
> nginx fully buffers, and this is why I can only recommend nginx for slow
> clients.
> The proxy -> unicorn transfer speed should _never_ be dependent by the
> client -> proxy transfer speed.
> 1) client        ------------------ (slow) --------------> proxy
> 2) proxy (nginx) --- (as fast as the connection goes) ---> unicorn
> With nginx, 1) and 2) are decoupled and serialized.  This adds latency,
> but is the only way for multiprocess servers like unicorn to efficiently
> handle slow clients.

I've some new information. Heroku buffers the headers of a HTTP
request but it doesn't buffer the body of POST requests. Because of
that I switched to Rainbows! and the responsiveness of the application
increased dramatically.

Right now I'm using this configuration:

worker_processes 4
timeout 15
preload_app true

Rainbows! do
  use :EventMachine
  worker_connections 50
  client_max_body_size 5*1024*1024 # 5 megabytes
  client_header_buffer_size 2 * 1024 # 2 kilobytes
  timeout 10

With high load the performance drops, is EventMachine the right choice
for this situation (Rails application with slow POST requests and with
up to 50% POST requests)? Will increasing the worker_connections help?

I'm planning to make a blog post about this and tell Heroku not to
advise Unicorn for Rails applications but to use Rainbows and suggest
a correct/optimised configuration.



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