PAB's blog

HAProxy in front of Unicorn

By Pierre-Alain B on Sunday, May 6 2018, 07:19 - Permalink

Unicorn, the "Rack HTTP server for fast clients", is really for "fast clients". Fast clients mean clients on the same local network (or even same machine) with very low latency and near-zero chance of packets loss. Slow clients are the usual web users: far away from the server. Unicorn has been designed to serve fast clients and if you try to use it to serve slow clients then the performance may be dismal. This is why the Unicorn developers recommend to use Nginx in front of Unicorn. To know more on the Unicorn philosophy, visit this page.

I recently tried to use HAProxy in front of Unicorn and was disappointed to see that:

  • the system was slow and unresponsive
  • a lot of 502 Gateway errors popped up for seemingly no reason (and this popped up unconsistently)

I came to the conclusion that the default configuration of HAProxy was not appropriate for Unicorn. After some web digging, I discovered the "http-buffer-request" option.

Here is what the HAProxy 1.8 documentation says about the "http-buffer-request" option :

It is sometimes desirable to wait for the body of an HTTP request before taking a decision. This is what is being done by "balance url_param" for example. The first use case is to buffer requests from slow clients before connecting to the server. Another use case consists in taking the routing decision based on the request body's contents. This option placed in a frontend or backend forces the HTTP processing to wait until either the wholebody is received, or the request buffer is full, or the first chunk is complete in case of chunked encoding. It can have undesired side effects with some applications abusing HTTP by expecting unbuffered transmissions between the frontend and the backend, so this should definitely not be used by default.

It seems to be the best fit with Unicorn's philosophy! Let's activate the option in each backend corresponding to a Unicorn-run application:

backend unicorn-app
	option http-buffer-request
	server unicorn-app 1.2.3.4:3000

and restart HAProxy:

/etc/init.d/haproxy reload