Nginx + unicorn (rails) often gives "Connection refused" in nginx error log

At work we're running some high traffic sites in rails. We often get a problem with the following being spammed in the nginx error log:

2011/05/24 11:20:08 [error] 90248#0: *468577825 connect() to unix:/app_path/production/shared/system/unicorn.sock failed (61: Connection refused) while connecting to upstream

Our setup is nginx on the frontend server (load balancing), and unicorn on our 4 app servers. Each unicorn is running with 8 workers. The setup is very similar to the one GitHub uses.

Most of our content i开发者_开发技巧s cached, and when the request hits nginx it looks for the page in memcached and serves that it if can find it - otherwise the request goes to rails.

I can solve the above issue - SOMETIMES - by doing a pkill of the unicorn processes on the servers followed by a:

cap production unicorn:check (removing all the pid's)
cap production unicorn:start

Do you guys have any clue to how I can debug this issue? We don't have any significantly high load on our database server when these problems occurs..

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Something killed your unicorn process on one of the servers, or it timed out. Or you have an old app server in your upstream app_server { } block that is no longer valid. Nginx will retry it from time to time. The default is to re-try another upstream if it gets a connection error, so hopefully your clients didn't notice anything.

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I don't think this is a nginx issue for me, restarting nginx didn't help. It seems to be gunicorn...A quick and dirty way to avoid this is to recycle the gunicorn instances when the system is not being used, say 1AM for example if that is an acceptable maintenance window. I run gunicorn as a service that will come back up if killed so a pkill script takes care of the recycle/respawn:

start on runlevel [2345]
stop on runlevel [06]
respawn
respawn limit 10 5
exec /var/web/proj/server.sh

I am starting to wonder if this is at all related to memory allocation. I have MongoDB running on the same system and it reserves all the memory for itself but it is supposed to yield if other applications require more memory.

Other things worth a try is getting rid of eventlet or other dependent modules when running gunicorn. uWSGI can also be used as an alternative to gunicorn.