From: Eric Wong <normalperson@yhbt.net>
To: unicorn list <mongrel-unicorn@rubyforge.org>
Subject: Re: after_fork and redis
Date: Mon, 1 Oct 2012 19:31:15 +0000 [thread overview]
Message-ID: <20121001193115.GA23907@dcvr.yhbt.net> (raw)
In-Reply-To: <CAEbKVFSAgG=-EhWeVMEGrG9_gLGPg1yS-KXoCpaB0rtJ7E9pPw@mail.gmail.com>
bradford <fingermark@gmail.com> wrote:
> I'm using unicorn w/ a rails app. I have the following in my
> environment.rb $redis = MyApplication::Application.config.redis and in
> production.rb I have config.redis = Redis.new(host: "localhost").
>
> I've read I'm supposed to $redis = Redis.new(host: "localhost") in
> after_fork when preload_app is true.
>
> When I don't do this, each worker/pid seems to have their own redis
> instance. So, why is this needed? Here's the logs of me printing out
> $redis.client.inspect when both $redis = Redis.new in the after_fork
> and just $redis = Redis.new in the environment.rb.
(Disclaimer: I still haven't used Redis, but similar knowledge applies
to every TCP-based service)
If a Redis client opens a TCP (or any stream) socket connection before
forking, all the children that are forked will share that _same_ local
TCP socket. Sharing stream sockets across processes/threads is nearly
always a bad idea: data streams become interleaved and impossible to
separate in either direction.
> No after fork
> https://gist.github.com/bc2c2a3bda01c35730e2
>
> After fork
> https://gist.github.com/0c96550660d3926ffe16
>
> The only thing different I notice is Connection::TCPSocket:fd is
> always 13 w/ the after fork.
FD shouldn't really matter. FDs get recycled ASAP once they're closed
(and GC can automatically close them). What you want is a different
local port for the TCPSocket on every worker process, and the after_fork
hook will give you that.
Btw, you can demonstrate FD recycling with:
require 'socket'
loop do
c = TCPSocket.new("example.com", 80)
p [ :fileno, c.fileno ]
p [ :addr, c.addr ] # (local address)
c.close
sleep 1 # be nice to the remote server
end
You'll see the same FD recycled over and over, but c.addr will
have a different local port.
You can see if after_fork is working correctly by checking the output of
tools like `ss' or `netstat':
(something like: ss -tan | grep -F :$PORT_OF_REDIS)
You can also check connections on the Redis server/process itself
using "lsof -p $PID_OF_REDIS_DAEMON"
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next prev parent reply other threads:[~2012-10-01 19:36 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-01 15:31 after_fork and redis bradford
2012-10-01 19:31 ` Eric Wong [this message]
2012-10-01 20:28 ` Ben Somers
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