From: Ken Bloom <kbloom@...>
Date: 2009-05-21T01:10:04+09:00
Subject: Re: mysql vs. fork

On Wed, 20 May 2009 22:14:53 +0900, Daniel DeLorme wrote:

> It's commonly known that database connections do not play well with
> Process.fork; I thought that using at_exit{exit!} was the correct way to
> solve this problem, but it appears imperfect. Take this pseudo-code:
> 
> 1  db1 = mysql_connect()
> 2  Process.fork do
> 3    at_exit{exit!}
> 4    db2 = mysql_connect()
> 5    db2.query(...)
> 6  end
> 7  Process.wait
> 8  db1.query(...)
> 
> With line 3, only the at_exit handlers defined in the child process are
> run when the child exits, ensuring that db1 stays alive and line 8 keeps
> working.
> 
> At least that's what I thought. But in the past 2 days I've been getting
> some "MySQL server has gone away" errors from our servers, indicating
> that at_exit{exit!} is not enough to protect the db1 connection. It's
> not consistently reproducible; I can't get the error when directly
> testing but the flow of errormails is unmistakable. So what exactly was
> wrong in my understanding of at_exit, sockets, mysql, and fork?
> 
> -Daniel

One solution is to encapsulate the database connection in a DRb server. I 
have written about this in ruby-talk:296032 (see http://
blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/296032)

-- 
Chanoch (Ken) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/