From: Joel VanderWerf <vjoel@...>
Date: 2008-03-27T12:14:07+09:00
Subject: Re: fork

Joey Marino wrote:
> I see,
> I have a class to handle interfacing to the database. The initialization of
> the class will connect to the db and another method will close the
> connection and there are query functions for handling the queries and
> results. This object is then passed into new objects on their
> initialization. The problem comes in when I iterate through a set of objects
> creating a fork for each one. After the first object I am getting a
> "database went away error". My code looks like the following:
> 
> db = MySQL.new
> 
> fork do
>   obj1 = Object1.new(db)
>   obj1.addToDb

At this point in the child, the process is about to exit, the db object 
in the child is freed, and (my guess) a finalizer is executed which 
closes the database connection.

> end
> Process.wait
> 
> fork do
>   obj2 = Object2.new(db)   #Object1 and Object2 are inherited from the same
> superclass
>   ob2.addToDb   #db error occurs here

...which is the same as the connection you're trying to use here (same 
connection because of the fork), through _this_ copy of the db object.

But that's only my guess. I don't know how the MySQL ruby interface 
works, or if there is a way to tell it not to close the connection when 
the child exits.

Possibly informative:
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/117013

If you really need the forks to access the database, they could start 
new connections. If there's a performance problem with that, you could 
keep a pool of child processes and send them commands from the parent. 
(The slave gem might be a good way to do that.)

> end
> Process.wait
> 
> db.closeDB



-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407