From: Robert Klemme <bob.news@...>
Date: 2006-01-25T21:03:12+09:00
Subject: Re: threading sync

Mage wrote:
>           Hello,
>
> I was thinking of writing my first threaded app in Ruby. One thread
> should fetch data from the database, another should write it to a
> file.
>
> I would like to use an array, which has hash elements. I append new
> elements to the end, and I write to the file from the beginning
> (FIFO).
>
> My question is that do I really need mutex? Is appending a hash to the
> end of an array atomic, or not? How can I know which method is atomic?

In your case you probably better use a Queue which saves you the
synchronization overhead (the Queue does it internally). Something along
the lines of this (untested):

require 'thread'

QUEUE = Queue.new

threads = [
  Thread.new(QUEUE) do |queue|
    File.open("foo", "w") do |io|
      while x = queue.deq
        io.puts x
      end
    end
  end,
  Thread.new(QUEUE) do |queue|
    while rec = fetch_from_db()
      queue.enq rec
    end
    queue.enc false
  end
]

thread.each {|th|th.join}

Note, if you like you can increase the number of DB writer and / or file
reader threads. You can as well leave the db reader in the main loop if
you need only one. Depending on your situation you may need a different
end of processing detection.

Kind regards

    robert