From: Brian Candler <B.Candler@...>
Date: 2003-03-07T08:44:16+09:00
Subject: DRB and threads

I wonder if anyone can give me some hints on the interactions between dRuby
(drb) and threads.

I have been having a bit of a play, and it seems if you set up a dRuby
server in the way described in the Pickaxe, then each incoming request is
handled as a separate thread, but method calls are made to the *same*
object concurrently.

Example:

  # server.rb
  require 'drb'
  class SlowServer
    def doit(x)
      puts "Starting #{x}..."
      sleep 3
      puts "Finished #{x}"
      nil
    end
  end
  a = SlowServer.new
  DRb.start_service('druby://localhost:9000', a)
  DRb.thread.join

  # client.rb
  require 'drb'
  DRb.start_service    # not sure what this does for a client?
  obj = DRbObject.new(nil, 'druby://localhost:9000')
  10.times {
    obj.doit($$)
  }

Now, if I run two copies of client.rb concurrently as two separate
processes, they both only take 30 seconds to complete. The 'doit' method is
being run twice on the same object instance.

If I understand this rightly, then I can't see how you can use this with any
object which carries state unless you rewrite or wrap all the methods to
make them thread-safe. Is this correct? Unfortunately I can't find much
documentation in English, and I can't see if there's another way of running
it - even simple serialisation, so that method calls are handled one at a
time.

Anyway, the pattern I really want to implement is a pool of objects, with
the method call handed to any free object (and that object does not get
another request until it has finished handling the first one).

The aplication is that I have a database front-end class where each instance
keeps a DBI handle open (amongst other things) but each method call is
independent of the previous one, i.e. there's no state kept
*between* calls, other than inside the database itself of course. So
incoming requests can be farmed off to any object which is free.

Do I have to write a wrapper object which duplicates every method in my
database class (or method_missing)? Or is there a simpler way of handling
this?

Another solution would be if I could run a DRb server under FastCGI, so that
I had a pool of processes (maintained by Apache) and requests would be
farmed out to those processes. Is that possible?

Thanks,

Brian.

[P.S. I am using dRuby-2.0.2 with ruby-1.6.8]