From: Brian Candler <B.Candler@...>
Date: 2003-08-08T05:30:07+09:00
Subject: Re: [DRb] Using DRb to implement object database


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On Fri, Aug 08, 2003 at 04:36:42AM +0900, Steve Tuckner wrote:
> I had some trouble understanding your solution, so I will attempt to explain
> how PStore works and what the exact problem is. PStore is just a set of
> marshalled objects on the server's local disk. To store the objects on disk,
> just create the pstore
> 
> require "pstore"
> 
> store = PStore.new("test.db")
> store.transaction do
> 	store["objects"] = object_list	# where object_list is the array of objects
> in the database

Sorry if I wasn't clear :-)

So 'store' looks like a hash?  store[key] = some_object

If you update store[] or one of the objects it refers to, is there anything
you need to do to write it back to disk? Hmm, probably that's what the
transaction<block> syntax achieves.

OK, I've had a play. The attached code hopefully demonstrates what I was
trying to say.

DRb provides a remote 'facade' to a single object on a particular port, 9000
in this case. So you create a single object which contains a set of methods
which do all the different work parcels you want - including invoking
methods on other objects if necessary.

That's unless you want a separate DRb front-end to each individual Foo
object, which would mean binding them to separate TCP ports. That's not an
approach I would recommend unless you've only got two or three objects you
want to access remotely.

If you decided to make the PStore object itself available by DRb, then the
only methods you would get would be those methods which PStore itself
provides - such as [] and []= to fetch and set the contained objects. And
running over DRb, in its normal operation, you would get *copies* of those
objects. Updating state on a local copy of an object will do nothing on the
server side of course; you would have to explicitly overwrite the old object
with the new one using []=.

> When I do that however foo can't be dumped because its _dump function has
> been replaced by a function that raises a TypeError exception (and produces
> the following output).

I don't know anything about DRbUndumped (have you got a reference to a
description? [*]) Standard DRb just marshals a method call and its arguments
across a TCP stream, and marshals the method result back over the stream. So
it's like a local method call except the arguments and the result are
'dup'ed (deep-copied, actually)

I don't know anything about 'Linda' or Java either, so Rinda doesn't mean
much to me.

If DRbUndumped automatically puts in some kind of proxy-object for each
remote object on the server, then that would be very cool indeed. But then I
see your problem with PStore, which relies on being able to dump objects to
get them to disk. Have you tried just commenting out the _dump function? It
looks like DRb only tests whether the module has been included:

      obj = DRbObject.new(obj) if obj.kind_of? DRbUndumped

Hence it probably doesn't actually need _dump to be overridden.

Regards,

Brian.

[*] In the source tarball there's a reference in doc/drb-ref.rd.ja , but the
text in that file looks like it has been marshaled in a strange binary
format that I'm not able to read :-)

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class Foo
    attr_accessor :x,:y

    def initialize(x,y)
        @x, @y = x,y
    end
end

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require 'pstore'
require 'drb'
require 'foo'

# Note: this is probably not thread-safe as written (unless
# PStore#transaction has a serialisation mechanism of its own)

class StoreApplication
  def initialize
    @store = PStore.new("test.db")
    @store.transaction do
      @store["objects"] = [Foo.new(1,2), Foo.new(3,4), Foo.new(5,6)]
    end
  end
  def setx(index, xval)
    @store.transaction do
      o = @store["objects"]
      o[index].x = xval
    end
  end
  def getx(index)
    @store.transaction do
      @store["objects"][index].x
    end
  end
end

app = StoreApplication.new
DRb.start_service('druby://0.0.0.0:9000', app)
DRb.thread.join

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require 'drb'
require 'pstore' # so any pstore exceptions are reported correctly
require 'foo'
DRb.start_service
app = DRbObject.new(nil, 'druby://localhost:9000')

p app.getx(0)
app.setx(0,99)
p app.getx(0)
app.setx(0,33)
p app.getx(0)

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