Forum: Ruby Append (<<) to Array attributes of DRb objects?

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Jay McGavren (Guest)
on 2007-07-30 03:42
(Received via mailing list)
When working locally with an object whose attribute is meant to store
an array, I have no problem making calls like:

object.items << new_item

If I try to do the same thing with a DRb object, though, appending has
no effect on the remote object.  I've distilled the problem down to
this example...


My server:

require 'drb'
class TestServer
attr_accessor :items
def initialize
  @items = []
end
def add_item(item)
  @items << item
end
def add
  sum = 0
  @items.each {|item| sum += item}
  sum
end
end
server = TestServer.new
DRb.start_service('druby://localhost:9000', server)
DRb.thread.join


My client:

require 'drb'
DRb.start_service()
obj = DRbObject.new(nil, 'druby://localhost:9000')
obj.items << 1 #No effect.
obj.items << 2 #No effect.
items = [1, 2]
obj.items = items #Works.
obj.add_item(1) #Works.
obj.add_item(2) #Works.
puts obj.add #Gives '6' instead of '9'.


I'd rather my client programs be able to use append (<<) instead of an
add_item(item) method or direct assignment.  How can I do this?
Joel VanderWerf (Guest)
on 2007-07-30 03:54
(Received via mailing list)
Jay McGavren wrote:
> When working locally with an object whose attribute is meant to store
> an array, I have no problem making calls like:
>
> object.items << new_item
>
> If I try to do the same thing with a DRb object, though, appending has
> no effect on the remote object.

The #items method returns an Array. That Array is not DRbUndumped (i.e.,
not a proxy object), so it gets _copied_ back to the client. Modifying
that copy in the client can't affect the original in the server.

Your #add_item method is a good way around this.

Or, you could extend the array with DRbUndumped, to keep it on the
server.
Phlip (Guest)
on 2007-07-30 03:58
(Received via mailing list)
Jay McGavren wrote:

> When working locally with an object whose attribute is meant to store
> an array, I have no problem making calls like:
>
> object.items << new_item
>
> If I try to do the same thing with a DRb object, though, appending has
> no effect on the remote object.  I've distilled the problem down to
> this example...

The BackgrounDRb mailing list will give you better odds of a good
answer.

In general, I treat my background threads with the strictest isolation,
and
I don't go passing arrays back and forth. Consider the marshalling that
must
occur just for one stinkin' <<. So I don't know the answer to your
question,
even though I have used BackgrounDRb on 2 projects so far...
Chris C. (Guest)
on 2007-07-30 19:35
(Received via mailing list)
On 7/29/07, Phlip <removed_email_address@domain.invalid> wrote:
> The BackgrounDRb mailing list will give you better odds of a good answer.
>
> In general, I treat my background threads with the strictest isolation, and
> I don't go passing arrays back and forth. Consider the marshalling that must
> occur just for one stinkin' <<. So I don't know the answer to your question,
> even though I have used BackgrounDRb on 2 projects so far...
>
> --
>   Phlip

Note that he is not using BackgroundDRb, but DRb itself, so I believe
this list is the most appropriate.

You should probably just extend the array with DRbUndumped, that will
(IIRC) speed up the appending, and allow << to work properly.
Jay McGavren (Guest)
on 2007-07-31 00:36
(Received via mailing list)
On Jul 30, 8:33 am, "Chris C." <removed_email_address@domain.invalid> wrote:
> You should probably just extend the array with DRbUndumped, that will
> (IIRC) speed up the appending, and allow << to work properly.

Thusly, yes?

Server:

  require 'drb'
  class UndumpedArray < Array
    include DRbUndumped
  end
  class TestServer
    attr_accessor :items
    def initialize
      @items = UndumpedArray.new
    end
    def add
      sum = 0
      @items.each {|item| sum += item}
      sum
    end
  end
  server = TestServer.new
  DRb.start_service('druby://localhost:9000', server)
  DRb.thread.join

Client:

  require 'drb'
  DRb.start_service()
  obj = DRbObject.new(nil, 'druby://localhost:9000')
  obj.items << 1
  obj.items << 2
  puts obj.add #Gives '3'.

Seems to work pretty well.  DRbUndumped may be the solution to my
concerns about network overhead as well.

Thanks to everyone for the assistance!

-Jay
Eric H. (Guest)
on 2007-07-31 03:56
(Received via mailing list)
On Jul 30, 2007, at 13:35, Jay McGavren wrote:

>     include DRbUndumped
>   end
>   class TestServer
>     attr_accessor :items
>     def initialize
>       @items = UndumpedArray.new

Get rid of UndumpedArray.

@items = Array.new
@items.extend DRbUndumped
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