Why can't I return "Threads.list" Array in DRb?

Ruby
#1

Hi, I can’t understand why I can’t return the current “Threads.list”
array in
a DRb server method.

  • In the DRb server object I just do:

    def current_threads
    Thread.list
    end

  • And the DRb client calls:

    drb_client.current_threads

but instead of an Array object I get a DRb::DRbObject, and in each call
to
this method I get a different “id”:

irb> drb_client.current_threads
=> #<DRb::DRbObject:0xb7b8d384 @ref=-608642478,
@uri=“druby://0.0.0.0:10001”>
irb> drb_client.current_threads
=> #<DRb::DRbObject:0xb7b8b675 @ref=-608644410,
@uri=“druby://0.0.0.0:10001”>

Of course, I’m 100% sure that “Thread.list” returns an Array. If I
return any
other Array all works ok:

  • DRb server:

    def current_threads
    [1,2,3,4,5]
    end

irb> drb_client.current_threads
[1,2,3,4,5]

Why does this issue occur when returning “Thread.list”?

Thanks for any help.

#2

El Domingo, 29 de Junio de 2008, Iñaki Baz C. escribió:

but instead of an Array object I get a DRb::DRbObject, and in each call to
this method I get a different “id”:

irb> drb_client.current_threads
=> #<DRb::DRbObject:0xb7b8d384 @ref=-608642478,
@uri=“druby://0.0.0.0:10001”>
irb> drb_client.current_threads
=> #<DRb::DRbObject:0xb7b8b675 @ref=-608644410,
@uri=“druby://0.0.0.0:10001”>

Opsss, and more extrange:
Note the following:

irb> drb_client.current_threads
=> #<DRb::DRbObject:0xb7b8d384 @ref=-608642478,
@uri=“druby://0.0.0.0:10001”>

irb> puts drb_client.current_threads
#Thread:0xb7c8f700
#Thread:0xb76f9598
#Thread:0xb76f9a98
#Thread:0xb76f9890
#Thread:0xb76f91b0
=> nil

How is it possible? why “puts” gets this?

irb> puts drb_client.current_threads.class
DRb::DRbObject
=> nil

#3

2008/6/29 Iñaki Baz C. [email protected]:

Of course, I’m 100% sure that “Thread.list” returns an Array. If I return any

Why does this issue occur when returning “Thread.list”?

Threads are local to a process so it does not make any sense to
transfer them to another process.

What are you trying to achieve?

Cheers

robert

#4

On Jun 29, 7:00 pm, Iñaki Baz C. [email protected] wrote:

El Domingo, 29 de Junio de 2008, Iñaki Baz C. escribió:

El Domingo, 29 de Junio de 2008, Iñaki Baz C. escribió:

but instead of an Array object I get a DRb::DRbObject, and in each call
to this method I get a different “id”:

Well, I am understanding that if DRb server return an object unknown for the
DRb client, the the only way the client hast to use/show it is as raw value,
is it?

It seems more like a situation where DRb detects that the return
object (the Array) is not fully serializable (contains non-
derializable objects), and hence it returns a “server reference” or
“proxy object” for the object instead. Whenever you invoke method on
the proxy object, they are invoked on the server.

This is really the beauty of DRb: Objects that can’t be marshalled /
serialized, become remote objects automatically. You can even ‘yield’
to blocks on the client, which is what happens when you “puts” the
array.

Here’s a simpler example:

class Thing
include DRb::DRbUndumped # make sure instance are not marshalled and
transferred
def each
puts “each called on server”
3.times { |i| yield(i) }
end
end
class Server
def create_thing
Thing.new
end
end
DRb::start_service(“druby://:12000”, Server.new)
DRb.thread.join

And a client:

DRb.start_service
remoteServer = DRb::DRbObject.new(nil, “druby://localhost:12000”)
remoteThing = remoteServer.create_thing
remoteThing.each { |i| p i }

The ‘remoteThing’ becomes a DRb::DRbObject which serves as a proxy to
the Thing object on the server. Thing#each is actally run on the
server and yields back to the block on the client.

Lars

#5

El Domingo, 29 de Junio de 2008, Iñaki Baz C. escribió:

Opsss, and more extrange:
#Thread:0xb76f9890
#Thread:0xb76f91b0
=> nil

How is it possible? why “puts” gets this?

irb> puts drb_client.current_threads.class
DRb::DRbObject
=> nil

Well, I am understanding that if DRb server return an object unknown for
the
DRb client, the the only way the client hast to use/show it is as raw
value,
is it?

#6

El Lunes, 30 de Junio de 2008, Robert K.
escribió:> Threads are local to a process so it does not make any sense to

transfer them to another process.

What are you trying to achieve?

My projects binds for TCP connections (threads) and I want to monitorize
them,
so I get them via DRb.

Anyway I’ve changed the data transferred: instead of “Thread.list” (that
is
not serializable) I do something as:
Thread.list.map {|t| {“id”=>t.id, “status”=>t.status … }

I have enough with that :slight_smile:

#7

El Lunes, 30 de Junio de 2008, Lars C.
escribió:> The ‘remoteThing’ becomes a DRb::DRbObject which serves as a proxy to

the Thing object on the server. Thing#each is actally run on the
server and yields back to the block on the client.

Really espectacular :slight_smile: