From: Robert Klemme <shortcutter@...>
Date: 2009-11-25T16:15:05+09:00
Subject: Re: DRb::DRbObject.alive?

On 11/25/2009 12:04 AM, Benoit Daloze wrote:
> I found a feature that seem to be missing to me in DRb.
> 
> I'm just creating a chat with a client and a server, using only DRb to
> interact between them.
> 
> My problem is I didn't found a nice way to verify if a client is alive.
> I would like to be able to update my clients' list, but I need to know if
> they are alive.
> 
> Currently, I'm getting around with catching the Exception DRb::DRbConnError,
> but that looks really ugly:
> @clients.each { |c|
>   begin
>     c.respond_to?(:some_method_defined_in_client) #Trying to use any method
> of c will raise the exception (except if not defined)
>   rescue DRb::DRbConnError
>     @clients.delete(c)
>   end
> }
> 
> Using Exceptions would be meaningful when I try to send(it would raise the
> exception if I call #send), but then I would have to manage Exceptions
> everywhere.

Well, but this is the usual approach.

> The method alive? is defined for the DRbServer and DRbTCPSocket, but not for
> DRbObject! (in fact if the Client just got disconnected, alive? will still
> return true)

It should not be too difficult to add method alive? to DRbUndumped. 
However, there is a more fundamental problem that cannot be solved: even 
if you have method alive? and it is working properly what does it tell 
you?  It tells you that the object was alive when you invoked that 
method.  You have no guarantees that it will be alive when you make the 
next remote method call.  Hence you need to handle exceptions anyway. 
Now you have exception handling plus additional code that checks 
liveness beforehand.  Your code is complexer and slower.

> Does somebody already met this?

This is a common issue people stumble over all the time (typically with 
things that involve interprocess communication such as for database 
connections and the like).  The issue is always the same: you do not 
gain anything with this method because you have to deal with failure anyway.

Kind regards

	robert

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