# DRb::DRbObject.alive?

**URL:** <https://www.ruby-forum.com/t/drb-drbobject-alive/179438>\
**Category:** Ruby\
**Created:** [November 25, 2009, 12:05am UTC](https://www.ruby-forum.com/t/drb-drbobject-alive/179438 "2009-11-25T00:05:08Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![benoitdinocourt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/87869e/32.png) [@benoitdinocourt](https://www.ruby-forum.com/u/benoitdinocourt)\
**Post date:** [November 25, 2009, 12:05am UTC](https://www.ruby-forum.com/t/drb-drbobject-alive/179438/1 "2009-11-25T00:05:08Z")

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Hi,

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©  
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.

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)

Does somebody already met this? (And does somebody knows what DRb is?^^  
Please have a look it’s pretty cool, but the documentation could even be  
better…)

Benoit

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<div class="post-metadata">

**Author:** ![benoitdinocourt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/87869e/32.png) [@benoitdinocourt](https://www.ruby-forum.com/u/benoitdinocourt)\
**Post date:** [November 25, 2009, 8:15am UTC](https://www.ruby-forum.com/t/drb-drbobject-alive/179438/2 "2009-11-25T08:15:35Z")

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On 11/25/2009 12:04 AM, Benoit D. wrote:

> but that looks really ugly:  
> 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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<div class="post-metadata">

**Author:** ![benoitdinocourt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/87869e/32.png) [@benoitdinocourt](https://www.ruby-forum.com/u/benoitdinocourt)\
**Post date:** [November 25, 2009, 10:48pm UTC](https://www.ruby-forum.com/t/drb-drbobject-alive/179438/3 "2009-11-25T22:48:50Z")

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Yeah, you’re right.

So here’s how I did to not duplicate the code:

```
 def try_on_clients
    @clients.each { |client|
        begin
            yield client
        rescue DRb::DRbConnError
            del_drb_client(client)
        end
    }
end

def receive_message(msg)
    try_on_clients { |client| client.show_message(msg) }
end

```

That looks already more beautiful, and doesn’t spend time looking if  
alive.  
Anyway, it’s a little too bad for the client, because any connection  
error  
will drop him away, but he can reconnect easily.

Regards,  
Benoit

2009/11/25 Robert K. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)

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<div class="post-metadata">

**Author:** ![benoitdinocourt](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/87869e/32.png) [@benoitdinocourt](https://www.ruby-forum.com/u/benoitdinocourt)\
**Post date:** [November 26, 2009, 10:17am UTC](https://www.ruby-forum.com/t/drb-drbobject-alive/179438/4 "2009-11-26T10:17:03Z")

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2009/11/25 Benoit D. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> ```
> end
> }
> 
> ```
> 
> end
> 
> def receive\_message(msg)  
> try\_on\_clients { |client| client.show\_message(msg) }  
> end
> 
> That looks already more beautiful, and doesn’t spend time looking if alive.  
> Anyway, it’s a little too bad for the client, because any connection error  
> will drop him away, but he can reconnect easily.

Well, you could place them in a closed list (or queue) or even return  
the array of closed / removed clients.

Kind regards

robert