[erlang-questions] Confused about the "rex" server

Raimo Niskanen <>
Mon May 12 09:58:02 CEST 2008

On Fri, May 09, 2008 at 05:29:46PM +0200, Alexander Lamb wrote:
> Hello list,
> I am a bit confused about references of what is called "rex server".
> I wrote a module with exported functions. I need to call these  
> functions from a JInterface. So as I discovered in the documentation,  
> I create a node and a mailbox (I will have several mailboxes in  
> separate threads).
> However, what I understood from a previous reply from Raimo, is that a  
> rpc is actually a message send with some particular way of giving the  
> parameters. In this message send, there is a reference to "rex". What  
> does that actually mean? I didn't find the documentation related to  
> this.

I am afraid the source code is the documentation in this case...

"rex" is the registered name of the Remote EXecution server
aka RPC server on every erlang node. It is part of the
Kernel application.

The Kernel application client code in the 'rpc' module
makes gen_server calls to that registered name. But there
is an alternative to gen_server calls that e.g C and
Java nodes use, and that is to send a message
{self(),{call,Mod,Fun,Args,Gleader}} to the registered
name "rex" on your desired node, and then wait for
a reply message {rex,Reply}.

> Now, on the receiving side, how do things happen?

Read the source code for the 'rpc' module, function handle_call/3
clause ({call,...}, ...):
The RPC server spawns a worker process for each
RPC call, and waits for it to return a reply that
the RPC server in its turn returns to the caller.

The worker proceess protects the RPC server from
odd things that may happen in the applied RPC function,
and frees the RPC server so serve new calls before
the previous is complete.

> Do I need to spawn a process for each call received? Or does "rex"  
> handle this for me? Or would it be better to send messages instead of  
> calling functions?

You can always customize a server. But if you only
want to call existing functions remotely, and you are
happy with the rather heavy footed and safe way
the RPC server does its calls, use it!

> Alex
> --
> Alexander Lamb
> Founding Associate
> 4 ch. de la Tour de Champel
> 1206 Geneva
> Switzerland
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/ Raimo Niskanen, Erlang/OTP, Ericsson AB

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