[erlang-questions] Fwd: Question about registry semantics for the remote processes.

Tim Watson watson.timothy@REDACTED
Wed Jan 21 11:40:33 CET 2015

Can anyone shed any light on this??

On 14 January 2015 at 10:57, Alexander V Vershilov <
alexander.vershilov@REDACTED> wrote:

> Hello.
> I want to understand the reasons for the current semantics
> for saving remote processes in local registry.
> According to the specification for register function [1] it's
> impossible to save remote process in local registry:
>     Failure: badarg if PidOrPort is not an existing,
>     local process or port, if RegName is already in use,
>     if the process or port is already registered (already has a name),
>     or if RegName is the atom undefined.
> As a result it's not possible to construct expression Name ! Message,
> where Name is evaluated to a atom stored in registry that points
> to the remote process.
> Another approach is described in unified semantics paper (Section 2.3)
>     However, for uniformity, in this semantics names can be registered
>     for remote processes (i.e., register(name,pid) does not fail if pid
>     is a remote process), and registering a local process at a remote
>     node is supported too (using the operation register(node,name,pid)).
>     As a consequence, when a message is sent to a remote node using the
> syntax
>     {atom,node}!msg there is no guarantee that the process that should
> receive the mes-
>     sage is located at node; thus it may be necessary to relay the message
>     to a process on yet another node
> So I have two questions:
> 1. Were there any technical reasons for forbidding remote pids in local
>     registry as we have in current erlang, or it's a historical reasons?
> 2. If there are technical reasons for that, then why uniformed
> specification
>     introduces such behaviour, or it provides some mechanisms to solve
>     those problems?
> One can argue that the reason is monitoring, and case when we are sending
> to the remote process from registry that already dead, but I don't see any
> difference with sending to remote Pid that already died, semantics of all
> the functions and guarantees are the same.
> Thanks
> [1] http://www.erlang.org/doc/man/erlang.html#register-2
> [2] http://happy-testing.com/hans/papers/EW2010-UnifiedSemantics.pdf
> --
> Alexander
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