nng_rep(7)
==========
:doctype: manpage
:manmanual: nng
:mansource: nng
:icons: font
:source-highlighter: pygments
:copyright: Copyright 2017 Garrett D'Amore <garrett@damore.org> \
            Copyright 2017 Capitar IT Group BV <info@capitar.com> \
            This software is supplied under the terms of the MIT License, a \
            copy of which should be located in the distribution where this \
            file was obtained (LICENSE.txt).  A copy of the license may also \
            be found online at https://opensource.org/licenses/MIT.

NAME
----
nng_rep - reply protocol

SYNOPSIS
--------

[source,c]
----------
#include <nng/protocol/reqrep0/rep.h>

int nng_rep0_open(nng_socket *s);
----------

DESCRIPTION
-----------

The _nng_rep_ protocol is one half of a request/reply pattern.
In this pattern, a requester sends a message to one replier, who
is expected to reply.  The request is resent if no reply arrives,
until a reply is received or the request times out.

TIP: This protocol is useful in setting up RPC-like services.  It
is also "reliable", in that a the requester will keep retrying until
a reply is received.

The _nng_rep_ protocol is the replier side, and the
<<nng_req.adoc#,nng_req(7)>> protocol is the requester side.

Socket Operations
~~~~~~~~~~~~~~~~~

The `nng_rep0_open()` call creates a requester socket.  This socket
may be used to receive messages (requests), and then to send replies.  Generally
a reply can only be sent after receiving a request.  (Attempts to receive
a message will result in `NNG_ESTATE` if there is no outstanding request.)

Attempts to send on a socket with no outstanding requests will result
in `NNG_ESTATE`. 

Raw mode sockets (set with `NNG_OPT_RAW`) ignore all these restrictions.

Protocol Versions
~~~~~~~~~~~~~~~~~

Only version 0 of this protocol is supported.  (At the time of writing,
no other versions of this protocol have been defined.)

Protocol Options
~~~~~~~~~~~~~~~~

The following protocol-specific options are available.

`NNG_OPT_MAXTTL`::

   Maximum time-to-live.  This option is an integer value
   between 0 and 255,
   inclusive, and is the maximum number of "hops" that a message may
   pass through until it is discarded.  The default value is 8.  A value
   of 0 may be used to disable the loop protection, allowing an infinite
   number of hops.

Protocol Headers
~~~~~~~~~~~~~~~~

The _nng_rep_ protocol uses a _backtrace_ in the header.  This
form uses an array of 32-bit big-endian identifiers, where the first
element in the array
identifies the local peer identifier to which the message will next be sent.
This is a hop-by-hop header where each element in a path adds routing
information to the end when sending a request, and when replying removes
elements to obtain the next hop information.  The request ID is at the
end of this header and is inserted into the header as its first element
by the originating surveyor.  (Request IDs are distinguished from hops by
having their high order bit set to one.  They are generated automatically
and randomly when a request is first issued.)

// TODO: Insert reference to RFC.

    
AUTHORS
-------
link:mailto:garrett@damore.org[Garrett D'Amore]

SEE ALSO
--------
<<nng.adoc#,nng(7)>>
<<nng_req.adoc#,nng_req(7)>>

COPYRIGHT
---------

Copyright 2017 mailto:garrett@damore.org[Garrett D'Amore] +
Copyright 2017 mailto:info@capitar.com[Capitar IT Group BV]

This document is supplied under the terms of the
https://opensource.org/licenses/LICENSE.txt[MIT License].
