= nng_respondent(7)
:doctype: manpage
:manmanual: nng
:mansource: nng
:copyright: Copyright 2018 mailto:info@staysail.tech[Staysail Systems, Inc.] + \
	    Copyright 2018 mailto:info@capitar.com[Capitar IT Group BV] + \
	    {blank} + \
	    This document is supplied under the terms of the \
	    https://opensource.org/licenses/MIT[MIT License].

== NAME

nng_respondent - respondent protocol

== SYNOPSIS

[source,c]
----------
#include <nng/protocol/survey0/respond.h>

int nng_respondent0_open(nng_socket *s);
----------

== DESCRIPTION

The _nng_respondent_ protocol is one half of a survey pattern.
In this pattern, a surveyor sends a survey, which is broadcast to all
peer respondents.  The respondents then have a chance to reply (but after
not obliged to).  The survey itself is a timed event, so that responses
received after the survey has finished are discarded.

TIP: This protocol is useful in solving voting problems, such as leader
election in cluster configurations, as well as certain kinds of service
discovery problems.

The _nng_respondent_ protocol is the respondent side, and the
<<nng_surveyor#,nng_surveyor(7)>> protocol is the surveyor side.

=== Socket Operations

The `nng_respondent0_open()` call creates a respondent socket.  This socket
may be used to receive messages, and then to send replies.  Generally
a reply can only be sent after receiving a survey, and generally the
reply will be sent to surveyor from whom the last survey was received.

Respondents may discard a survey by simply not replying to it. 

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.  An earlier and
incompatible version of the protocol was used in older pre-releases of
http://nanomsg.org[nanomsg], but was not released in any production
version.)

=== 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_respondent_ 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 survey, and when replying removes
elements to obtain the next hop information.  The survey ID is at the
end of this header and is inserted into the header as its first element
by the originating surveyor.  (Survey IDs are distinguished from hops by
having their high order bit set to one.)

// TODO: Insert reference to RFC.

    
== SEE ALSO

<<nng#,nng(7)>>,
<<nng_surveyor#,nng_surveyor(7)>>

== COPYRIGHT

{copyright}
