SYNOPSIS
#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.
| 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(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 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.)