SYNOPSIS
#include <nng/protocol/survey0/survey.h>
int nng_surveyor0_open(nng_socket *s);DESCRIPTION
The nng_surveyor 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_surveyor protocol is the surveyor side, and the nng_respondent(7) protocol is the respondent side.
Socket Operations
The nng_surveyor0_open() call creates a respondent socket. This socket
may be used to send messages (surveys), and then to receive replies. Generally
a reply can only be received after sending a survey. Generally a surveyor
can expect to receive at most one reply from each responder. (Messages
can be duplicated in some topologies, so there is no guarantee of this.)
Attempts to receive on a socket with no outstanding survey will result
in NNG_ESTATE. If the survey times out while the surveyor is waiting
for replies, then the result will be NNG_ETIMEDOUT.
Only one survey can be outstanding at a time; sending another survey will cancel the prior one, and any responses from respondents from the prior survey that arrive after this will be discarded.
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_SURVEYOR_SURVEYTIME-
This read/write option is a duration (32-bit unsigned integer) representing a relative number of milliseconds that following surveys will last. When a new survey is started, a timer of this duration is also started. Any responses arriving this time will be discarded. Attempts to receive after the timer expires with no other surveys started will result in
NNG_ESTATE. Attempts to receive when this timer expires will result inNNG_ETIMEDOUT. 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_surveyor 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.)