nng_req(7)
==========
:doctype: manpage
:manmanual: nng
:mansource: nng
:icons: font
:source-highlighter: pygments
:copyright: Copyright 2018 Garrett D'Amore <garrett@damore.org> \
            Copyright 2018 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_req - request protocol

SYNOPSIS
--------

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

int nng_req0_open(nng_socket *s);
----------

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

The _nng_req_ 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.

NOTE: Because requests are resent, it is important that they be idempotent
to ensure predictable and repeatable behavior even in the face of duplicated
requests, which can occur (for example if a reply message is lost for
some reason.)

The requester generally only has one outstanding request at a time unless
in "raw" mode (via `NNG_OPT_RAW`), and it will generally attempt to spread
work requests to different peer repliers.

TIP: This property, when combined with a <<nng_device#,device>> can
help provide a degree of load-balancing.

The _nng_req_ protocol is the requester side, and the
<<nng_rep#,nng_rep(7)>> protocol is the replier side.

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

The `nng_req0_open()` call creates a requester socket.  This socket
may be used to send messages (requests), and then to receive replies.  Generally
a reply can only be received after sending a request.  (Attempts to receive
a message will result in `NNG_ESTATE` if there is no outstanding request.)

Requests may be canceled by sending a different request.  This will
cause the requester to discard any reply from the earlier request,
but it will not stop a replier
from processing a request it has already received or terminate a request
that has already been placed on the wire.

Attempts to receive 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_REQ_RESENDTIME`::

   This read/write option is a duration (32-bit unsigned integer) representing
   a relative number of milliseconds. 
   When a new request is started, a timer of this duration is also started.
   If no reply is received before this timer expires, then the request will
   be resent.  (Requests are also automatically resent if the peer to whom
   the original request was sent disconnects, or if a peer becomes available
   while the requester is waiting for an available peer.)

`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_req_ 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.

    
SEE ALSO
--------
<<nng#,nng(7)>>,
<<nng_rep#,nng_rep(7)>>

COPYRIGHT
---------

Copyright 2018 mailto:garrett@damore.org[Garrett D'Amore] +
Copyright 2018 mailto:info@capitar.com[Capitar IT Group BV]

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