= nng_req(7)
//
// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
// Copyright 2018 Capitar IT Group BV <info@capitar.com>
//
// This document 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

This protocol uses a _backtrace_ in the header.  This
form uses a "stack" of 32-bit big-endian identifiers.  There *must* be
at least one identifier, the __request ID__, which will be the last
element in the array, and *must* have the most significant bit set.

There may be additional __peer ID__s preceeding the request ID.  These
will be distinguishable from the request ID by having their most
significant bit clear.

When a request message is received by a forwarding node (see
<<nng_device#,nng_device(3)>>), the forwarding node prepends a
32-bit peer ID (which *must* have the most significant bit clear),
which is the forwarder's way of identifying the directly connected
peer from which it received the message.  (This peer ID, except for the
most significant bit, has meaning only to the forwarding node itself.)

It may help to think of prepending a peer ID as "pushing" a peer ID onto the
front of the stack of headers for the message.  (It will use the peer ID
it popped from the front to determine the next intermediate destination
for the reply.)

When a reply message is created, it is created using the same headers
that the request contained.

A forwarding node can "pop" the peer ID it originally pushed on the
message, stripping it from the front of the message as it does so.

When the reply finally arrives back at the initiating requestor, it
should have only a single element in the message, which will be the
request ID it originally used for the request.

// TODO: Insert reference to RFC.

== SEE ALSO

<<nng_device(3)#,nng_device(3)>>,
<<nng#,nng(7)>>,
<<nng_rep#,nng_rep(7)>>
