= 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>
----

== DESCRIPTION

(((protocol, _req_)))
The ((_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.)

(((load-balancing)))
The requester generally only has one outstanding request at a time unless
in raw mode (via
xref:nng_options.5.adoc#NNG_OPT_RAW[`NNG_OPT_RAW`]),
and it will generally attempt to spread work requests to different peer repliers.

TIP: This property, when combined with xref:nng_device.3.adoc[`nng_device()`]
can help provide a degree of load-balancing.

The _req_ protocol is the requester side, and the
xref:nng_rep.7.adoc[_rep_] protocol is the replier side.

=== Socket Operations

The xref:nng_req_open.3.adoc[`nng_req0_open()`] functions create 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.)

Furthermore, only a single receive operation may be pending at a time.
Attempts to post more receive operations concurrently will result in
`NNG_ESTATE`.

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.

xref:nng.7.adoc#raw_mode[Raw] mode sockets ignore all these restrictions.

=== Context Operations

This protocol supports the creation of xref:nng_ctx.5.adoc[contexts] for concurrent
use cases using xref:nng_ctx_open.3.adoc[`nng_ctx_open()`].

The `NNG_OPT_REQ_RESENDTIME` value may be configured differently
on contexts created this way.

Each context may have at most one outstanding request, and operates
independently from the others.

The restrictions for order of operations with sockets apply equally
well for contexts, except that each context will be treated as if it were
a separate socket.

=== 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 option is available.

((`NNG_OPT_REQ_RESENDTIME`))::

   (xref:nng_duration.5.adoc[`nng_duration`])
   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.)

=== Protocol Headers

(((backtrace)))
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 preceding 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
xref:nng_device.3.adoc[`nng_device()`]), 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 requester, 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

[.text-left]
xref:nng_ctx_open.3.adoc[nng_ctx_open(3)],
xref:nng_device.3.adoc[nng_device(3)],
xref:nng_req_open.3.adoc[nng_req_open(3)],
xref:nng_ctx.5.adoc[nng_ctx(5)],
xref:nng.7.adoc[nng(7)],
xref:nng_rep.7.adoc[nng_rep(7)]
