= nng_device(3)
//
// 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_sendmsg - send message

== SYNOPSIS

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

int nng_device(nng_socket s1, nng_socket s2);
----

== DESCRIPTION

The `nng_device()` function forwards messages received from one
<<nng_socket.5#,socket>> _s1_ to another socket _s2_, and vice versa.

This function is used to create forwarders, which can be used to create
complex network topologies to provide for improved ((horizontal scalability)),
reliability, and isolation.

The `nng_device()` function does not return until one of the sockets
is closed.

=== Reflectors

One of the sockets may be passed the special value `-1` (cast to an,
<<nng_socket.5#,`nng_socket`>> of course).
If this is the case, then the other socket must be valid, and must be
a protocol that is bidirectional and can peer with itself (such as
<<nng_pair.7#,_pair_>> or
<<nng_bus.7#,_bus_>>.)
In this case the device acts as a ((reflector)) or loop-back device,
where messages received from the valid socket are merely returned
back to the sender.

=== Forwarders

When both sockets are valid, then the result is a ((forwarder)) or proxy.
In this case sockets _s1_ and _s2_ must be "`compatible`" with each other,
which is to say that they should represent the opposite halves of a two
protocol pattern, or both be the same protocol for a single protocol
pattern.
For example, if _s1_ is a <<nng_pub.7#,_pub_>> socket, then _s2_ must 
be a <<nng_sub.7#,_sub_>> socket.
Or, if _s1_ is a <<nng_bus.7#,_bus_>> socket, then _s2_ must also
be a _bus_ socket.

=== Operation

This `nng_device()` function puts each socket into raw mode
(see <<nng_options.5#NNG_OPT_RAW,`NNG_OPT_RAW`>>), and then moves messages
between them.
When a protocol has a ((backtrace)) style header, routing information is
added as the message crosses the forwarder, allowing replies to be
returned to requestors, and responses to be routed back to surveyors.

Additionally, some protocols have a maximum ((time-to-live)) to protect
against forwarding loops and especially amplification loops.
In these cases, the default limit (usually 8), ensures that messages will
self-terminate when they have passed through too many forwarders,
protecting the network from unlimited message amplification that can arise
through misconfiguration.
This is controlled via the <<nng_options.5#NNG_OPT_MAXTTL,`NNG_OPT_MAXTTL`>>
option.

IMPORTANT: Not all protocols have support for guarding against forwarding loops,
and even for those that do, forwarding loops can be extremely determintal
to network performance.

NOTE: Devices (forwarders and reflectors) act in best effort delivery mode only.
If a message is received from one socket that cannot be accepted by the
other (due to backpressure or other issues), then the message is discarded.

TIP: Use the request/reply pattern, which includes automatic retries by
the requestor, if reliable delivery is needed.

== RETURN VALUES

This function returns 0 on success, and non-zero otherwise.

== ERRORS

`NNG_ECLOSED`:: At least one of the sockets is not open.
`NNG_ENOMEM`:: Insufficient memory is available.
`NNG_EINVAL`:: The sockets are not compatible, or are both invalid.

== SEE ALSO

<<nng_options.5#,nng_options(5)>>,
<<nng_socket.5#,nng_socket(5)>>,
<<nng.7#,nng(7)>>