zmq_device(3)
=============

NAME
----
zmq_device - start built-in 0MQ device


SYNOPSIS
--------
*int zmq_device (int 'device', const void '*frontend', const void '*backend');*


DESCRIPTION
-----------
The _zmq_device()_ function starts a built-in 0MQ device. The 'device' argument
is one of:

'ZMQ_QUEUE'::
    starts a queue device
'ZMQ_FORWARDER'::
    starts a forwarder device
'ZMQ_STREAMER'::
    starts a streamer device

The device connects a frontend socket to a backend socket. Conceptually, data
flows from frontend to backend. Depending on the socket types, replies may flow
in the opposite direction.

Before calling _zmq_device()_ you must set any socket options, and connect or
bind both frontend and backend sockets. The two conventional device models are:

*proxy*::
    bind frontend socket to an endpoint, and connect backend socket to
    downstream components. A proxy device model does not require changes to
    the downstream topology but that topology is static (any changes require
    reconfiguring the device).
*broker*::
    bind frontend socket to one endpoint and bind backend socket to a second
    endpoint. Downstream components must now connect into the device. A broker
    device model allows a dynamic downstream topology (components can come and 
    go at any time).

_zmq_device()_ runs in the current thread and returns only if/when the current
context is closed.


QUEUE DEVICE
------------
'ZMQ_QUEUE' creates a shared queue that collects requests from a set of clients,
and distributes these fairly among a set of services. Requests are fair-queued
from frontend connections and load-balanced between backend connections. 
Replies automatically return to the client that made the original request.

This device is part of the 'request-reply' pattern. The frontend speaks to
clients and the backend speaks to services. You should use 'ZMQ_QUEUE' with a
'ZMQ_XREP' socket for the frontend and a 'ZMQ_XREQ' socket for the backend. 
Other combinations are not documented.

Refer to linkzmq:zmq_socket[3] for a description of these socket types.


FORWARDER DEVICE
----------------
'ZMQ_FORWARDER' collects messages from a set of publishers and forwards these to
a set of subscribers. You will generally use this to bridge networks, e.g. read
on TCP unicast and forward on multicast.

This device is part of the 'publish-subscribe' pattern. The frontend speaks to
publishers and the backend speaks to subscribers. You should use
'ZMQ_FORWARDER' with a 'ZMQ_SUB' socket for the frontend and a 'ZMQ_PUB' socket
for the backend. Other combinations are not documented.

Refer to linkzmq:zmq_socket[3] for a description of these socket types.


STREAMER DEVICE
---------------
'ZMQ_STREAMER' collects tasks from a set of pushers and forwards these to a set
of pullers. You will generally use this to bridge networks. Messages are
fair-queued from pushers and load-balanced to pullers.

This device is part of the 'pipeline' pattern. The frontend speaks to pushers
and the backend speaks to pullers. You should use 'ZMQ_STREAMER' with a
'ZMQ_PULL' socket for the frontend and a 'ZMQ_PUSH' socket for the backend. 
Other combinations are not documented.

Refer to linkzmq:zmq_socket[3] for a description of these socket types.


RETURN VALUE
------------
The _zmq_device()_ function always returns `-1` and 'errno' set to *ETERM* (the
0MQ 'context' associated with either of the specified sockets was terminated).


EXAMPLE
-------
.Creating a queue broker
----
//  Create frontend and backend sockets
void *frontend = zmq_socket (context, ZMQ_XREP);
assert (backend);
void *backend = zmq_socket (context, ZMQ_XREQ);
assert (frontend);
//  Bind both sockets to TCP ports
assert (zmq_bind (frontend, "tcp://*:5555") == 0);
assert (zmq_bind (backend, "tcp://*:5556") == 0);
//  Start a queue device
zmq_device (ZMQ_QUEUE, frontend, backend);
----


SEE ALSO
--------
linkzmq:zmq_bind[3]
linkzmq:zmq_connect[3]
linkzmq:zmq_socket[3]
linkzmq:zmq[7]


AUTHORS
-------
This manual page was written by the 0MQ community.
