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 model - accept inward connections to frontend socket (by binding it to
  an endpoint), and make onward connections through backend socket (connecting
  to endpoints on other nodes). A proxy device model can fit well into an
  existing topology.
* broker model - accept connections on both frontend and backend sockets (by
  binding both to endpoints).  A broker device model creates a star topology
  where nodes 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 shall not return if successful. Otherwise it shall
return `-1` and set 'errno' to one of the values defined below.


ERRORS
------
*ETERM*::
The 0MQ 'context' associated with the specified 'frontend' or 'backend' was 
terminated.
*EFAULT*::
The provided 'frontend' or 'backend' was not valid (NULL).


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);
----

.Creating a pubsub proxy
----
//  Create frontend and backend sockets
void *frontend = zmq_socket (context, ZMQ_SUB);
assert (backend);
void *backend = zmq_socket (context, ZMQ_PUB);
assert (frontend);
//  Connect frontend to publisher
assert (zmq_bind (frontend, "tcp://192.68.55.112:4444") == 0);
//  Bind backend to TCP port
assert (zmq_bind (backend, "tcp://*:5556") == 0);
//  Start a forwarder device
zmq_device (ZMQ_FORWARDER, frontend, backend);
----


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