= zmq_proxy(3)

== NAME
zmq_proxy - start built-in 0MQ proxy


== SYNOPSIS
*int zmq_proxy (void '*frontend', void '*backend', void '*capture');*


== DESCRIPTION
The _zmq_proxy()_ function starts the built-in 0MQ proxy in the current
application thread.

The proxy 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. The direction is conceptual only; the proxy is fully
symmetric and there is no technical difference between frontend and backend.

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

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

If the capture socket is not NULL, the proxy shall send all messages, received
on both frontend and backend, to the capture socket. The capture socket should
be a 'ZMQ_PUB', 'ZMQ_DEALER', 'ZMQ_PUSH', or 'ZMQ_PAIR' socket.

Refer to xref:zmq_socket.adoc[zmq_socket] for a description of the available socket types.

== EXAMPLE USAGE

Shared Queue
~~~~~~~~~~~~

When the frontend is a ZMQ_ROUTER socket, and the backend is a ZMQ_DEALER
socket, the proxy shall act as a shared queue that collects requests from a
set of clients, and distributes these fairly among a set of services.
Requests shall be fair-queued from frontend connections and distributed evenly
across backend connections. Replies shall automatically return to the client
that made the original request.

Forwarder
~~~~~~~~~

When the frontend is a ZMQ_XSUB socket, and the backend is a ZMQ_XPUB socket,
the proxy shall act as a message forwarder that collects messages from a set
of publishers and forwards these to a set of subscribers. This may be used to
bridge networks transports, e.g. read on tcp:// and forward on pgm://.

Streamer
~~~~~~~~

When the frontend is a ZMQ_PULL socket, and the backend is a ZMQ_PUSH socket,
the proxy shall collect tasks from a set of clients and forwards these to a set
of workers using the pipeline pattern.

== RETURN VALUE
The _zmq_proxy()_ function always returns `-1` and 'errno' set to *ETERM* or
*EINTR* (the 0MQ 'context' associated with either of the specified sockets was
terminated) or *EFAULT* (the provided 'frontend' or 'backend' was invalid).


== EXAMPLE
.Creating a shared queue proxy
----
//  Create frontend and backend sockets
void *frontend = zmq_socket (context, ZMQ_ROUTER);
assert (frontend);
void *backend = zmq_socket (context, ZMQ_DEALER);
assert (backend);
//  Bind both sockets to TCP ports
assert (zmq_bind (frontend, "tcp://*:5555") == 0);
assert (zmq_bind (backend, "tcp://*:5556") == 0);
//  Start the queue proxy, which runs until ETERM
zmq_proxy (frontend, backend, NULL);
----


== SEE ALSO
* xref:zmq_bind.adoc[zmq_bind]
* xref:zmq_connect.adoc[zmq_connect]
* xref:zmq_socket.adoc[zmq_socket]
* xref:zmq.adoc[zmq]


== AUTHORS
This page was written by the 0MQ community. To make a change please
read the 0MQ Contribution Policy at <https://zeromq.org/how-to-contribute/>.
