libzmq/doc/zmq_socket.txt

zmq_socket(3)
=============


NAME
----
zmq_socket - create 0MQ socket


SYNOPSIS
--------
*void *zmq_socket (void '*context', int 'type');*


DESCRIPTION
-----------
The 'zmq_socket()' function shall create a 0MQ socket within the specified
'context' and return an opaque handle to the newly created socket. The 'type'
argument specifies the socket type, which determines the semantics of
communication over the socket.

The newly created socket is initially unbound, and not associated with any
endpoints. In order to establish a message flow a socket must first be
connected to at least one endpoint with linkzmq:zmq_connect[3], or at least one
endpoint must be created for accepting incoming connections with
linkzmq:zmq_bind[3].

.Key differences to conventional sockets
Generally speaking, conventional sockets present a _synchronous_ interface to
either connection-oriented reliable byte streams (SOCK_STREAM), or
connection-less unreliable datagrams (SOCK_DGRAM). In comparison, 0MQ sockets
present an abstraction of an asynchronous _message queue_, with the exact
queueing semantics depending on the socket type in use. Where conventional
sockets transfer streams of bytes or discrete datagrams, 0MQ sockets transfer
discrete _messages_.

0MQ sockets being _asynchronous_ means that the timings of the physical
connection setup and tear down, reconnect and effective delivery are transparent
to the user and organized by 0MQ itself. Further, messages may be _queued_ in
the event that a peer is unavailable to receive them.

Conventional sockets allow only strict one-to-one (two peers), many-to-one
(many clients, one server), or in some cases one-to-many (multicast)
relationships. With the exception of 'ZMQ_PAIR', 0MQ sockets may be connected
*to multiple endpoints* using _zmq_connect()_, while simultaneously accepting
incoming connections *from multiple endpoints* bound to the socket using
_zmq_bind()_, thus allowing many-to-many relationships.

.Thread safety
0MQ 'sockets' are _not_ thread safe. Applications MUST NOT use a socket
from multiple threads except after migrating a socket from one thread to 
another with a "full fence" memory barrier.

.Socket types
0MQ defines several messaging patterns which encapsulate exact semantics of
a particular topology. For example, publush-subscribe pattern defines data
distribution trees while request-reply defines networks of shared stateless
services. Each pattern defines several socket types (roles in the pattern).

The following sections present the socket types defined by 0MQ:


Request-reply pattern
~~~~~~~~~~~~~~~~~~~~~
The request-reply pattern is used for sending requests from a _client_ to one
or more instances of a stateless _service_, and receiving subsequent replies
to each request sent.


ZMQ_REQ
^^^^^^^
A socket of type 'ZMQ_REQ' is used by a _client_ to send requests to and
receive replies from a _service_. This socket type allows only an alternating
sequence of _zmq_send(request)_ and subsequent _zmq_recv(reply)_ calls. Each
request sent is load-balanced among all _services_, and each reply received is
matched with the last issued request.

When a 'ZMQ_REQ' socket enters an exceptional state due to having reached the
high water mark for all _services_, or if there are no _services_ at all, then
any linkzmq:zmq_send[3] operations on the socket shall block until the
exceptional state ends or at least one _service_ becomes available for sending;
messages are not discarded.

[horizontal]
.Summary of ZMQ_REQ characteristics
Compatible peer sockets:: 'ZMQ_REP'
Send/receive pattern:: Send, Receive, Send, Receive, ...
Outgoing routing strategy:: Load-balanced
Incoming routing strategy:: Last peer
ZMQ_HWM option action:: Block


ZMQ_REP
^^^^^^^
A socket of type 'ZMQ_REP' is used by a _service_ to receive requests from and
send replies to a _client_. This socket type allows only an alternating
sequence of _zmq_recv(request)_ and subsequent _zmq_send(reply)_ calls. Each
request received is fair-queued from among all _clients_, and each reply sent
is routed to the _client_ that issued the last request. If the original
requester doesn't exist any more the reply is silently discarded.

When a 'ZMQ_REP' socket enters an exceptional state due to having reached the
high water mark for a _client_, then any replies sent to the _client_ in
question shall be dropped until the exceptional state ends.

[horizontal]
.Summary of ZMQ_REP characteristics
Compatible peer sockets:: 'ZMQ_REQ'
Send/receive pattern:: Receive, Send, Receive, Send, ...
Incoming routing strategy:: Fair-queued
Outgoing routing strategy:: Last peer
ZMQ_HWM option action:: Drop


ZMQ_XREQ
^^^^^^^^
A socket of type 'ZMQ_XREQ' is a socket type underlying 'ZMQ_REQ'. It doesn't
impose the strict order of sends and recvs as 'ZMQ_REQ' does and it is
intended for use in intermediate devices in request-reply topologies.

Each message sent is load-balanced among all connected
peers, and each message received is fair-queued from all connected peers.

When a 'ZMQ_XREQ' socket enters an exceptional state due to having reached the
high water mark for all peers, or if there are no peers at all, then any
linkzmq:zmq_send[3] operations on the socket shall block until the exceptional
state ends or at least one peer becomes available for sending; messages are not
discarded.

[horizontal]
.Summary of ZMQ_XREQ characteristics
Compatible peer sockets:: 'ZMQ_XREP', 'ZMQ_REP'
Send/receive pattern:: Unrestricted
Outgoing routing strategy:: Load-balanced
Incoming routing strategy:: Fair-queued
ZMQ_HWM option action:: Block


ZMQ_XREP
^^^^^^^^
A socket of type 'ZMQ_XREP' is a socket type underlying 'ZMQ_REP'. It doesn't
impose the strict order of sends and recvs as 'ZMQ_REQ' does and it is
intended for use in intermediate devices in request-reply topologies.

Messages received are fair-queued from among all connected peers. The outbound
messages are routed to a specific peer, as explained below.

When a 'ZMQ_XREP' socket enters an exceptional state due to having reached the
high water mark for all peers, or if there are no peers at all, then any
messages sent to the socket shall be dropped until the exceptional state ends.
Likewise, any messages to be routed to a non-existent peer or a peer for which
the individual high water mark has been reached shall also be dropped.

[horizontal]
.Summary of ZMQ_XREP characteristics
Compatible peer sockets:: 'ZMQ_XREQ', 'ZMQ_REQ'
Send/receive pattern:: Unrestricted
Outgoing routing strategy:: See text
Incoming routing strategy:: Fair-queued
ZMQ_HWM option action:: Drop


Publish-subscribe pattern
~~~~~~~~~~~~~~~~~~~~~~~~~
The publish-subscribe pattern is used for one-to-many distribution of data from
a single _publisher_ to multiple _subscribers_ in a fan out fashion.


ZMQ_PUB
^^^^^^^
A socket of type 'ZMQ_PUB' is used by a _publisher_ to distribute data.
Messages sent are distributed in a fan out fashion to all connected peers.
The linkzmq:zmq_recv[3] function is not implemented for this socket type.

When a 'ZMQ_PUB' socket enters an exceptional state due to having reached the
high water mark for a _subscriber_, then any messages that would be sent to the
_subscriber_ in question shall instead be dropped until the exceptional state
ends. The _zmq_send()_ function shall never block for this socket type.

[horizontal]
.Summary of ZMQ_PUB characteristics
Compatible peer sockets:: 'ZMQ_SUB', 'ZMQ_XSUB'
Send/receive pattern:: Send only
Incoming routing strategy:: N/A
Outgoing routing strategy:: Fan out
ZMQ_HWM option action:: Drop


ZMQ_SUB
^^^^^^^
A socket of type 'ZMQ_SUB' is used by a _subscriber_ to subscribe to data
distributed by a _publisher_. Initially a 'ZMQ_SUB' socket is not subscribed to
any messages, use the 'ZMQ_SUBSCRIBE' option of linkzmq:zmq_setsockopt[3] to
specify which messages to subscribe to. The _zmq_send()_ function is not
implemented for this socket type.

[horizontal]
.Summary of ZMQ_SUB characteristics
Compatible peer sockets:: 'ZMQ_PUB', 'ZMQ_XPUB'
Send/receive pattern:: Receive only
Incoming routing strategy:: Fair-queued
Outgoing routing strategy:: N/A
ZMQ_HWM option action:: Drop


ZMQ_XPUB
^^^^^^^^
Same as ZMQ_PUB except that you can receive subscriptions from the peers
in form of incoming messages. Subscription message is a byte 1 (for
subscriptions) or byte 0 (for unsubscriptions) followed by the subscription
body.

[horizontal]
.Summary of ZMQ_XPUB characteristics
Compatible peer sockets:: 'ZMQ_SUB', 'ZMQ_XSUB'
Send/receive pattern:: Send messages, receive subscriptions
Incoming routing strategy:: N/A
Outgoing routing strategy:: Fan out
ZMQ_HWM option action:: Drop


ZMQ_XSUB
^^^^^^^^
Same as ZMQ_SUB except that you subscribe by sending subscription messages to
the socket. Subscription message is a byte 1 (for subscriptions) or byte 0
(for unsubscriptions) followed by the subscription body.

[horizontal]
.Summary of ZMQ_XSUB characteristics
Compatible peer sockets:: 'ZMQ_PUB', 'ZMQ_XPUB'
Send/receive pattern:: Receive messages, send subscriptions
Incoming routing strategy:: Fair-queued
Outgoing routing strategy:: N/A
ZMQ_HWM option action:: Drop


Pipeline pattern
~~~~~~~~~~~~~~~~
The pipeline pattern is used for distributing data to _nodes_ arranged in
a pipeline. Data always flows down the pipeline, and each stage of the pipeline
is connected to at least one _node_. When a pipeline stage is connected to
multiple _nodes_ data is load-balanced among all connected _nodes_.


ZMQ_PUSH
^^^^^^^^
A socket of type 'ZMQ_PUSH' is used by a pipeline _node_ to send messages
to downstream pipeline _nodes_. Messages are load-balanced to all connected
downstream _nodes_. The _zmq_recv()_ function is not implemented for this
socket type.

When a 'ZMQ_PUSH' socket enters an exceptional state due to having reached the
high water mark for all downstream _nodes_, or if there are no downstream
_nodes_ at all, then any linkzmq:zmq_send[3] operations on the socket shall
block until the exceptional state ends or at least one downstream _node_
becomes available for sending; messages are not discarded.

[horizontal]
.Summary of ZMQ_PUSH characteristics
Compatible peer sockets:: 'ZMQ_PULL'
Direction:: Unidirectional
Send/receive pattern:: Send only
Incoming routing strategy:: N/A
Outgoing routing strategy:: Load-balanced
ZMQ_HWM option action:: Block


ZMQ_PULL
^^^^^^^^
A socket of type 'ZMQ_PULL' is used by a pipeline _node_ to receive messages
from upstream pipeline _nodes_. Messages are fair-queued from among all
connected upstream _nodes_. The _zmq_send()_ function is not implemented for
this socket type.

[horizontal]
.Summary of ZMQ_PULL characteristics
Compatible peer sockets:: 'ZMQ_PUSH'
Direction:: Unidirectional
Send/receive pattern:: Receive only
Incoming routing strategy:: Fair-queued
Outgoing routing strategy:: N/A
ZMQ_HWM option action:: N/A


Exclusive pair pattern
~~~~~~~~~~~~~~~~~~~~~~
The exclusive pair is an advanced pattern used for communicating exclusively
between two peers.


ZMQ_PAIR
^^^^^^^^
A socket of type 'ZMQ_PAIR' can only be connected to a single peer at any one
time.  No message routing or filtering is performed on messages sent over a
'ZMQ_PAIR' socket.

When a 'ZMQ_PAIR' socket enters an exceptional state due to having reached the
high water mark for the connected peer, or if no peer is connected, then
any linkzmq:zmq_send[3] operations on the socket shall block until the peer
becomes available for sending; messages are not discarded.

NOTE: 'ZMQ_PAIR' sockets are experimental, and are currently missing several
features such as auto-reconnection.

[horizontal]
.Summary of ZMQ_PAIR characteristics
Compatible peer sockets:: 'ZMQ_PAIR'
Direction:: Bidirectional
Send/receive pattern:: Unrestricted
Incoming routing strategy:: N/A
Outgoing routing strategy:: N/A
ZMQ_HWM option action:: Block


RETURN VALUE
------------
The _zmq_socket()_ function shall return an opaque handle to the newly created
socket if successful. Otherwise, it shall return NULL and set 'errno' to one of
the values defined below.


ERRORS
------
*EINVAL*::
The requested socket 'type' is invalid.
*EFAULT*::
The provided 'context' is invalid.
*EMFILE*::
The limit on the total number of open 0MQ sockets has been reached.
*ETERM*::
The context specified was terminated.

SEE ALSO
--------
linkzmq:zmq_init[3]
linkzmq:zmq_setsockopt[3]
linkzmq:zmq_bind[3]
linkzmq:zmq_connect[3]
linkzmq:zmq_send[3]
linkzmq:zmq_recv[3]
linkzmq:zmq[7]


AUTHORS
-------
The 0MQ documentation was written by Martin Sustrik <sustrik@250bpm.com> and
Martin Lucina <martin@lucina.net>.