ZMQ_SOCKET(3) 0MQ Manual ZMQ_SOCKET(3)NAMEzmq_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 zmq_connect(3), or at least
one endpoint must be created for accepting incoming connections with
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. The following sections present the socket types defined
by 0MQ, grouped by the general messaging pattern which is built from
related socket types.
Request-reply pattern
The request-reply pattern is used for sending requests from a client to
one or more instances of a 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 round-robined 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 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.
Table 1. Summary of ZMQ_REQ characteristics
Compatible peer sockets ZMQ_REP
Direction Bidirectional
Send/receive pattern Send, Receive, Send,
Receive, ...
Outgoing routing strategy Round-robin
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.
Table 2. Summary of ZMQ_REP characteristics
Compatible peer sockets ZMQ_REQ
Direction Bidirectional
Send/receive pattern Receive, Send, Receive,
Send, ...
Incoming routing strategy Fair-queued
Outgoing routing strategy Last peer
ZMQ_HWM option action Drop
ZMQ_DEALER
A socket of type ZMQ_DEALER is an advanced pattern used for
extending request/reply sockets. Each message sent is round-robined
among all connected peers, and each message received is fair-queued
from all connected peers.
Previously this socket was called ZMQ_XREQ and that name remains
available for backwards compatibility.
When a ZMQ_DEALER 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 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.
When a ZMQ_DEALER socket is connected to a ZMQ_REP socket each
message sent must consist of an empty message part, the delimiter,
followed by one or more body parts.
Table 3. Summary of ZMQ_DEALER characteristics
Compatible peer sockets ZMQ_ROUTER, ZMQ_REQ,
ZMQ_REP
Direction Bidirectional
Send/receive pattern Unrestricted
Outgoing routing strategy Round-robin
Incoming routing strategy Fair-queued
ZMQ_HWM option action Block
ZMQ_ROUTER
A socket of type ZMQ_ROUTER is an advanced pattern used for
extending request/reply sockets. When receiving messages a
ZMQ_ROUTER socket shall prepend a message part containing the
identity of the originating peer to the message before passing it
to the application. Messages received are fair-queued from among
all connected peers. When sending messages a ZMQ_ROUTER socket
shall remove the first part of the message and use it to determine
the identity of the peer the message shall be routed to. If the
peer does not exist anymore the message shall be silently
discarded.
Previously this socket was called ZMQ_XREP and that name remains
available for backwards compatibility.
When a ZMQ_ROUTER 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 routed to a
non-existent peer or a peer for which the individual high water
mark has been reached shall also be dropped.
When a ZMQ_REQ socket is connected to a ZMQ_ROUTER socket, in
addition to the identity of the originating peer each message
received shall contain an empty delimiter message part. Hence, the
entire structure of each received message as seen by the
application becomes: one or more identity parts, delimiter part,
one or more body parts. When sending replies to a ZMQ_REQ socket
the application must include the delimiter part.
Table 4. Summary of ZMQ_ROUTER characteristics
Compatible peer sockets ZMQ_DEALER, ZMQ_REQ,
ZMQ_REP
Direction Bidirectional
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 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.
Table 5. Summary of ZMQ_PUB characteristics
Compatible peer sockets ZMQ_SUB
Direction Unidirectional
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
zmq_setsockopt(3) to specify which messages to subscribe to. The
zmq_send() function is not implemented for this socket type.
Table 6. Summary of ZMQ_SUB characteristics
Compatible peer sockets ZMQ_PUB
Direction Unidirectional
Send/receive pattern Receive only
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 round-robined 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 round-robined
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 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.
Deprecated alias: ZMQ_DOWNSTREAM.
Table 7. 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 Round-robin
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.
Deprecated alias: ZMQ_UPSTREAM.
Table 8. 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 pattern is used to connect a peer to precisely one
other peer. This pattern is used for inter-thread communication across
the inproc transport.
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 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 designed for inter-thread communication
across the zmq_inproc(7) transport and do not implement
functionality such as auto-reconnection. ZMQ_PAIR sockets are
considered experimental and may have other missing or broken
aspects.
Table 9. 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 ALSOzmq_init(3)zmq_setsockopt(3)zmq_bind(3)zmq_connect(3)zmq_send(3)zmq_recv(3)zmq_inproc(7)zmq(7)AUTHORS
This manual page was written by the 0MQ community.
0MQ 2.2.0 04/04/2012 ZMQ_SOCKET(3)
