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
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 ALSO¶
zmq_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.