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 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 teardown, 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.

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 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 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 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.; 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 stratagy Last peer
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 fanout fashion.; ZMQ_PUB
A socket of type ZMQ_PUB is used by a publisher to distribute data. Messages sent are distributed in a fanout 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.

Table 3. 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 Fanout

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 4. 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 N/A
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_DOWNSTREAM
A socket of type ZMQ_DOWNSTREAM 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_DOWNSTREAM 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.

Table 5. Summary of ZMQ_DOWNSTREAM characteristics Compatible peer sockets ZMQ_UPSTREAM

Direction Unidirectional

Send/receive pattern Send only

Incoming routing strategy N/A

Outgoing routing strategy Load-balanced

ZMQ_HWM option action Block; ZMQ_UPSTREAM
A socket of type ZMQ_UPSTREAM 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.

Table 6. Summary of ZMQ_UPSTREAM characteristics Compatible peer sockets ZMQ_DOWNSTREAM

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 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.

Table 7. 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.
EMTHREAD: The maximum number of sockets within this context has been exceeded.

AUTHORS

The 0MQ documentation was written by Martin Sustrik
<sustrik@250bpm.com[1]> and Martin Lucina <mato@kotelna.sk[2]>.

NOTES

1. sustrik@250bpm.com: mailto:sustrik@250bpm.com; 2. mato@kotelna.sk
mailto:mato@kotelna.sk

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See also