zmq_socket(3) Manual Page

void *zmq_socket (void *context, int type);

The zmq_socket() function shall create a ØMQ 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).

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, ØMQ 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, ØMQ sockets transfer discrete messages.

ØMQ 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 ØMQ 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, ØMQ 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.

The following sections present the socket types defined by ØMQ, grouped by the general messaging pattern which is built from related socket types.

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.

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.

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.

A socket of type ZMQ_XREQ is an advanced pattern used for extending request/reply sockets. 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 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_XREQ 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.

A socket of type ZMQ_XREP is an advanced pattern used for extending request/reply sockets. When receiving messages a ZMQ_XREP 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_XREP 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.

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 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_XREP 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.

The publish-subscribe pattern is used for one-to-many distribution of data from a single publisher to multiple subscribers in a fanout fashion.

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.

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.

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.

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 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.

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.

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

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.

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.

zmq_init(3) zmq_setsockopt(3) zmq_bind(3) zmq_connect(3) zmq_send(3) zmq_recv(3) zmq(7)

The ØMQ documentation was written by Martin Sustrik <sustrik@250bpm.com> and Martin Lucina <mato@kotelna.sk>.