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 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 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
    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.

    Table 2. 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 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.

    Table 3. 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.

    Table 4. 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 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, 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 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, 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.

    Table 7. 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.

    Table 8. 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 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 9. 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.

    Table 10. 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 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 11. 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(7)

AUTHORS

The 0MQ documentation was written by Martin Sustrik <sustrik@250bpm.com[1]> and Martin Lucina <martin@lucina.net[2]>.

NOTES

1.  sustrik@250bpm.com
    mailto:sustrik@250bpm.com

2.  martin@lucina.net
    mailto:martin@lucina.net