zmq_socket (3)

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

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 ZMQ_REQ client to one or more ZMQ_REP services, and receiving subsequent replies to each request sent.

The request-reply pattern is formally defined by \m[blue]http://rfc.zeromq.org/spec:28\m[].

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.

If no services are available, then any send operation on the socket shall block until at least one service becomes available. The REQ socket shall not discard messages.

Table 1. Summary of ZMQ_REQ characteristics

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 does not exist any more the reply is silently discarded.

Table 2. Summary of ZMQ_REP characteristics

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.

When a ZMQ_DEALER socket enters the mute 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 mute 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

ZMQ_ROUTER

A socket of type ZMQ_ROUTER is an advanced socket type 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 by default, unless ZMQ_ROUTER_MANDATORY socket option is set to 1.

When a ZMQ_ROUTER socket enters the mute state due to having reached the high water mark for all peers, then any messages sent to the socket shall be dropped until the mute state ends. Likewise, any messages routed to 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

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.

The publish-subscribe pattern is formally defined by \m[blue]http://rfc.zeromq.org/spec:29\m[].

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 the mute 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 mute state ends. The zmq_send() function shall never block for this socket type.

Table 5. Summary of ZMQ_PUB characteristics

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

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. Messages without a sub/unsub prefix are also received, but have no effect on subscription status.

Table 7. Summary of ZMQ_XPUB characteristics

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. Messages without a sub/unsub prefix may also be sent, but have no effect on subscription status.

Table 8. Summary of ZMQ_XSUB characteristics

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.

The pipeline pattern is formally defined by \m[blue]http://rfc.zeromq.org/spec:30\m[].

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 the mute 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 mute state ends or at least one downstream node becomes available for sending; messages are not discarded.

Table 9. Summary of ZMQ_PUSH characteristics

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

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.

The exclusive pair pattern is formally defined by \m[blue]http://rfc.zeromq.org/spec:31\m[].

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 the mute 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 11. Summary of ZMQ_PAIR characteristics

Native Pattern

The native pattern is used for communicating with TCP peers and allows asynchronous requests and replies in either direction.

ZMQ_STREAM

A socket of type ZMQ_STREAM is used to send and receive TCP data from a non-0MQ peer, when using the tcp:// transport. A ZMQ_STREAM socket can act as client and/or server, sending and/or receiving TCP data asynchronously.

When receiving TCP data, a ZMQ_STREAM 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 TCP data, a ZMQ_STREAM 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, and unroutable messages shall cause an EHOSTUNREACH or EAGAIN error.

To open a connection to a server, use the zmq_connect call, and then fetch the socket identity using the ZMQ_IDENTITY zmq_getsockopt call.

To close a specific client connection, as a server, send the identity frame followed by a zero-length message (see EXAMPLE section).

The ZMQ_MSGMORE flag is ignored on data frames. You must send one identity frame followed by one data frame.

Also, please note that omitting the ZMQ_MSGMORE flag will prevent sending further data (from any client) on the same socket.

Table 12. Summary of ZMQ_STREAM characteristics

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.

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.

Creating a simple HTTP server using ZMQ_STREAM.

void *ctx = zmq_ctx_new ();
assert (ctx);
/* Create ZMQ_STREAM socket */
void *socket = zmq_socket (ctx, ZMQ_STREAM);
assert (socket);
int rc = zmq_bind (socket, "tcp://*:8080");
assert (rc == 0);
/* Data structure to hold the ZMQ_STREAM ID */
uint8_t id [256];
size_t id_size = 256;
while (1) {
        /*  Get HTTP request; ID frame and then request */
        id_size = zmq_recv (socket, id, 256, 0);
        assert (id_size > 0);
        /* Prepares the response */
        char http_response [] =
                "HTTP/1.0 200 OK\r\n"
                "Content-Type: text/plain\r\n"
                "\r\n"
                "Hello, World!";
        /* Sends the ID frame followed by the response */
        zmq_send (socket, id, id_size, ZMQ_SNDMORE);
        zmq_send (socket, http_response, strlen (http_response), ZMQ_SNDMORE);
        /* Closes the connection by sending the ID frame followed by a zero response */
        zmq_send (socket, id, id_size, ZMQ_SNDMORE);
        zmq_send (socket, 0, 0, ZMQ_SNDMORE);
        /* NOTE: If we don't use ZMQ_SNDMORE, then we won't be able to send more */
        /* message to any client */
}
zmq_close (socket);
zmq_ctx_destroy (ctx);

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

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