ZMQ_SOCKET

Section: 0MQ Manual (3)
Updated: 12/31/2016
Index Return to Main Contents

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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 has both thread safe socket type and not thread safe socket types. Applications MUST NOT use a not thread safe socket from multiple threads except after migrating a socket from one thread to another with a "full fence" memory barrier.

Following are the thread safe sockets: * ZMQ_CLIENT * ZMQ_SERVER * ZMQ_DISH * ZMQ_RADIO * ZMQ_SCATTER * ZMQ_GATHER

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.  

Client-server pattern

The client-server pattern is used to allow a single ZMQ_SERVER server talk to one or more ZMQ_CLIENT clients. The client always starts the conversation, after which either peer can send messages asynchronously, to the other.

The client-server pattern is formally defined by m[blue]http://rfc.zeromq.org/spec:41m[].

Note: this pattern is meant to eventually deprecate the use of ZMQ_DEALER and ZMQ_ROUTER to build client-server architectures, as well as ZMQ_REP and ZMQ_REQ for request-reply.

ZMQ_CLIENT

A ZMQ_CLIENT socket talks to a ZMQ_SERVER socket. Either peer can connect, though the usual and recommended model is to bind the ZMQ_SERVER and connect the ZMQ_CLIENT.

If the ZMQ_CLIENT socket has established a connection, zmq_send(3) will accept messages, queue them, and send them as rapidly as the network allows. The outgoing buffer limit is defined by the high water mark for the socket. If the outgoing buffer is full, or if there is no connected peer, zmq_send(3) will block, by default. The ZMQ_CLIENT socket will not drop messages.

When a ZMQ_CLIENT socket is connected to multiple ZMQ_SERVER sockets, outgoing messages are distributed between connected peers on a round-robin basis. Likewise, the ZMQ_CLIENT socket receives messages fairly from each connected peer. This usage is sensible only for stateless protocols.

ZMQ_CLIENT sockets are threadsafe and can be used from multiple threads at the same time. Note that replies from a ZMQ_SERVER socket will go to the first client thread that calls libzmq:zmq_msg_recv. If you need to get replies back to the originating thread, use one ZMQ_CLIENT socket per thread.

Note

ZMQ_CLIENT sockets are threadsafe. They do not accept the ZMQ_SNDMORE option on sends not ZMQ_RCVMORE on receives. This limits them to single part data. The intention is to extend the API to allow scatter/gather of multi-part data.

Table 1. Summary of ZMQ_CLIENT characteristics

Compatible peer sockets	ZMQ_SERVER
Direction	Bidirectional
Send/receive pattern	Unrestricted
Outgoing routing strategy	Round-robin
Incoming routing strategy	Fair-queued
Action in mute state	Block

ZMQ_SERVER

A ZMQ_SERVER socket talks to a set of ZMQ_CLIENT sockets. A ZMQ_SERVER socket can only reply to an incoming message: the ZMQ_CLIENT peer must always initiate a conversation.

Each received message has a routing_id that is a 32-bit unsigned integer. The application can fetch this with zmq_msg_routing_id(3). To send a message to a given ZMQ_CLIENT peer the application must set the peercqs routing_id on the message, using zmq_msg_set_routing_id(3).

If the routing_id is not specified, or does not refer to a connected client peer, the send call will fail with EHOSTUNREACH. If the outgoing buffer for the client peer is full, the send call shall block, unless ZMQ_DONT_WAIT is used in the send, in which case it shall fail with EAGAIN. The ZMQ_SERVER socket shall not drop messages in any case.

Note

ZMQ_SERVER sockets are threadsafe. They do not accept the ZMQ_SNDMORE option on sends not ZMQ_RCVMORE on receives. This limits them to single part data. The intention is to extend the API to allow scatter/gather of multi-part data.

Table 2. Summary of ZMQ_SERVER characteristics

Compatible peer sockets	ZMQ_CLIENT
Direction	Bidirectional
Send/receive pattern	Unrestricted
Outgoing routing strategy	See text
Incoming routing strategy	Fair-queued
Action in mute state	Return EAGAIN

 

Radio-dish pattern

The radio-dish pattern is used for one-to-many distribution of data from a single publisher to multiple subscribers in a fan out fashion.

Radio-dish is using groups (vs Pub-sub topics), Dish sockets can join a group and each message sent by Radio sockets belong to a group.

Groups are null terminated strings limited to 16 chars length (including null). The intention is to increase the length to 40 chars (including null).

Groups are matched using exact matching (vs prefix matching of PubSub).

Note

Radio-dish is still in draft phase.

Note: this pattern is meant to eventually deprecate the use of ZMQ_PUB and ZMQ_SUB to build pub-sub architectures.

ZMQ_RADIO

A socket of type ZMQ_RADIO is used by a publisher to distribute data. Each message belong to a group, a group is specified with linkzmq_zmq_msg_set_group[3]. Messages are distributed to all members of a group. The zmq_recv(3) function is not implemented for this socket type.

When a ZMQ_RADIO 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.

Note

ZMQ_RADIO sockets are threadsafe. They do not accept the ZMQ_SNDMORE option on sends. This limits them to single part data.

Table 3. Summary of ZMQ_RADIO characteristics

Compatible peer sockets	ZMQ_DISH
Direction	Unidirectional
Send/receive pattern	Send only
Incoming routing strategy	N/A
Outgoing routing strategy	Fan out
Action in mute state	Drop

ZMQ_DISH

A socket of type ZMQ_DISH is used by a subscriber to subscribe to groups distributed by a radio. Initially a ZMQ_DISH socket is not subscribed to any groups, use zmq_join(3) to join a group. To get the group the message belong to call zmq_msg_group(3). The zmq_send() function is not implemented for this socket type.

Note

ZMQ_DISH sockets are threadsafe. They do not accept ZMQ_RCVMORE on receives. This limits them to single part data.

Table 4. Summary of ZMQ_DISH characteristics

Compatible peer sockets	ZMQ_RADIO
Direction	Unidirectional
Send/receive pattern	Receive only
Incoming routing strategy	Fair-queued
Outgoing routing strategy	N/A

 

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:29m[].

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

Compatible peer sockets	ZMQ_SUB, ZMQ_XSUB
Direction	Unidirectional
Send/receive pattern	Send only
Incoming routing strategy	N/A
Outgoing routing strategy	Fan out
Action in mute state	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
Direction	Unidirectional
Send/receive pattern	Receive only
Incoming routing strategy	Fair-queued
Outgoing routing strategy	N/A

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

Compatible peer sockets	ZMQ_SUB, ZMQ_XSUB
Direction	Unidirectional
Send/receive pattern	Send messages, receive subscriptions
Incoming routing strategy	N/A
Outgoing routing strategy	Fan out
Action in mute state	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. Messages without a sub/unsub prefix may also be sent, but have no effect on subscription status.

Table 8. Summary of ZMQ_XSUB characteristics

Compatible peer sockets	ZMQ_PUB, ZMQ_XPUB
Direction	Unidirectional
Send/receive pattern	Receive messages, send subscriptions
Incoming routing strategy	Fair-queued
Outgoing routing strategy	N/A
Action in mute state	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.

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

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

Compatible peer sockets	ZMQ_PULL
Direction	Unidirectional
Send/receive pattern	Send only
Incoming routing strategy	N/A
Outgoing routing strategy	Round-robin
Action in mute state	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
Action in mute state	Block

 

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:31m[].

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.

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
Action in mute state	Block

 

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 connection, send the identity frame followed by a zero-length message (see EXAMPLE section).

When a connection is made, a zero-length message will be received by the application. Similarly, when the peer disconnects (or the connection is lost), a zero-length message will be received by the application.

You must send one identity frame followed by one data frame. The ZMQ_SNDMORE flag is required for identity frames but is ignored on data frames.

Table 12. Summary of ZMQ_STREAM characteristics

Compatible peer sockets	none.
Direction	Bidirectional
Send/receive pattern	Unrestricted
Outgoing routing strategy	See text
Incoming routing strategy	Fair-queued
Action in mute state	EAGAIN

 

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:28m[].

Note: this pattern will be deprecated in favor of the client-server pattern.

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 13. Summary of ZMQ_REQ characteristics

Compatible peer sockets	ZMQ_REP, ZMQ_ROUTER
Direction	Bidirectional
Send/receive pattern	Send, Receive, Send, Receive, ...
Outgoing routing strategy	Round-robin
Incoming routing strategy	Last peer
Action in mute state	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 does not exist any more the reply is silently discarded.

Table 14. Summary of ZMQ_REP characteristics

Compatible peer sockets	ZMQ_REQ, ZMQ_DEALER
Direction	Bidirectional
Send/receive pattern	Receive, Send, Receive, Send, ...
Incoming routing strategy	Fair-queued
Outgoing routing strategy	Last peer

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 15. Summary of ZMQ_DEALER characteristics

Compatible peer sockets	ZMQ_ROUTER, ZMQ_REP, ZMQ_DEALER
Direction	Bidirectional
Send/receive pattern	Unrestricted
Outgoing routing strategy	Round-robin
Incoming routing strategy	Fair-queued
Action in mute state	Block

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, unless ZMQ_ROUTER_MANDATORY socket option is set.

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 16. Summary of ZMQ_ROUTER characteristics

Compatible peer sockets	ZMQ_DEALER, ZMQ_REQ, ZMQ_ROUTER
Direction	Bidirectional
Send/receive pattern	Unrestricted
Outgoing routing strategy	See text
Incoming routing strategy	Fair-queued
Action in mute state	Drop (see text)

 

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.

 

EXAMPLE

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;
/* Data structure to hold the ZMQ_STREAM received data */
uint8_t raw [256];
size_t raw_size = 256;
while (1) {
        /*  Get HTTP request; ID frame and then request */
        id_size = zmq_recv (socket, id, 256, 0);
        assert (id_size > 0);
        do {
                raw_size = zmq_recv (socket, raw, 256, 0);
                assert (raw_size >= 0);
        } while (raw_size == 256);
        /* 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), 0);
        /* 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, 0);
}
zmq_close (socket);
zmq_ctx_destroy (ctx);

 

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 page was written by the 0MQ community. To make a change please read the 0MQ Contribution Policy at m[blue]http://www.zeromq.org/docs:contributingm[].

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Index

NAME

SYNOPSIS

DESCRIPTION

Client-server pattern

Radio-dish pattern

Publish-subscribe pattern

Pipeline pattern

Exclusive pair pattern

Native Pattern

Request-reply pattern

RETURN VALUE

ERRORS

EXAMPLE

SEE ALSO

AUTHORS