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 linkzmq:zmq_connect[3], or at least one endpoint must be created for accepting incoming connections with linkzmq: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 http://rfc.zeromq.org/spec:41. 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, linkzmq: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, linkzmq: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 linkzmq:zmq_msg_recv[3]. 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. [horizontal] .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 linkzmq:zmq_msg_routing_id[3]. To send a message to a given 'ZMQ_CLIENT' peer the application must set the peer's 'routing_id' on the message, using linkzmq: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. [horizontal] .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). The encoding of groups shall be UTF8. Groups are matched using exact matching (vs prefix matching of PubSub). NOTE: Radio-dish is still in draft phase. 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 linkzmq: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. [horizontal] .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 linkzmq:zmq_join[3] to join a group. To get the group the message belong to call linkzmq: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. [horizontal] .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 http://rfc.zeromq.org/spec:29. 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 linkzmq: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. [horizontal] .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 linkzmq:zmq_setsockopt[3] to specify which messages to subscribe to. The _zmq_send()_ function is not implemented for this socket type. [horizontal] .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. [horizontal] .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. [horizontal] .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 http://rfc.zeromq.org/spec:30. 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 linkzmq: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. [horizontal] .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. [horizontal] .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 http://rfc.zeromq.org/spec:31. 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 linkzmq: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 linkzmq:zmq_inproc[7] transport and do not implement functionality such as auto-reconnection. [horizontal] .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. [horizontal] .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 http://rfc.zeromq.org/spec:28. 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. [horizontal] .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. [horizontal] .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 linkzmq: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_. [horizontal] .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, or has never existed, the message shall be silently discarded. However, if 'ZMQ_ROUTER_MANDATORY' socket option is set to '1', the socket shall fail with EHOSTUNREACH in both cases. 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. If, 'ZMQ_ROUTER_MANDATORY' is set to '1', the socket shall block or return EAGAIN in both cases. When a 'ZMQ_ROUTER' socket has 'ZMQ_ROUTER_MANDATORY' flag set to '1', the socket shall generate 'ZMQ_POLLIN' events upon reception of messages from one or more peers. Likewise, the socket shall generate 'ZMQ_POLLOUT' events when at least one message can be sent to one or more peers. 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. [horizontal] .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 -------- linkzmq:zmq_init[3] linkzmq:zmq_setsockopt[3] linkzmq:zmq_bind[3] linkzmq:zmq_connect[3] linkzmq:zmq_send[3] linkzmq:zmq_recv[3] linkzmq:zmq_inproc[7] linkzmq:zmq[7] AUTHORS ------- This page was written by the 0MQ community. 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