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 '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 _client_ to one or more instances of a _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 round-robined 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 linkzmq: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. [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 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. [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_HWM option action:: Drop 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 an exceptional 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 exceptional 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_. Deprecated alias: 'ZMQ_XREQ'. [horizontal] .Summary of ZMQ_DEALER characteristics Compatible peer sockets:: 'ZMQ_ROUTER', 'ZMQ_REP' Direction:: Bidirectional Send/receive pattern:: Unrestricted Outgoing routing strategy:: Round-robin Incoming routing strategy:: Fair-queued ZMQ_HWM option action:: 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. When a 'ZMQ_ROUTER' 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_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. Deprecated alias: 'ZMQ_XREP'. [horizontal] .Summary of ZMQ_ROUTER characteristics Compatible peer sockets:: 'ZMQ_DEALER', 'ZMQ_REQ Direction:: Bidirectional 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 linkzmq: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. [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 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 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_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. [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 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. [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 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 round-robined 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 round-robined 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 linkzmq: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. [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 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. [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 ZMQ_HWM option action:: N/A 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. 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 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. 'ZMQ_PAIR' sockets are considered experimental and may have other missing or broken aspects. [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 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 -------- 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 0MQ manual page was written by Martin Sustrik , Martin Lucina , and Pieter Hintjens .