diff options
Diffstat (limited to 'doc/zmq_socket.txt')
| -rw-r--r-- | doc/zmq_socket.txt | 172 |
1 files changed, 146 insertions, 26 deletions
diff --git a/doc/zmq_socket.txt b/doc/zmq_socket.txt index 119ac31..23cc317 100644 --- a/doc/zmq_socket.txt +++ b/doc/zmq_socket.txt @@ -19,30 +19,89 @@ The 'zmq_socket()' function shall create a 0MQ socket within the specified 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 teardown, 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. + +.Socket types The following sections present the socket types defined by 0MQ, grouped by the -general _messaging pattern_ built from related socket types. +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 a -_service_, and receiving subsequent replies to each request sent. +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. -Socket type:: 'ZMQ_REQ' -Compatible peer sockets:: 'ZMQ_REP' +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 connected _services_. +request sent is load-balanced among all _services_, and each reply received is +matched with the last issued request. -Socket type:: 'ZMQ_REP' -Compatible peer sockets:: 'ZMQ_REQ' +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' +Direction:: Bidirectional +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 +send replies to a _client_. This socket type allows only an alternating sequence of _zmq_recv(request)_ and subsequent _zmq_send(reply)_ calls. Each -reply is routed to the _client_ that issued the last received request. +request received is fair-queued from among all _clients_, and each reply sent +is routed to the _client_ that issued the last request. + +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' +Direction:: Bidirectional +Send/receive pattern:: Receive, Send, Receive, Send, ... +Incoming routing strategy:: Fair-queued +Outgoing routing stratagy:: Last peer +ZMQ_HWM option action:: Drop Publish-subscribe pattern @@ -50,21 +109,44 @@ 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 fanout fashion. -Socket type:: 'ZMQ_PUB' -Compatible peer sockets:: 'ZMQ_SUB' +ZMQ_PUB +^^^^^^^ A socket of type 'ZMQ_PUB' is used by a _publisher_ to distribute data. Messages sent are distributed in a fanout fashion to all connected peers. -The _zmq_recv()_ function is not implemented for this socket type. +The linkzmq:zmq_recv[3] function is not implemented for this socket type. -Socket type:: 'ZMQ_SUB' -Compatible peer sockets:: 'ZMQ_PUB' +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. +[horizontal] +.Summary of ZMQ_PUB characteristics +Compatible peer sockets:: 'ZMQ_SUB' +Direction:: Unidirectional +Send/receive pattern:: Send only +Incoming routing strategy:: N/A +Outgoing routing strategy:: Fanout +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()_ to specify -which messages to subscribe to. The _zmq_send()_ function is not implemented -for this socket type. +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' +Direction:: Unidirectional +Send/receive pattern:: Receive only +Incoming routing strategy:: Fair-queued +Outgoing routing strategy:: N/A +ZMQ_HWM option action:: N/A Pipeline pattern @@ -74,38 +156,76 @@ 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_. -Socket type:: 'ZMQ_DOWNSTREAM' -Compatible peer sockets:: 'ZMQ_UPSTREAM' +ZMQ_DOWNSTREAM +^^^^^^^^^^^^^^ A socket of type 'ZMQ_DOWNSTREAM' 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. -Socket type:: 'ZMQ_UPSTREAM' -Compatible peer sockets:: 'ZMQ_DOWNSTREAM' +When a 'ZMQ_DOWNSTREAM' 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_DOWNSTREAM characteristics +Compatible peer sockets:: 'ZMQ_UPSTREAM' +Direction:: Unidirectional +Send/receive pattern:: Send only +Incoming routing strategy:: N/A +Outgoing routing strategy:: Load-balanced +ZMQ_HWM option action:: Block + +ZMQ_UPSTREAM +^^^^^^^^^^^^ A socket of type 'ZMQ_UPSTREAM' 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_UPSTREAM characteristics +Compatible peer sockets:: 'ZMQ_DOWNSTREAM' +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. -Socket type:: 'ZMQ_PAIR' -Compatible peer sockets:: 'ZMQ_PAIR' +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 experimental, and are currently missing several features 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 +ZMQ_HWM option action:: Block + RETURN VALUE ------------ @@ -120,8 +240,7 @@ ERRORS The requested socket 'type' is invalid. *EMTHREAD*:: -The number of application threads using sockets within this 'context' has been -exceeded. See the 'app_threads' parameter of the _zmq_init()_ function. +The maximum number of sockets within this 'context' has been exceeded. SEE ALSO @@ -132,6 +251,7 @@ linkzmq:zmq_bind[3] linkzmq:zmq_connect[3] linkzmq:zmq_send[3] linkzmq:zmq_recv[3] +linkzmq:zmq[7] AUTHORS |