xs_socket(3) ============ NAME ---- xs_socket - create Crossroads socket SYNOPSIS -------- *void *xs_socket (void '*context', int 'type');* DESCRIPTION ----------- The 'xs_socket()' function shall create a Crossroads 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 linkxs:xs_connect[3], or at least one endpoint must be created for accepting incoming connections with linkxs:xs_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, Crossroads 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, Crossroads sockets transfer discrete _messages_. Crossroads 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 Crossroads library 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 'XS_PAIR', Crossroads sockets may be connected *to multiple endpoints* using _xs_connect()_, while simultaneously accepting incoming connections *from multiple endpoints* bound to the socket using _xs_bind()_, thus allowing many-to-many relationships. .Thread safety Crossroads '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 Crossroads defines several messaging patterns which encapsulate exact semantics of a particular topology. For example, publish-subscribe pattern defines data distribution trees while request-reply defines networks of shared stateless services. Each pattern defines several socket types (roles in the pattern). The following sections present the socket types defined by Crossroads library: Request-reply pattern ~~~~~~~~~~~~~~~~~~~~~ The request-reply pattern is used for sending requests from a _client_ to one or more instances of a stateless _service_, and receiving subsequent replies to each request sent. XS_REQ ^^^^^^ A socket of type 'XS_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 _xs_send(request)_ and subsequent _xs_recv(reply)_ calls. Each request sent is load-balanced among all _services_, and each reply received is matched with the last issued request. When a 'XS_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 linkxs:xs_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 XS_REQ characteristics Compatible peer sockets:: 'XS_REP' Send/receive pattern:: Send, Receive, Send, Receive, ... Outgoing routing strategy:: Load-balanced Incoming routing strategy:: Last peer XS_HWM option action:: Block XS_REP ^^^^^^ A socket of type 'XS_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 _xs_recv(request)_ and subsequent _xs_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 'XS_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 XS_REP characteristics Compatible peer sockets:: 'XS_REQ' Send/receive pattern:: Receive, Send, Receive, Send, ... Incoming routing strategy:: Fair-queued Outgoing routing strategy:: Last peer XS_HWM option action:: Drop XS_XREQ ^^^^^^^ A socket of type 'XS_XREQ' is a socket type underlying 'XS_REQ'. It doesn't impose the strict order of sends and recvs as 'XS_REQ' does and it is intended for use in intermediate devices in request-reply topologies. Each message sent is load-balanced among all connected peers, and each message received is fair-queued from all connected peers. When a 'XS_XREQ' 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 linkxs:xs_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. [horizontal] .Summary of XS_XREQ characteristics Compatible peer sockets:: 'XS_XREP', 'XS_REP' Send/receive pattern:: Unrestricted Outgoing routing strategy:: Load-balanced Incoming routing strategy:: Fair-queued XS_HWM option action:: Block XS_XREP ^^^^^^^ A socket of type 'XS_XREP' is a socket type underlying 'XS_REP'. It doesn't impose the strict order of sends and recvs as 'XS_REQ' does and it is intended for use in intermediate devices in request-reply topologies. Messages received are fair-queued from among all connected peers. The outbound messages are routed to a specific peer, as explained below. When a 'XS_XREP' 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 to be routed to a non-existent peer or a peer for which the individual high water mark has been reached shall also be dropped. [horizontal] .Summary of XS_XREP characteristics Compatible peer sockets:: 'XS_XREQ', 'XS_REQ' Send/receive pattern:: Unrestricted Outgoing routing strategy:: See text Incoming routing strategy:: Fair-queued XS_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. XS_PUB ^^^^^^ A socket of type 'XS_PUB' is used by a _publisher_ to distribute data. Messages sent are distributed in a fan out fashion to all connected peers. The linkxs:xs_recv[3] function is not implemented for this socket type. When a 'XS_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 _xs_send()_ function shall never block for this socket type. [horizontal] .Summary of XS_PUB characteristics Compatible peer sockets:: 'XS_SUB', 'XS_XSUB' Send/receive pattern:: Send only Incoming routing strategy:: N/A Outgoing routing strategy:: Fan out XS_HWM option action:: Drop XS_SUB ^^^^^^ A socket of type 'XS_SUB' is used by a _subscriber_ to subscribe to data distributed by a _publisher_. Initially a 'XS_SUB' socket is not subscribed to any messages, use the 'XS_SUBSCRIBE' option of linkxs:xs_setsockopt[3] to specify which messages to subscribe to. The _xs_send()_ function is not implemented for this socket type. [horizontal] .Summary of XS_SUB characteristics Compatible peer sockets:: 'XS_PUB', 'XS_XPUB' Send/receive pattern:: Receive only Incoming routing strategy:: Fair-queued Outgoing routing strategy:: N/A XS_HWM option action:: Drop XS_XPUB ^^^^^^^ Same as XS_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 XS_XPUB characteristics Compatible peer sockets:: 'XS_SUB', 'XS_XSUB' Send/receive pattern:: Send messages, receive subscriptions Incoming routing strategy:: N/A Outgoing routing strategy:: Fan out XS_HWM option action:: Drop XS_XSUB ^^^^^^^ Same as XS_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 XS_XSUB characteristics Compatible peer sockets:: 'XS_PUB', 'XS_XPUB' Send/receive pattern:: Receive messages, send subscriptions Incoming routing strategy:: Fair-queued Outgoing routing strategy:: N/A XS_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 load-balanced among all connected _nodes_. XS_PUSH ^^^^^^^ A socket of type 'XS_PUSH' is used by a pipeline _node_ to send messages to downstream pipeline _nodes_. Messages are load-balanced to all connected downstream _nodes_. The _xs_recv()_ function is not implemented for this socket type. When a 'XS_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 linkxs:xs_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 XS_PUSH characteristics Compatible peer sockets:: 'XS_PULL' Direction:: Unidirectional Send/receive pattern:: Send only Incoming routing strategy:: N/A Outgoing routing strategy:: Load-balanced XS_HWM option action:: Block XS_PULL ^^^^^^^ A socket of type 'XS_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 _xs_send()_ function is not implemented for this socket type. [horizontal] .Summary of XS_PULL characteristics Compatible peer sockets:: 'XS_PUSH' Direction:: Unidirectional Send/receive pattern:: Receive only Incoming routing strategy:: Fair-queued Outgoing routing strategy:: N/A XS_HWM option action:: N/A Survey pattern ~~~~~~~~~~~~~~ Survey pattern can be used to post a survey to a set of notes and collect responses from them. The survey is distributed from surveyor to all connected respondents. Responses are routed back to the original surveyor. XS_SURVEYOR ^^^^^^^^^^^ XS_SURVEYOR socket type can be used to send surveys to all respondents in the topology and receive the replies from all of them. Each survey sent is distributed to all connected peers, and incoming replies are fair-queue. As you don't know the number of respondents in the topology you don't know the number of responses you are going to get, therefore you should use XS_SURVEY_TIMEOUT socket option to set the deadline for the survey. [horizontal] .Summary of XS_SURVEYOR characteristics Compatible peer sockets:: 'XS_RESPONDENT', 'XS_XRESPONDENT' Direction:: Bidirectional Send/receive pattern:: Send one message, receive many messages. Incoming routing strategy:: Fair-queued Outgoing routing strategy:: Fan out XS_HWM option action:: Drop XS_RESPONDENT ^^^^^^^^^^^^^ This socket type receives surveys from surveyors and sends responses. Incoming surveys are fair-queued. Outgoing responses are routed back to the original surveyor. [horizontal] .Summary of XS_RESPONDENT characteristics Compatible peer sockets:: 'XS_SURVEYOR', 'XS_XSURVEYOR' Direction:: Bidirectional Send/receive pattern:: Receive a survey, send one response. Incoming routing strategy:: Fair-queued Outgoing routing strategy:: Last peer XS_HWM option action:: Drop XS_XSURVEYOR ^^^^^^^^^^^^ A socket of type 'XS_XSURVEYOR' is a socket type underlying 'XS_SURVEYOR'. It doesn't impose the strict order of sends and recvs as 'XS_SURVEYOR' does and it is intended for use in intermediate devices in survey topologies. [horizontal] .Summary of XS_XSURVEYOR characteristics Compatible peer sockets:: 'XS_RESPONDENT', 'XS_XRESPONDENT' Direction:: Bidirectional Send/receive pattern:: Send surveys, receive responses. Incoming routing strategy:: Fair-queued Outgoing routing strategy:: Fan out XS_HWM option action:: Drop XS_XRESPONDENT ^^^^^^^^^^^^^^ A socket of type 'XS_XRESPONDENT' is a socket type underlying 'XS_RESPONDENT'. It doesn't impose the strict order of sends and recvs as 'XS_RESPONDENT' does and it is intended for use in intermediate devices in survey topologies. Incoming surveys are fair-queued. Each survey is prefixed by a message part identifying the surveyor it was received from. Outgoing responses are routed to the original surveyor based on the first message part. [horizontal] .Summary of XS_XRESPONDENT characteristics Compatible peer sockets:: 'XS_SURVEYOR', 'XS_XSURVEYOR' Direction:: Bidirectional Send/receive pattern:: Receive surveys, send responses. Incoming routing strategy:: Fair-queued Outgoing routing strategy:: See text XS_HWM option action:: Drop Exclusive pair pattern ~~~~~~~~~~~~~~~~~~~~~~ The exclusive pair is an advanced pattern used for communicating exclusively between two peers. XS_PAIR ^^^^^^^ A socket of type 'XS_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 'XS_PAIR' socket. When a 'XS_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 linkxs:xs_send[3] operations on the socket shall block until the peer becomes available for sending; messages are not discarded. NOTE: 'XS_PAIR' sockets are experimental, and are currently missing several features such as auto-reconnection. [horizontal] .Summary of XS_PAIR characteristics Compatible peer sockets:: 'XS_PAIR' Direction:: Bidirectional Send/receive pattern:: Unrestricted Incoming routing strategy:: N/A Outgoing routing strategy:: N/A XS_HWM option action:: Block RETURN VALUE ------------ The _xs_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 Crossroads sockets has been reached. *ETERM*:: The context specified was terminated. SEE ALSO -------- linkxs:xs_init[3] linkxs:xs_setsockopt[3] linkxs:xs_bind[3] linkxs:xs_connect[3] linkxs:xs_send[3] linkxs:xs_recv[3] linkxs:xs[7] AUTHORS ------- The Crossroads documentation was written by Martin Sustrik and Martin Lucina .