xs_socket(3)

# xs_socket(3) Manual Page

## 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 xs_connect(3), or at least one endpoint must be created for accepting incoming connections with 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 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.

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.

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 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.

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.

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 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.

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 xs_setsockopt(3) to specify which messages to subscribe to. The xs_send() function is not implemented for this socket type.

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.

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.

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 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.

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.

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.

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.

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.

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.

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 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.

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

xs_init(3) xs_setsockopt(3) xs_bind(3) xs_connect(3) xs_send(3) xs_recv(3) xs(7)

## AUTHORS

The Crossroads documentation was written by Martin Sustrik <sustrik@250bpm.com> and Martin Lucina <martin@lucina.net>.