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.

Table 1. Summary of XS_REQ characteristics

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.

Table 5. Summary of XS_PUB characteristics

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.

Table 9. Summary of XS_PUSH characteristics

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.

Table 11. Summary of XS_SURVEYOR characteristics

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.

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.

AUTHORS

The Crossroads documentation was written by Martin Sustrik <m[blue][email protected]m[][1]> and Martin Lucina <m[blue][email protected]m[][2]>.

NOTES

1.

[email protected]

mailto:[email protected]

2.

[email protected]

mailto:[email protected]