xs(7)
NAME
xs - Crossroads I/O, a lightweight messaging layer
SYNOPSIS
#include <xs/xs.h> cc [flags] files -lxs [libraries]
DESCRIPTION
Crossroads I/O is a library for building scalable and high performance
distributed applications. It fits between classic BSD sockets,
JMS/AMQP-style message queues, and enterprise message-oriented
middleware.
Crossroads I/O extends the standard socket interfaces with features
traditionally provided by specialised messaging middleware products.
Crossroads sockets provide an abstraction of asynchronous message
queues, multiple messaging patterns, message filtering (subscriptions),
seamless access to multiple transport protocols and more.
Crossroads I/O provides a native C API for applications. Support for
many more languages is provided by the community through language
bindings which can be found at the Crossroads website.
This documentation presents an overview of Crossroads concepts,
describes how Crossroads abstract standard sockets and provides a
reference manual for the functions provided by the Crossroads library.
Context
Before using any Crossroads library functions the caller must
initialise a context using xs_init(). The following functions are
provided to handle initialisation and termination of a context:
Initialise Crossroads context
xs_init(3)
Terminate Crossroads context
xs_term(3)
Set Crossroads context options
xs_setctxopt(3)
Thread safety
A context is thread safe and may be shared among as many
application threads as necessary, without any additional locking
required on the part of the caller.
The individual sockets within a context are not thread safe ---
applications may not use a single socket concurrently from multiple
threads.
A socket may be migrated from one thread to another, by issuing a
full memory barrier between individual calls on the socket. For
example, this means applications can create a socket in one thread
with xs_socket() and then pass it to a newly created thread as part
of thread initialization via a structure passed as an argument to
pthread_create().
Multiple contexts
Multiple contexts may coexist within a single application. Thus, an
application can use Crossroads directly and at the same time make
use of any number of additional libraries or components which
themselves make use of Crossroads.
Messages
A Crossroads message is a discrete unit of data passed between
applications or components of the same application. Crossroads messages
have no internal structure and from the point of view of Crossroads
themselves they are considered to be opaque binary data.
Applications using the Crossroads library send and receive messages
directly from/to buffers provided by the application, using the
Crossroads functions xs_send() and xs_recv().
Alternatively, applications desiring zero-copy messaging and/or
reference counted allocation of messages can use the message handling
functions described in this section, and send and receive messages
using xs_sendmsg() and xs_recvmsg() respectively. These two approaches
are interchangeable.
The following functions are provided to work with messages using
zero-copy and/or reference-counted allocation of messages:
Initialise a message
xs_msg_init(3) xs_msg_init_size(3) xs_msg_init_data(3)
Release a message
xs_msg_close(3)
Access message content
xs_msg_data(3) xs_msg_size(3)
Message manipulation
xs_msg_copy(3) xs_msg_move(3)
Retrieve message option
xs_getmsgopt(3)
Sockets
Crossroads sockets present an abstraction of a asynchronous message
queue, with the exact queueing semantics depending on the socket type
in use. See xs_socket(3) for the socket types provided.
The following functions are provided to work with sockets:
Creating a socket
xs_socket(3)
Closing a socket
xs_close(3)
Manipulating socket options
xs_getsockopt(3) xs_setsockopt(3)
Creating and modifiying topologies
xs_bind(3) xs_connect(3) xs_shutdown(3)
Sending and receiving messages
xs_send(3) xs_recv(3)
Sending and receiving messages (zero-copy)
xs_sendmsg(3) xs_recvmsg(3)
Input/output multiplexing. Crossroads provides a mechanism for
applications to multiplex input/output events over a set containing
both Crossroads sockets and standard sockets. This mechanism mirrors
the standard poll() system call, and is described in detail in
xs_poll(3).
Transports
A Crossroads socket can use multiple different underlying transport
mechanisms. Each transport mechanism is suited to a particular purpose
and has its own advantages and drawbacks.
The following transport mechanisms are provided:
Unicast transport using TCP
xs_tcp(7)
Reliable multicast transport using PGM
xs_pgm(7)
Local inter-process communication transport
xs_ipc(7)
Local in-process (inter-thread) communication transport
xs_inproc(7)
ERROR HANDLING
The Crossroads library functions handle errors using the standard conventions found on POSIX systems. Generally, this means that upon failure a Crossroads library function shall return either a NULL value (if returning a pointer) or a negative value (if returning an integer), and the actual error code shall be stored in the errno variable. On non-POSIX systems some users may experience issues with retrieving the correct value of the errno variable. The xs_errno() function is provided to assist in these cases; for details refer to xs_errno(3). The xs_strerror() function is provided to translate Crossroads-specific error codes into error message strings; for details refer to xs_strerror(3).
MISCELLANEOUS
The following miscellaneous functions are provided:
Report Crossroads library version
xs_version(3)
LANGUAGE BINDINGS
The Crossroads library provides interfaces suitable for calling from programs in any language; this documentation documents those interfaces as they would be used by C programmers. The intent is that programmers using Crossroads from other languages shall refer to this documentation alongside any documentation provided by the vendor of their language binding.
ZEROMQ COMPATIBILITY
The Crossroads library provides an optional drop-in libzmq compatibility library for ZeroMQ applications. See xs_zmq(7) for documentation on this option.
AUTHORS
The Crossroads documentation was written by Martin Sustrik <sustrik@250bpm.com[1]> and Martin Lucina <martin@lucina.net[2]>.
COPYING
Free use of the Crossroads library software is granted under the terms of the GNU Lesser General Public License (LGPL). For details see the files COPYING and COPYING.LESSER included with the libxs distribution. As a special exception, the copyright holders of libxs grant you the right to link the library statically with your software. Refer to the end of the COPYING.LESSER file included with the libxs distribution for details.
NOTES
1. sustrik@250bpm.com
mailto:sustrik@250bpm.com
2. martin@lucina.net
mailto:martin@lucina.net
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