xpaintro(7)							SAORD Documentation						       xpaintro(7)

NAME

       XPAIntro -  Introduction to the XPA Messaging System

SYNOPSIS

       A brief introduction to the XPA messaging system, which provides seamless communication between all kinds of Unix event-driven programs,
       including X programs, Tcl/Tk programs, and Perl programs.

DESCRIPTION

       The XPA messaging system provides seamless communication between all kinds of Unix programs, including X programs, Tcl/Tk programs, and
       Perl programs.  It also provides an easy way for users to communicate with these XPA-enabled programs by executing XPA client commands in
       the shell or by utilizing such commands in scripts.  Because XPA works both at the programming level and the shell level, it is a powerful
       tool for unifying any analysis environment: users and programmers have great flexibility in choosing the best level or levels at which to
       access XPA services, and client access can be extended or modified easily at any time.

       A program becomes an XPA-enabled server by defining named points of public access through which data and commands can be exchanged with
       other client programs (and users).  Using standard TCP sockets as a transport mechanism, XPA supports both single-point and broadcast mes-
       saging to and from these servers.  It supports direct communication between clients and servers, or indirect communication via an interme-
       diate message bus emulation program. Host-based access control is implemented, as is as the ability to communicate with XPA servers across
       a network.

       XPA implements a layered interface that is designed to be useful both to software developers and to users.  The interface consists of a
       library of XPA client and server routines for use in programs and a suite of high-level user programs built on top of these libraries.
       Using the XPA library, access points can be added to Tcl/Tk programs, Xt programs, or to Unix programs that use the XPA event loop or any
       event loop based on select().  Client access subroutines can be added to any Tcl/Tk or Unix program. Client access also is supported at the
       command line via a suite of high-level programs.

       The major components of the XPA layered interface are:

       o   A set of XPA server routines, centered on XPANew(), which are used by XPA server programs to tag public access points with string iden-
	   tifiers and to register send and receive callbacks for these access points.

       o   A set of XPA client routines, centered on the XPASet() and XPAGet(), which are used by external client applications to exchange data
	   and commands with an XPA server.

       o   High-level programs, centered on xpaset and xpaget, which allow data and information to be exchanged with XPA server programs from the
	   command line and from scripts.  These programs have the command syntax:

	     [data] | xpaset  [qualifiers ...]
		      xpaget  [qualifiers ...]

       o   An XPA name server program, xpans, through which XPA access point names are registered by servers and distributed to clients.

       Defining an XPA access point is easy: a server application calls XPANew(), XPACmdNew(), or the experimental XPAInfoNew() routine to create
       a named public access point.  An XPA service can specify "send" and "receive" callback procedures (or an "info" procedure in the case of
       XPAInfoNew()) to be executed by the program when an external process either sends data or commands to this access point or requests data or
       information from this access point.  Either of the callbacks can be omitted, so that a particular access point can be specified as
       read-only, read-write, or write-only.  Application-specific client data can be associated with these callbacks.  Having defined one or
       more public access points in this way, an XPA server program enters its usual event loop (or uses the standard XPA event loop).

       Clients communicate with these XPA public access points using programs such as xpaget, xpaset, and xpainfo (at the command line), or rou-
       tines such as XPAGet(), XPASet(), and XPAInfo() within a program.  Both methods require specification of the name of the access point.  The
       xpaget program returns data or other information from an XPA server to its standard output, while the xpaset program sends data or commands
       from its standard input to an XPA application. The corresponding API routines set/get data to/from memory, returning error messages and
       other info as needed.  If a template is used to specify the access point name (e.g., "ds9*"), then communication will take place with all
       servers matching that template.

       Please note that XPA currently is not thread-safe. All XPA calls must be in the same thread.

SEE ALSO

       See xpa(7) for a list of XPA help pages

version 2.1.14							   June 7, 2012 						       xpaintro(7)