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 messaging to and from these servers.
       It supports direct communication between clients and servers, or indirect communication via an
       intermediate 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:

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

       •   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.

       •   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 ...]

       •   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 routines 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