NAME

       xpamb - the XPA Message Bus

SYNOPSIS

       The xpamb program can act as a "classical" message bus interface between clients and servers. A client
       can send a data request to the message bus, which then interfaces with multiple servers and returns the
       data back to the client.

OPTIONS

       For xpaset, several optional switches are used to save data and manipulate the stored data:

       •   -data [name]

           Add  the  supplied data buffer to a pool of stored data buffers, using the specified name as a unique
           identifier for later retrieval.  An error occurs if the name already exists (use  either  replace  or
           del to rectify this). The -add switch is supported for backwards compatibility with xpa 2.0.

       •   -replace [name]

           Replace  previously  existing stored data having the same unique name with new data. This essentially
           is a combination of the del and data commands.

       •   -info ["'info string'"]

           When adding a data buffer, you can specify an informational string to be stored with that data.  This
           string will be returned by xpaget:

             xpaget xpamb foo -info

           (along with other information such as the date/time of storage and the size of the  data  buffer)  if
           the -info switch is specified. If the info string contains spaces, you must enclose it in two sets of
           quotes:

             cat foo | xpaset xpamb -data foo -info "'this is info on foo'"

           The  first  set  of  quotes  is  removed  by the shell while the second is used to delineate the info
           string.

       •   -send [name]

           Broadcast the stored data buffer to the named template.

       •   -del [name]

           Delete the named data buffer and free all allocated space.

       Switches can be used in any combination that makes sense. For example:

         cat foo.fits | xpaset xpamb -data foo -info "FITS" "DS9:*" fits foo.fits

       will broadcast the foo.fits image to all access points of class DS9.  In addition, the foo.fits file will
       be stored under the name of foo for later manipulation such as:

         xpaset -p xpamb -send foo "DS9:*" fits foo.fits

       will re-broadcast the foo.fits image to all access points of class "DS9".

DESCRIPTION

       A "classical" message bus (such as ToolTalk) consists of servers and  clients,  along  with  a  mediating
       program  that transfers data between different processes. XPA takes a slightly different approach in that
       communication between clients and servers is direct.  This generally is the correct technique when  there
       is  only  one  connection  (or  even  a  small number of connections), but can become inefficient for the
       serving program if a large amount of data is being transferred to many clients. For example, if  a  real-
       time  data acquisition program is broadcasting a FITS image to several clients, it would need to transmit
       that image to each client individually.  This might  interfere  with  its  own  processing  cycles.   The
       preferable  mechanism  would  be to pass the image off to an intermediate program that can then broadcast
       the data to the several clients.

       The xpamb program can alleviate such problems by functioning as a message bus  in  cases  where  such  an
       intermediary process is wanted.  It pre-defines a single access point named XPAMB:xpamb to which data can
       be  sent  for  re-broadcast. You also can tell xpamb to save the data, and associate with that data a new
       access point, so that it can be retrieved later on.

       All interaction with xpamb is performed through xpaset and xpaget (or  the  corresponding  API  routines,
       XPASet()  and  XPAGet())  to  the  XPAMB:xpamb  access  point. That is, xpamb is just another XPA-enabled
       program that responds to requests from clients. The paramlist is used to specify the targets to which the
       data will be for re-broadcast, as well as the re-broadcast paramlist:

         data | xpaset xpamb [switches] broadcast-target broadcast-paramlist

       Optional switches are used to store data, and manipulate stored data, and are described below.

       In its simplest form, you can, for example, send a FITS image to xpamb for broadcasting to all ds9  image
       simply by executing:

         cat foo.fits | xpaset xpamb "DS9:*" fits foo.fits

       Since  DS9  is the class name for the ds9 image display program, this will result in the FITS image being
       re-sent to all fits access points for all active image display programs.

       You can send stored data and new data to the same set of access points at the same time.  The stored data
       always is sent first, followed by the new data:

         cat foo2.fits | xpaset xpamb -send foo "DS9:*" fits foo.fits

       will first send the foo.fits file, and then the foo2.fits file to all access points of class DS9.  Notice
       that in this example, the foo2.fits file is not stored, but it could be stored by using the -store [name]
       switch on the command line.

       The xpaget command can be used to retrieve a data from XPA access points or from a stored data buffer, or
       retrieve information about a stored data buffer.  If no arguments are given:

         xpaget xpamb

       then information about all currently stored data buffers is returned. This information includes the  data
       and time at which the data was stored, the size in bytes of the data, and the supplied info string.

       If arguments are specified, they will be in the form:

         xpaget xpamb [-info] [-data] [name [paramlist]]

       If  the optional -info and/or -data switches are specified, then information and/or data will be returned
       for the named data buffer following the switches. You can use either or  both  of  these  switches  in  a
       single command. For example, if the -info switch is used:

         xpaget xpamb -info foo

       then the info about that stored data buffer will be returned.  If the -data is used with a specific name:

         xpaget xpamb -data foo

       then the stored data itself will be returned. If both are used:

         xpaget xpamb -info -data foo

       then  the  info  will be returned, followed by the data. Note that it is an error to specify one of these
       switches without a data buffer name and that the paramlist will be ignored.

       If neither the -info or -data switch is specified, then the name refers to an XPA access point  (with  an
       optional paramlist following).  For example:

         xpaget xpamb ds9 file

       is equivalent to:

         xpaget ds9 file

SEE ALSO

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

version 2.1.18                                    May 12, 2017                                          xpamb(1)