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authorWilliam Joye <wjoye@cfa.harvard.edu>2018-01-23 16:53:51 (GMT)
committerWilliam Joye <wjoye@cfa.harvard.edu>2018-01-23 16:53:51 (GMT)
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-=pod
-
-=head1 NAME
-
-
-
-B<xpamb: the XPA Message Bus>
-
-
-
-=head1 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.
-
-
-
-=head1 OPTIONS
-
-
-
-
-
-For xpaset, several optional switches are used to save data and
-manipulate the stored data:
-
-
-=over 4
-
-
-
-
-
-
-=item *
-
-B<-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 B<replace>
-or B<del> to rectify this). The B<-add> switch is supported
-for backwards compatibility with xpa 2.0.
-
-
-
-
-=item *
-
-B<-replace [name]>
-
-
-Replace previously existing stored data having the same unique name
-with new data. This essentially is a combination of the B<del>
-and B<data> commands.
-
-
-
-
-=item *
-
-B<-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 B<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.
-
-
-
-
-=item *
-
-B<-send [name]>
-
-
-Broadcast the stored data buffer to the named template.
-
-
-
-
-=item *
-
-B<-del [name]>
-
-
-Delete the named data buffer and free all allocated space.
-
-
-=back
-
-
-
-
-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
-B<DS9>. In addition, the foo.fits file will be stored under the
-name of B<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".
-
-
-
-=head1 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 B<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
-B<XPAMB:xpamb> to which data can be sent for re-broadcast. You
-also can tell B<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 B<xpamb> is performed through
-B<xpaset> and B<xpaget> (or the corresponding API
-routines, B<XPASet()> and B<XPAGet()>) to the
-B<XPAMB:xpamb> access point. That is, B<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 B<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 B<DS9>. Notice that in this example,
-the foo2.fits file is not stored, but it could be stored by using the
-B<-store [name]> switch on the command line.
-
-
-The B<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 B<-info> and/or B<-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 B<-info> or B<-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
-
-
-
-
-=head1 SEE ALSO
-
-
-
-See xpa(n) for a list of XPA help pages
-
-
-
-=cut