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 \-store 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 \-store 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 send 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(7) for a list of XPA help pages