1xpamb(1)                      SAORD Documentation                     xpamb(1)
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NAME

6       xpamb: the XPA Message Bus
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SYNOPSIS

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

15       For xpaset, several optional switches are used to save data and manipu‐
16       late the stored data:
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18       ·   -data [name]
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20           Add the supplied data buffer to a pool of stored data buffers,
21           using the specified name as a unique identifier for later
22           retrieval.  An error occurs if the name already exists (use either
23           replace or del to rectify this). The -add switch is supported for
24           backwards compatibility with xpa 2.0.
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26       ·   -replace [name]
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28           Replace previously existing stored data having the same unique name
29           with new data. This essentially is a combination of the del and
30           data commands.
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32       ·   -info ["'info string'"]
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34           When adding a data buffer, you can specify an informational string
35           to be stored with that data.  This string will be returned by
36           xpaget:
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38             xpaget xpamb foo -info
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40           (along with other information such as the date/time of storage and
41           the size of the data buffer) if the -info switch is specified. If
42           the info string contains spaces, you must enclose it in two sets of
43           quotes:
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45             cat foo ⎪ xpaset xpamb -store foo -info "'this is info on foo'"
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47           The first set of quotes is removed by the shell while the second is
48           used to delineate the info string.
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50       ·   -send [name]
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52           Broadcast the stored data buffer to the named template.
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54       ·   -del [name]
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56           Delete the named data buffer and free all allocated space.
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58       Switches can be used in any combination that makes sense. For example:
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60         cat foo.fits ⎪ xpaset xpamb -store foo -info "FITS" "DS9:*" fits foo.fits
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62       will broadcast the foo.fits image to all access points of class DS9.
63       In addition, the foo.fits file will be stored under the name of foo for
64       later manipulation such as:
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66         xpaset -p xpamb -send foo "DS9:*" fits foo.fits
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68       will re-broadcast the foo.fits image to all access points of class
69       "DS9".
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DESCRIPTION

72       A "classical" message bus (such as ToolTalk) consists of servers and
73       clients, along with a mediating program that transfers data between
74       different processes. XPA takes a slightly different approach in that
75       communication between clients and servers is direct.  This generally is
76       the correct technique when there is only one connection (or even a
77       small number of connections), but can become inefficient for the serv‐
78       ing program if a large amount of data is being transferred to many
79       clients. For example, if a real-time data acquisition program is broad‐
80       casting a FITS image to several clients, it would need to transmit that
81       image to each client individually.  This might interfere with its own
82       processing cycles.  The preferable mechanism would be to pass the image
83       off to an intermediate program that can then broadcast the data to the
84       several clients.
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86       The xpamb program can alleviate such problems by functioning as a mes‐
87       sage bus in cases where such an intermediary process is wanted.  It
88       pre-defines a single access point named XPAMB:xpamb to which data can
89       be sent for re-broadcast. You also can tell xpamb to save the data, and
90       associate with that data a new access point, so that it can be
91       retrieved later on.
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93       All interaction with xpamb is performed through xpaset and xpaget (or
94       the corresponding API routines, XPASet() and XPAGet()) to the
95       XPAMB:xpamb access point. That is, xpamb is just another XPA-enabled
96       program that responds to requests from clients. The paramlist is used
97       to specify the targets to which the data will be for re-broadcast, as
98       well as the re-broadcast paramlist:
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100         data ⎪ xpaset xpamb [switches] broadcast-target broadcast-paramlist
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102       Optional switches are used to store data, and manipulate stored data,
103       and are described below.
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105       In its simplest form, you can, for example, send a FITS image to xpamb
106       for broadcasting to all ds9 image simply by executing:
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108         cat foo.fits ⎪ xpaset xpamb "DS9:*" fits foo.fits
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110       Since DS9 is the class name for the ds9 image display program, this
111       will result in the FITS image being re-sent to all fits access points
112       for all active image display programs.
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114       You can send stored data and new data to the same set of access points
115       at the same time.  The stored data always is send first, followed by
116       the new data:
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118         cat foo2.fits ⎪ xpaset xpamb -send foo "DS9:*" fits foo.fits
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120       will first send the foo.fits file, and then the foo2.fits file to all
121       access points of class DS9.  Notice that in this example, the foo2.fits
122       file is not stored, but it could be stored by using the -store [name]
123       switch on the command line.
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125       The xpaget command can be used to retrieve a data from XPA access
126       points or from a stored data buffer, or retrieve information about a
127       stored data buffer.  If no arguments are given:
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129         xpaget xpamb
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131       then information about all currently stored data buffers is returned.
132       This information includes the data and time at which the data was
133       stored, the size in bytes of the data, and the supplied info string.
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135       If arguments are specified, they will be in the form:
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137         xpaget xpamb [-info] [-data] [name [paramlist]]
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139       If the optional -info and/or -data switches are specified, then infor‐
140       mation and/or data will be returned for the named data buffer following
141       the switches. You can use either or both of these switches in a single
142       command. For example, if the -info switch is used:
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144         xpaget xpamb -info foo
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146       then the info about that stored data buffer will be returned.  If the
147       -data is used with a specific name:
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149         xpaget xpamb -data foo
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151       then the stored data itself will be returned. If both are used:
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153         xpaget xpamb -info -data foo
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155       then the info will be returned, followed by the data. Note that it is
156       an error to specify one of these switches without a data buffer name
157       and that the paramlist will be ignored.
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159       If neither the -info or -data switch is specified, then the name refers
160       to an XPA access point (with an optional paramlist following).  For
161       example:
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163         xpaget xpamb ds9 file
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165       is equivalent to:
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167         xpaget ds9 file
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SEE ALSO

170       See xpa(n) for a list of XPA help pages
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174version 2.1.12                 January 26, 2010                       xpamb(1)
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