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106 lines
4.2 KiB
Plaintext
106 lines
4.2 KiB
Plaintext
/*! \page dap4 DAP4 Support
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\section dap4_accessing The DAP4 Prototocol
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\tableofcontents
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\subsection dap4_introduction DAP4 Introduction
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Beginning with netCDF version 4.2.2, optional support is provided for
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accessing data through DAP4 servers using the DAP4 protocol.
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DAP4 support is automatically enabled if a usable curl library can be
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set using the LDFLAGS environment variable (similar to the way
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that the HDF5 libraries are referenced).
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DAP2 plus DAP4 support can forcibly be enabled or disabled using the --enable-dap
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flag or the --disable-dap flag, respectively. If enabled,
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then DAP4 support requires access to the curl library.
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Refer to the installation manual for details
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DAP4 uses a data model that is similar to, but not identical with,
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the netCDF-Enhanced (aka netcdf-4) model.
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Generically, the DAP4 data model is encoded in XML document
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called a DMR.
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For detailed information about the DAP4 DMR, refer to
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the DAP4 specification Volume 1:
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http://docs.opendap.org/index.php/DAP4:_Specification_Volume_1
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\subsection dap4_access Accessing Data Using the DAP4 Prototocol
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In order to access a DAP4 data source through the netCDF API, the
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file name normally used is replaced with a URL with a specific
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format. The URL is composed of three parts.
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- URL - this is a standard form URL with specific markers to indicate that
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it refers to a DAP4 encoded dataset. The markers can be one of the following
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examples.
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+ [dap4]http://remotetest.unidata.ucar.edu/d4ts/test.01
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+ [protocol=dap4]http://remotetest.unidata.ucar.edu/d4ts/test.01
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+ http://remotetest.unidata.ucar.edu/d4ts/test.01#dap4
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+ http://remotetest.unidata.ucar.edu/d4ts/test.01#protocol=dap4
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+ dap4://remotetest.unidata.ucar.edu/d4ts/test.01
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Note that the last case is converted internally to one of the other forms.
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- Constraints - these are suffixed to the URL and take the form
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“?dap4.ce=\<expression\>”. The form of the constraint expression
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is somewhat complicated, and the specification should be consulted.
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- Client parameters - these may be specified in either of
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two ways. The older, deprecated form prefixes text to the
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front of the url and is of the the general form [\<name>]
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or [\<name>=value]. Examples include [show=fetch].
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The newer, preferred form prefixes the
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parameters to the end of the url using the semi-standard '#'
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format: e.g. http://....#show=fetch&noprefetch.
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It is possible to see what the translation does to a particular DAP4
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data source in two steps. First, one can examine the DMR
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source through a web browser and then second, one can examine
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the translation using the "ncdump -h" command to see the
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corresponding netCDF-4 representation.
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For example, if a web browser is given the following (fictional) URL,
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it will return the DMR for the specified dataset
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\code
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http://remotetest.unidata.ucar.edu/d4ts/test.01.dmr#dap4
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\endcode
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By using the following ncdump command, it is possible to see the
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equivalent netCDF-4 translation.
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\code
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ncdump -h http://remotetest.unidata.ucar.edu/d4ts/test.01#dap4
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\endcode
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\subsection dap4_defined_params Defined Client Parameters
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Currently, a limited set of client parameters is
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recognized. Parameters not listed here are
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ignored, but no error is signalled.
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Parameter Name Legal Values Semantics
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- "log" | "log=<file>" - Turn on logging and send the log output to
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the specified file. If no file is specified, then output is sent to standard
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error.
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- "show=fetch" - This parameter causes the netCDF code to log a copy
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of the complete url for every HTTP get request. If logging is
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enabled, then this can be helpful in checking to see the access
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behavior of the netCDF code.
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\subsection dap4_debug Notes on Debugging DAP4 Access
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The DAP4 support has a logging facility.
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Note that this is currently separate from the
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existing netCDF logging facility. Turning on this logging can
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sometimes give important information. Logging can be enabled by
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using the client parameter "log" or "log=filename",
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where the first case will send log output to standard error and the
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second will send log output to the specified file.
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Users should also be aware that if one is
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accessing data over an NFS mount, one may see some .nfsxxxxx files;
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those can be ignored.
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*/
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