Greetings,
So I am using the THREDDS API in an attempt to get the <property>
elements for a dataset. I've run into a couple of (possibly related)
problems.
** 1) I can't get the dataset information without searching.
In the HttpServletRequest I have the URL for the dataset, say:
http://localhost:8080/opendap/wcs/MODIS/Grid/test.hdf.html
In order for me to get THREDDS to divulge the <property> elements for
the dataset I have to:
- take the dataset name "wcs/MODIS/Grid/test.hdf.html" and back track
to the
collection name, "wcs/MODIS/Grid/".
- ask the DataRootHandler for the InvCatalog for "wcs/MODIS/Grid/"
- Ask the InvCatalog for the InvDataset for "wcs/MODIS/Grid/"
- Search the child datasets of the "wcs/MODIS/Grid/" InvDataset for the
one whose name (lexically) matches "wcs/MODIS/Grid/test.hdf.set"
- Read the properties of that InvDataset
That seems awfully complex. (Of course there may a more straight
forward way that I am not aware of.)
** 2) When I ask for a catalog I have to know the name of the XML
file in which it resides.
In the above example, when I ask the DataRootHandler for the
InvCatalog I ask for: " wcs/MODIS/Grid/catalog.xml" Which is all well
and good if all of the catalogs are stored in files called
catalog.xml. Essentially this means that anyone configuring a THREDDS
catalog has to create a hierarchy of directories that mimics the
organizatiopn of the collections, and all of the THREDDS information
must be stored in files called "catalog.xml".
THREDDS does not actually require this - I can make a complex
hierarchy of collections by using either a single (complex) top level
catalog.xml file, or a collection of XML files in a single directory
that employ <catalogRef> elements to create their organizations.
However the API breaks down in both cases.
If the catalog is composed of a collection of XML files in a single
directory that employ <catalogRef> elements to create their
organizations, then in order to retrieve catalog information I would
have to KNOW how the information was organized (file names, directory
hierarchy , etc.) But I don't know - since the catalog may be created
by a user after compile time (although THREDDS does know this since
it parsed all of the catalog information at start up) - and I
shouldn't have to know. For me to know would require that I parse the
top level catalog.xml file and build the XML doc tree myself. At
which point it I can get the elusive <property> elements from the XML
doc in memory.
If the catalog is composed of a single (complex) top level
catalog.xml file then I would have to know that and just ask for the
top level catalog.
(Searching the entire catalog from the top down for my dataset
doesn't seem to work either...)
All of these methods of writing and organizing catalogs are
legitimate in THREDDS, and users writing THREDDS catalogs would
likely employ one or more of these methods when writing their catalogs.
I propose that the THREDDS API be extended so that one can simply ask
the DataRootHandler for an InvDataset or an InvCatalog. Like:
InvDataset id = drh.getDataSet("wcs/MODIS/foo.nc");
InvCatalog id = drh.getCatalog("wcs/MODIS/");
or possible the InvDataset that represents a collection:
InvDataset id = drh.getDataSet("wcs/MODIS/");
If the DataRootHandler doesn't have it, return null.
Is that unreasonable?
Nathan
=
Nathan Potter ndp at opendap.org
OPeNDAP, Inc. 541.752.1852
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