On Jun 4, 2007, at 1:05 PM, Ethan Davis wrote:
Hi Nathan,
Can you explain the context for these questions. This is on the
server side (in Hyrax)?
Yes, server side.
Nathan Potter wrote:
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.
Just to clarify our terminology. When you say "THREDDS API" you
mean both the thredds.catalog and thredds.servlet packages? I
generally split those apart and call the thredds.catalog package
the "THREDDS Catalog API" and call the thredds.servlet package the
"THREDDS Servlet Framework" (TSF).
[Note: the TSF is probably only useful for those writing servers.]
I wasn't distinguishing. But since DataRootHandler is in the TSF then
that is where I am suggesting an API change.
** 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
Is this URL for an OPeNDAP HTML response?
Right, but the requested response isn't really meaningful in this
discussion since all I am really after is the THREDDS dataset
information for the atom/leaf/dataset test.hdf
Are you trying to get the property from the THREDDS catalog so you
can use it in the OPeNDAP response?
Well... In truth it's much more complex than that, but since I will
have to do that too we can roll with that vision for the moment.
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.)
That is about as simple as it gets. Though I would suggest you make
sure the THREDDS configuration (TSF) knows about this dataset first
by getting the CrawlableDataset that matches the dataset URL:
DataRootHandler.getCrawlableDataset("wcs/MODIS/Grid/test.hdf")
// I dropped of the trailing ".html" assuming it was the
OPeNDAP dataset URL extension
When I tried this I could only get CrawlableDataset objects for
catalogs that were part of a <datasetScan>
Are you using InvDataset.findDatasetByName( String name) to find
the child dataset?
No.
Also, depending on how you setup your dataset IDs, you could ask
the catalog to find the dataset by ID, like
cat.findDatasetByID( "wcs/MODIS/Grid/test.hdf")
Ahhh... I just tried that and it works. So, that greatly simplifies
that step, thanks!
** 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".
Why do you need to create this hierarchy of directories mimicking
the data collection hierarchy? The TSF should keep track of your
config catalogs and the automatically generated catalogs.
Right, but if all of the THREDDS catalog files have the name
"catalog.xml" they can't all be in the same directory, so they have
to live in some kind of directory hierarchy - I just figured it made
sense to mimic the collection organization, but that's not necessary.
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...)
I'm sorry, I'm having a hard time following here. What are you
trying to do and why?
For any request that is looking for one of the OPeNDAP data responses
I need to search the THREDDS catalog for the dataset, and if found, I
need to extract any metadata that may in the catalog for that dataset.
Is the problem that you may not know if the dataset is contained in
a catalog generated because of a datasetScan element or contained
directly in one of the THREDDS config catalogs?
I think that's a separate issue.
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?
I'll have to take a closer look at this.
Ethan
Nathan
=
Nathan Potter ndp at opendap.org
OPeNDAP, Inc. 541.752.1852
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Ethan R. Davis Telephone: (303)
497-8155
Software Engineer Fax: (303)
497-8690
UCAR Unidata Program Center E-mail:
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Boulder, CO 80307-3000 http://
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Nathan Potter ndp at opendap.org
OPeNDAP, Inc. 541.752.1852
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