Ethan,
I've been thinking about it too. It is probably the case that this is
an intractable issue. The THREDDS architecture benefits from the
ability to write catalogs independent of the data source/archive.
THREDDS is designed so that I can write a catalog and serve it on my
system, while the data sources referenced in the catalog may exist on
different systems - often far away and outside my sphere of influence.
The idea that the Access Link (the link that THREDDS generates that
allows data access) can be used to back track into the THREDDS
catalog is cross purposes to the THREDDS design. It would be possible
for a particular server implementation to have very carefully
constructed THREDDS catalogs that would allow it to do this, but a
general case is simply not possible.
At least as far as I can see it.
Nathan
On Jun 11, 2007, at 4:09 PM, Ethan Davis wrote:
Hi Nathan,
Sorry this is taking awhile. I'm trying to figure out some of the
trade offs and such involved in a variety of ways of handling this.
I should have a more detailed response tomorrow.
Ethan
Nathan Potter wrote:
Ethan et al.,
After talking with Ethan on the phone today I think I can state
the issue more clearly:
The current THREDDS Servlet Framework (TSF) does not allow the
collection/dataset information to be retrieved via the request URL.
The API method DataRootHandler.getCatalog(java.lang.String path,
java.net.URI baseURI) expects the "path" parameter to be the path
in the THREDDS catalog to the catalog file. There is no
restriction on the file name of the catalog file. The path in the
THREDDS catalog to the file may be different that the access URL.
What this means is that when a servlet receives an access request,
even one that comes from a valid access link in a THREDDS catalog
(.html), the servlet only knows about the request URL, nothing
more. If the servlet needs to get the THREDDS dataset/collection
information (and associated metadata if any) then it has no
recourse but to attempt to search the catalog from the highest
level looking for a dataset with a matching "urlPath" attribute.
This activity may fail if:
- The THREDDS catalog employs <catalogRef> elements.
- The "urlPath" is not unique within the catalog.
I think that the TSF API should be augmented with accessor methods
that allow the DataRootHandler to return InvDataset an InvCatalog
to be retrieved based on information that a servlet has access to
at run time, i.e. data that can be retrieved from the
HttpServletRequest object.
Nathan
On Jun 4, 2007, at 5:00 PM, Nathan Potter wrote:
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:
edavis@xxxxxxxx
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Boulder, CO 80307-3000 http://
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-------------------------------------------------------------------
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=
Nathan Potter ndp at opendap.org
OPeNDAP, Inc. 541.752.1852
=
Nathan Potter ndp at opendap.org
OPeNDAP, Inc. 541.752.1852
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=====================================================================
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Ethan R. Davis Telephone: (303)
497-8155
Software Engineer Fax: (303)
497-8690
UCAR Unidata Program Center E-mail:
edavis@xxxxxxxx
P.O. Box 3000
Boulder, CO 80307-3000 http://
www.unidata.ucar.edu/
----------------------------------------------------------------------
-----
=
Nathan Potter ndp at opendap.org
OPeNDAP, Inc. 541.752.1852
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