Re: THREDDS API Question

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:

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:


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:
// 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?


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/");
    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.



= Nathan Potter ndp at
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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= Nathan Potter ndp at
OPeNDAP, Inc.                        541.752.1852

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