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NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
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You are quite right: going to the extreme of one dataset per xml document is not very efficient. Having the ability to make a thredds catalog that contains a single URL does not mean that I am going to use it for all datasets: that is not my plan. I just want the ability to do it should it seem appropriate. Given that one server's collection is another server's dataset, you really ought to allow it. I think I can paraphrase your point about dataset aliases in that, since one can create an alias for a dataset, it does not matter that whatever document it is contained in may or may not contain a useless collection. True in a functional sense, I suppose, but rather ugly code that forces one to put in a collection even when it has no meaning in the sense of being a desired nesting. At the moment, I am requesting two small changes: let a catalog contain a collection or a dataset, and let suffix be a property of a service. The first one may not be particularly useful, but it is quite minor. The second one is extremely useful to me: I cannot make use of compound services without it. Please Please Please Benno Quoting Ethan Davis <edavis@xxxxxxxxxxxxxxxx>: > Hi Benno, > > At first I thought I agreed with you on the dataset aliasing issue. Now I'm > not > so sure. It seems like the problem boils down to the ability to reference a > dataset in an external catalog. Currently, the only external reference is a > catalogRef which dereferences into a collection. One solution would be to > switch > things around in one way or another so that catalogRef could be > dereferenced > into a dataset. On the other hand, an aliased dataset already gets > dereferenced > into a dataset. There are two issues at this point: first, dataset aliases > are > currently restricted to local datasets; second, datasets aren't complete > (i.e., > it doesn't necessarily contain a service). But that shouldn't matter, the > client > would need to parse the entire catalog doc to find the referenced dataset so > the > dereferencing step could also grab the required service. Here's an example: > > Top level catalog (cat1.xml) that references datasets (sorry, I don't know > XPointer well so I'm faking the actual references): > > <catalog name="catalog 1" version="0.6d"> > <collection name="collection 1"> > <service name="motherlode" serviceType="DODS" > base="http://motherlode.ucar.edu/cgi-bin/dods/nph-dods"; /> > <dataset alias="http://.../cat2.xml#id(ds1)" /> > <dataset alias="http://.../cat3.xml#id(ds2)" /> > <dataset alias="http://.../cat4.xml#id(ds3)" /> > </collection> > </catalog> > > Lower level catalog (cat2.xml) that contains info on a single dataset that > is > referenced by cat1.xml: > > <catalog name="catalog 2" version="0.6d"> > <collection name="collection 2"> > <service name="motherlode" serviceType="DODS" > base="http://motherlode.ucar.edu/cgi-bin/dods/nph-dods"; /> > <dataset name="dataset 1" ID="ds1" dataType="Grid"> > <access serviceName="motherlode" urlPath="ds1.nc" /> > <dataset alias="http://.../cat5.xml#id(ds5)" /> > <dataset alias="http://.../cat6.xml#id(ds6)" /> > </dataset> > </collection> > </catalog> > > Not sure if this is cleaner than figuring out how to get catalogRefs to > dereference into datasets rather than collections. The incompleteness of a > dataset seems to make it a bit harder on the client side. What does > everyone > think? > > > Also, one comment on efficiency of transmission. I certainly agree that it > won't > always be reasonable to represent an entire sites holdings in a single > THREDDS > catalog. But doesn't going to one dataset per step go to the other extreme > which > could also be somewhat inefficient? It certainly side-steps any opportunity > to > reduce the number of HTTP requests involved in the clients navigation of > the > hierarchy. Obviously, though, it will sometimes be desirable to do just > that. > > Ethan > > Benno Blumenthal wrote: > > > > Hi Ethan, > > > > I don't think so, but I don't know what you are talking about either, so > that > > is not definitive. Let me remind you what I am trying to do. I > contend > > (though people do not necessarily believe me) that it is not reasonable > to > > expect the THREDDS catalog at a particular site to fit into a single > file. > > It is certainly grossly inefficient to transmit the whole tree when the > user > > is only interested in a small branch. I think the IRI Data Library > catalog > > is one of those catalogs. The way I do that is to transmit one level of > > nesting at a time, using catalogREF to point to the sub-catalogs. Those > > subcatalogs may contain a collection or a dataset: there is no real > > difference as far as the Ingrid is concerned, though I am perfectly happy > to > > call objects without DODS access collections and objects with DODS access > > datasets. If you use datasetaliasing, then you still have the problem > of > > creating a THREDDS document that describes a dataset within a collection, > > which means a catalog must be able to contain a single dataset. No real > > progress. > > > > Benno > > > > > > Ethan Davis wrote: > > > > > Benno Blumenthal wrote: > > > > > > > > This does not quite work for me, unless you allow a catalog to > > > > contain exactly one collection or exactly one dataset. Right > > > > now the only thredds thing that can be pointed to is a catalog, > > > > with the added provision that a catalog with one collection is > > > > effectively that collection -- I would like to point to datasets > > > > with thredds, which requires either that a catalog can point to > > > > a dataset or a collection, or that you extend collection to be > > > > isomorphic to dataset. The catalog modification would be > > > > preferable. > > > > > > Currently dataset aliasing is confined to the current document. What do > > > people > > > think of extending dataset aliasing to reference datasets external to > the > > > current document. It means a bit higher maintenance risk as far as > broken > > > links > > > but not much more than the catalogRef already implies. Would that work > for > > > you > > > Benno? > > > > > > Ethan > > > > > > > > 6) access element can specify an absolute URL with a serverType -or- > a > > > > > reletive URL with a serverID. > > > > > > > > > > > > > If you are going to have a base element in service, it is only fair > that > > > you > > > > have a suffix element, too. Please leave the suffix element in. > > > > > > -- > > > 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/ > > > > -- > > Dr. M. Benno Blumenthal benno@xxxxxxxxxxxxxxxx > > International Research Institute for climate prediction > > Lamont-Doherty Earth Observatory of Columbia University > > Palisades NY 10964-8000 (845) 680-4450 > > > > > > -- > 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/ >
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