----- Original Message ----- From: "Benno Blumenthal" <address@hidden> To: "John Caron" <address@hidden> Cc: "THREDDS" <address@hidden> Sent: Tuesday, June 04, 2002 2:09 PM Subject: Re: New attempt > John Caron wrote: > > > > > Im trying to think what is the meaning of serviceType="Catalog" in general? > > What should the client assume? It seems that if you want the client to be > > able to get the collection as a dataset, then you add a dataset element. If > > you want the client to "drill down" further, then a collection or catalogRef > > element can do that. What I have removed is the clear association between > > the dataset element and the collection, eg that these are the same thing. I > > have also made it more cumbersome(need two elements). I agree these are > > weaknesses. > > The client does not know that these are two different ways of looking at the > same thing -- the key piece of information that was trying to be conveyed. > The client does not have to present both -- maybe the client only presents > THREDDS choices because it has no DODS capabilities, another client does not > present the drill-down because it does have DODS capabilities. > > > > > The fact that you can produce a dataset as COARDS vs DIF, etc is also for me > > not so great of an example. Rather than modifying the underlying data acess > > (eg DODS), it seems simpler to add a metadata element. I admit that this is > > just an idea which has not been done yet. And you already have a server that > > does in fact modify the data access. But think of it from a client POV. > > Should she search through the services looking for a service of type DODS, > > subtype COARDS? Or search through the metadata looking for COARDS metadata, > > independent of service type? > > My point was your metadata tag was services for metadata. clients that can > only handle COARDS metadata would ask for COARDS metadata services. > > > > > > > A more compelling example would be where the dataset is served up through > > FTP and DODS, and ADDE, etc. But then I wonder/doubt whether one URL is > > likely to be able to be used for all these services. > > > > DODS already has multiple services -- ascii is not necessarily present, some of > the selection interfaces are optional, metadata is optional. > > > > > > > > > I am also concerned about XYZT clients (4D world view) -- how can I > > protect > > > them against higher (and other) dimensional data (ensemble member count, > > > spectral, different kinds of time (forecast start, lead, target time)? I > > > could convert to multiple datasets, or spatial grids, but it would be nice > > to > > > advertise the service. As well as supporting various binary and ascii > > data > > > formats. Or the THREDDS dataset (as opposed to collection/catalog) > > > description... > > > > I am not clear of "protect", did you mean "project" ? > > Protect -- 4D world clients simply fail when given something else, I would like > them to have an alternative. This may be difficult to provide this kind of negotiation in a standard way. It may be pushing catalogs too far right now. But I would certainly be willing to discuss it for future versions. Its like the WMS capabilities feature. > > > > > > 2) The access for the dataset LEVITUS94 is again via THREDDS (the present > > > collection) or via DODS (the access statement). Adding another dataset > > inside > > > the collection called "Daily" is not the same meaning at all. > > > > Sorry, I should have had: > > > > <dataset name="LEVITUS94 dataset" urlPath="SOURCES/.LEVITUS94/dods"/> > > <catalogRef xlink:title="Drill down into dataset" > > xlink:href="http://iridl.ldeo.columbia.edu/SOURCES/LEVITUS94/thredds.xml" /> > > > > In this case the dataset is presented to the user for immediate selection > > AND a link is presented for drilling further down. > > This is the wrong example: LEVITUS94 was a collection that was also available > through DODS -- now you have lost it completely. Im not sure how i have lost it. The intention is that the user can select it as a DODS dataset (with the dataset element) or expand its details through the catalogRef. I admit there is no formal connection between the two things, the user may or may not be able to infer they are the same thing. > It is an example for the subdatasets ANNUAL, etc, with the flaw that clients no > longer can tell that these are two different ways of getting the same thing as > mentioned above. I agree its a weakness. > > Besides, multiple services also will show up in aggregations of > THREDDS catalogs -- multiple servers serving the same dataset could be > represented as a single entry with multiple services -- in this case, services > with identical attributes except for path information. > > > > > The main thing service does is to let you specify a type and factor out the > > common URL base. then this is passed to "protocol aware" code. Because the > > das,dds,dods,info,ascii subservice URLs are always regular in how they are > > formed, it seems unnecessary to actually specify them. In principle > > subservices are probably useful but some concrete examples are needed. > > As I mentioned earlier, not all DODS servers have all the services. Even if > they did, it would not hurt to be able to list them. Right now I dont envision presenting the DODS services seperately to the user in the catalog selection. The model is that 1) the user selects a dataset from the catalog (no protocol dependence, no contacting the server yet) then 2) the app. client reads the dataset and allows further selection or information (contact server, protocol aware). So it seems to me like it doesnt make sense to specify das,dds,dods,info,ascii in the catalog since it is not used until the non-catalog part 2. > > > While I have not given examples, different datasets will have different > > > services, which is why I kept specifying using the access tag. Some > > datasets > > > will be incompatible with certain representations, so the service lists > > will > > > vary. One could argue that the THREDDS standard collection is a dataset > > not > > > available via DODS -- certainly that is the case for me. > > > > I understand you want to compactly specify what services are available for > > datasets. Im not sure we have enough examples to make sure we are doing it > > right. I am also oriented towards incremental design, doing what we can get > > right and iterating. > > OK with me, but we have lost the structure I was trying to express -- alternate > ways of accessing the same object, with emphasis on the same object. I will think some more about that, and ask if others have ideas about this also. I also want to double check that this example: <dataset name="LEVITUS94 dataset" urlPath="SOURCES/.LEVITUS94/dods"/> <catalogRef xlink:title="Drill down into dataset" xlink:href="http://iridl.ldeo.columbia.edu/SOURCES/LEVITUS94/thredds.xml" /> accurately reflects the capability you envision, except that there is no indication that these two are the same object. > > Benno > > -- > Dr. M. Benno Blumenthal address@hidden > International Research Institute for climate prediction > Lamont-Doherty Earth Observatory of Columbia University > Palisades NY 10964-8000 (845) 680-4450 > > >
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