[Esip-discovery] Relating services to datasets served, and datasets to available services

Lynnes, Christopher S. (GSFC-6102) christopher.s.lynnes at nasa.gov
Wed Aug 31 09:14:39 EDT 2011

  Yes, that all makes a lot of sense.  But how do you see it impacting the DCP-2 proposal, which was targeted at alternate access methods (specifically) for individual files, not collections?

On Aug 30, 2011, at 8:07 PM, Wilson, Brian D (335G) wrote:

> Folks,
> I'd like to float a solution to two problems that are related to DCP-2:
> - how to have a service cast entry specify what datasets it "serves"
> - how to have a dataset (or collection) cast specify what services are
>    available to query, access, or transform each dataset.
> The proposed solution is to reuse the two casting standards and the
> OpenSearch protocol.
> The list of datasets that a particular service allows access to might be
> lengthy and computed using a search or semantic lookup process.
> Trying to 'name' datasets in the service cast is problematic since one
> has the problem of what names to use.
> So the idea is to hide this 'lookup' behind a URL, which could be
> an OpenSearch URL for example.
> So the scast entry would contain a <link> tag as follows:
> <link rel="http://esipfed.org/ns/discovery/1.1/collection#"
>     type="application/atom+xml"
>     xmlns:esip="http://esipfed.org/ns/discovery/1.1/"
>     esip:protocol="http://a9.com/-/spec/opensearch/1.1/"
>     href="<specific OpenSearch URL that does the appropriate search>" />
> Here I've reused the "collection" URI that has already been defined, but
> we could define a more specific one like "collectionsServed".  Either way,
> this is the known link (with rel=) to answer that question.
> The fact that this <link> is an OpenSearch is expressed in the 'protocol'
> attribute using the usual URI for versioned opensearch protocol.
> Of course, the <link> could be of some other type; e.g. a direct link to 
> a collection cast.
> And this is why I think we need both the 'rel' and 'protocol' attributes.
> One needs to specify both that the link's purpose is to answer collectionsServed
> question, and the protocol for getting the answer is an OpenSearch yielding a
> feed (the collection cast). By using two attributes, each of these URI's could
> also be de-referenceable and point to some additional information.
> The beauty of hiding the collectionsServed question behind a search link
> is that it nicely reuses the OpenSearch protocol and the collection cast format.
> The list of collectionsServed is available on demand, it can change without
> having to alter and re-publish the service cast, and metadata describing the
> collections is immediately available in the usual feed format.
> A GUI that wants to present metadata about the collections served has it
> immediately available.  But service metadata and dataset metadata are
> strictly separated into their respective casts, and reuse known formats.
> Behind the OpenSearch link, the collectionsServed lookup might be a
> SQL dbase lookup, or a SPARQL query, or involve some semantic reasoning.
> Implementers are free to innovate any way they want to, but in the meantime
> we can move forward with standardizing the casting formats.  If they don't
> want to use OpenSearch for this link, they can also choose an alternate
> protocol, at the risk of requiring users to understand additional request &
> response formats (besides OS and collection cast).
> The reverse problem can be solved in the same way.
> A link to 'servicesAvailable' could appear in each entry in a collection
> cast, as in:
> <link rel="http://esipfed.org/ns/discovery/1.1/service#"
>     type="application/atom+xml"
>     xmlns:esip="http://esipfed.org/ns/discovery/1.1/"
>     esip:protocol="http://a9.com/-/spec/opensearch/1.1/"
>     href="<specific OpenSearch URL that does the appropriate search>" />
> This time the opensearch yields a service cast listing the services available
> for that dataset, with the usual metadata in a known format.
> This also explains why I have been arguing for standardizing two attributes
> in the <link> tag:  rel and esip:protocol.  Given this flexibility and extra
> power, we can design solutions to thorny problems like above.  And all of
> our URI's will be cleanly defined or reused from W3C, and ultimately
> could be de-referenceable for more information if that serves additional
> purposes.
> It occurs to me that we might think about defining 'vendor-specific'
> MIME types for the casting (extended Atom) formats.  For example:
> "application/vnd.esip.discovery.cast.collection" and
> "application/vnd.esip.discovery.cast.service".  However, this seems a
> bit ugly and perhaps counterproductive.  For most purposes, it
> will be better to use the generic Atom or RSS mime type that more
> software will know.
> -- Brian
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Dr. Christopher Lynnes     NASA/GSFC, Code 610.2    phone: 301-614-5185

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