DCP-4

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DCP-4: OPeNDAP Links in the Atom <link/> element

  • Progress (fill in the dates as the process moves forward)
  1. Submitted on: 14 Feb 2012
  2. Review period: 14 Feb 2012 to 14 March 2012
  3. Revision: N/A
  4. Vote: TBD
  5. Final review: TBD
  6. Ratified: TBD
  7. Rejected: TBD

Description

To include URLs that reference OPeNDAP servers and/pr response objects in Atom-based casts, use the XLink 1.1 protocol. The technique presented relies on only standard behavior for XLink 1.1 conformant software. It can be trivially extended to other web-based data access protocols (e.g., WCS).

Problem Addressed

We have a number of datacast feeds that are, or have the capability to, include links that can be used to access those cast datasets using OPeNDAP servers. However, few feed readers will understand how to process the responses from those servers. Furthermore, each 'OPeNDAP URL' really specifies a collection of potential responses, each accessed using a specific suffix appended to the pathname component of a/the URI/URL.

Proposed Solution

Include multiple <link href=.../> elements for each dataset to be cast. Different <link/> elements can be categorized as follows:

  1. A <link/> with only the Atom 1.1 href, title and rel attributes will be interpreted to reference the 'raw' dataset. For this the atom 1.1 length attribute should be included and its value should be the size of the file in bytes.
  2. A <link/> with the attributes xlink:type="simple" with the additional attributes xlink:role="<<dap schema URL (dereferenceable)>>" and title="<<some human readable title that mentions OPeNDAP>>" will be interpreted to reference a DAP service endpoint. The URL that can be dereferenced to access the endpoint will be the value of the href attribute and the rel attribute will be "enclosure"
  3. For the OPeNDAP case, the only required link element is the service endpoint as described in the preceding item.
  4. It is assumes that every valid OPeNDAP server that implements DAP2 (and the DAP2 implementation is indicated using the xlink:role attribute) will provide the following response types which do not need to be enumerated. Any DAP2-capable client will know how to access these responses:
    • DAS
    • DDS
    • DODS (this is the 'data' response)
  5. The following responses are not part of the DAP2 specification but are commonly provided; When they are present and a provider wants to make sure clients know they can be accessed, The link element will use the following rel and type attribute values:
Server Response Type rel type xlink:arcrole
HTML describedby text/html #html
info describedby text/html #info
DDX describedby application/xml #ddx
RDF describedby application/rdf+xml #rdf
ASCII enclosure text/plain #ascii
NetCDF enclosure application/x-netcdf #netcdf

Examples

Access to the raw file; no OPeNDAP server involved
<link rel="enclosure" href="http://data.gsfc.nasa.gov/data/file.hdf" title="HDF5 data showing controversial things" type="application/x-hdf5"/>
The DAP2 service endpoint
<link rel="enclosure" href="http://data.gsfc.nasa.gov/opendap/hyrax/data/file.hdf" title="OPeNDAP acces to controversial data" type="text/plain" xlink:role="http://xml.opendap.org/dap/dap2.xsd" xlink:type="simple"/>
The DAP2 server's HTML Form interface for this file
<link rel="describedby" href="http://data.gsfc.nasa.gov/opendap/hyrax/data/file.hdf.html" title="OPeNDAP HTML Form interface acces to controversial data" type="application/rdf+xmll" xlink:role="http://xml.opendap.org/dap/dap2.xsd" xlink:arcrole="#html" xlink:type="simple"/>
The RDF response from the DAP2 server
<link rel="describedby" href="http://data.gsfc.nasa.gov/opendap/hyrax/data/file.hdf.html" title="OPeNDAP HTML Form interface acces to controversial data" type="text/html" xlink:role="http://xml.opendap.org/dap/dap2.xsd" xlink:arcrole="#rdf" xlink:type="simple"/>
The ASCII response from the DAP2 server
<link rel="enclosure" href="http://data.gsfc.nasa.gov/opendap/hyrax/data/file.hdf.html" title="OPeNDAP HTML Form interface acces to controversial data" type="text/plain" xlink:role="http://xml.opendap.org/dap/dap2.xsd" xlink:arcrole="#ascii" xlink:type="simple"/>
The netCDF response from the DAP2 server
<link rel="enclosure" href="http://data.gsfc.nasa.gov/opendap/hyrax/data/file.hdf.html" title="OPeNDAP HTML Form interface acces to controversial data" type="application/x-netcdf" xlink:role="http://xml.opendap.org/dap/dap2.xsd" xlink:arcrole="#netcdf" xlink:type="simple"/>

Notes:

  1. Pedro 19 March 2012 : The xlink:arcrole should probably only be used in the specific case where you need that information to differentiate something (HTML form / info responses). In the other cases it might bring the ambiguity because the link:arcrole is normally used to define the "purpose" of a link and not the type.
  2. JamesGallagher 13:33, 18 March 2012 (MDT): Added the xlink:arcrole idea. I think it's very useful to make disambiguating the different response types as easy as possible. Inferring the return type using a combination of rel and type is two tests, not one and won't actually work in the case of the HTML form and info responses.
  3. JamesGallagher 16:35, 14 February 2012 (MST): Updated to include the Atom 1.1 type attribute as per Pedro's suggestion.
  4. The xlink:type="simple" is not needed; if xlink:type is not present it defaults to simple
  5. JamesGallagher 20:59, 13 March 2012 (MDT) At today's meeting, we referred to the DDX response as part of DAP2. It's not actually in the specification that was approved by NASA. I think we should list it with the 'commonly provided' group of responses.
  6. Truslove 16:59, 13 April 2012 (MDT): To clarify point 1 of the proposed solution, would this be when links have only Atom attributes and rel="enclosure"?

Rationale for the Solution

Xlink is the best way to include arbitrary metadata about a link in a link. Using it takes advantage of existing XML standards and so it is within the realm of reason to assume feed readers will not behave in a way that is toxic to users.

Discussions

The Atom 1.1 title attribute is used rather than xlink:title because feed readers are more likely to display it (caution: this is a WAG on my part - jimg).

The fact that xlink includes a namespace-qualified href attribute and the Atom 1.1 specification requires that href be present in <link/> elements is cumbersome. Note that xlink:href is not required for xlink:type="simple".

NB: It is not an error for a simple-type element to have no locator (href) attribute value. If a value is not provided, the link is simply untraversable. Such a link may still be useful, for example, to associate properties with the resource by means of XLink attributes. See: http://www.w3.org/TR/xlink11/ section 5.2.

Notes (Pedro)

xlink:role in atom:link

from the information in

http://www.w3.org/TR/xlink11/#att-method

it seems that is "legal" to add any of the xlink attribute (e.g. xlink:role) to an element

JamesGallagher 16:32, 14 February 2012 (MST): Correct, but exactly which of the attributes are legal is a function of the value of xlink:type; it's a minor point because when xlink:type is simple all of the attributes we've been talking about using for both DCP-4 and DCP-5 are allowed. The table in section 4.1 of http://www.w3.org/TR/xlink11/ shows the supported collections of attributes depending on the value of xlink:type.

MIME-TYPES

In your examples we are missing the type attribute that gives us the mime-type of the resource

JamesGallagher 16:24, 14 February 2012 (MST): I'll update the examples. NB: The MIME types for the OPeNDAP responses will be different from application/x-hdf5

So the examples would be:

<link rel="enclosure" href="http://data.gsfc.nasa.gov/data/file.hdf" title="HDF5 data showing controversial things" type="application/x-hdf5"/>

<link rel="enclosure" href="http://data.gsfc.nasa.gov/opendap/hyrax/data/file.hdf" title="OPeNDAP acces to controversial data" xlink:role="http://xml.opendap.org/DAP2#" type="application/x-hdf5"/>

<link rel="enclosure" href="http://data.gsfc.nasa.gov/opendap/hyrax/data/file.hdf.ddx" title="OPeNDAP XML metadata response, for those weary of controversy" xlink:role="http://xml.opendap.org/DAP2#ddx" type="text/xml"/>

Consensus

Voting results.

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