Revised XRI resolution output proposal

["Drummond Reed" <drummond.reed@cordance.net>]

The discussion of the XML output format from the service endpoint selection
phase of XRI resolution has raised a larger issue. We started with the
assumption that the output of service endpoint selection was a single URI.
Then...

* We revised that to an array of URIs so that consuming applications could
make their own choices of transport protocols (such as choosing an HTTPS URI
vs. an HTTP URI).

* Then Steve raised the need for consuming applications to know the
CanonicalID(s) associated with the service endpoint, so we agreed on
Thursday's call to add a list of CanonicalIDs.

* Today, working on the issue of an XML output format for the results of
service endpoint selection, I realized that there are at least two other
items of metadata about a service endpoint that a consuming application may
need to know:

	1) The Type (which may not have been part of the input parameters
because a Path parameter or a MediaType parameter or even null parameters
may have been used to select the service endpoint).
	2) The ProviderID (for applications that need to know this to do
things like download SAML metadata regarding the service provider -- this is
already a hard requirement of the SAML-based XRI authentication service
being developed at XDI.org).

* Finally, I had the realization this morning that since XRDs are extensible
and may include other metadata describing a service endpoint, we can't even
make an arbitrary decision about which service endpoint elements/attributes
should be returned to an application as the output of service endpoint
selection.

Conclusion: 

1) The output of the service endpoint selection phase of XRI resolution
should be the selected <Service> element from the final XRD.

2) This Service element should contain all the metadata that the service
author wishes to make available to a consuming application (including, for
example, the CanonicalID).

3) When accessed via a local API, this should be expressed as an array of
name/value pairs in which the names are the fully-qualified XPath names of
the elements and attributes in the selected Service element and the value is
the contents of the element/attribute. Example:

xri://$xrd*($v*2.0)/ProviderID, xri://!!1000!1234.5678
xri://$xrd*($v*2.0)/CanonicalID, xri://=!A1B2.C3D4
xri://$xrd*($v*2.0)/Type, xri://+contact*($v*1.0)
xri://$xrd*($v*2.0)/Type/@select, true
xri://$xrd*($v*2.0)/URI, http://example.com/ctc/=!A1B2.C3D4/
xri://$xrd*($v*2.0)/URI/@append, qxri
xri://$xrd*($v*2.0)/URI, http://a.example.com/ctc/=!A1B2.C3D4/
xri://$xrd*($v*2.0)/URI/@append, qxri
xri://$xrd*($v*2.0)/URI, https://example.com/ctc/=!A1B2.C3D4/
xri://$xrd*($v*2.0)/URI/@append, qxri

4) When accessed via an HTTP proxy resolver, the output from the service
endpoint selection phase would be simply the selected <Service> element.
Example:

<Service xlmns="xri://$xrd*($v*2.0)">
	<ProviderID>xri://!!1000!1234.5678</ProviderID>
	<CanonicalID>xri://=!A1B2.C3D4</CanonicalID>
	<Type select="true">xri://+contact*($v*1.0)</Type>
	<URI append="qxri">http://example.com/ctc/=!A1B2.C3D4/</URI>
	<URI append="qxri">http://a.example.com/ctc/=!A1B2.C3D4/</URI>
	<URI append="qxri">https://example.com/ctc/=!A1B2.C3D4/</URI>
</Service>

This is the only model I can see that directly supports extensibility, i.e.,
an XRD author can add another element of metadata to a service endpoint
description and know that it will be passed to any application that asks any
XRI resolver (local or proxy) for this service endpoint. For example, say an
XRD author creates the following Service element:

<Service xlmns="xri://$xrd*($v*2.0)">
	<ProviderID>xri://!!1000!1234.5678</ProviderID>
	<CanonicalID>xri://=!A1B2.C3D4</CanonicalID>
	<Type select="true">xri://+contact*($v*1.0)</Type>
	<Pref xmlns="http://my.example.com/pref"; >nothot</Pref>
	<URI append="qxri">http://example.com/ctc/=!A1B2.C3D4/</URI>
	<URI append="qxri">http://a.example.com/ctc/=!A1B2.C3D4/</URI>
	<URI append="qxri">https://example.com/ctc/=!A1B2.C3D4/</URI>
</Service>

Now the output via a local API for an XRI that selected this service
endpoint would be:

xri://$xrd*($v*2.0)/ProviderID, xri://!!1000!1234.5678
xri://$xrd*($v*2.0)/CanonicalID, xri://=!A1B2.C3D4
xri://$xrd*($v*2.0)/Type, xri://+contact*($v*1.0)
xri://$xrd*($v*2.0)/Type/@select, true
http://my.example.com/Pref, nothot
xri://$xrd*($v*2.0)/URI, http://example.com/ctc/=!A1B2.C3D4/
xri://$xrd*($v*2.0)/URI/@append, qxri
xri://$xrd*($v*2.0)/URI, http://a.example.com/ctc/=!A1B2.C3D4/
xri://$xrd*($v*2.0)/URI/@append, qxri
xri://$xrd*($v*2.0)/URI, https://example.com/ctc/=!A1B2.C3D4/
xri://$xrd*($v*2.0)/URI/@append, qxri

*****

As always at this stage, please send feedback as soon as you can. Currently
this is the last remaining technical open issue to close WD 10.

=Drummond