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RE: a protocol analog to abstract/concrete identifiers

From: Drummond Reed <drummond.reed@cordance.net>
Date: Mon, 22 Sep 2008 00:03:11 -0700
To: "'Dan Connolly'" <connolly@w3.org>, "'www-tag'" <www-tag@w3.org>
Message-ID: <D8DEA954205B4EE6B396B779483A6096@ELROND>

> -----Original Message-----
> From: www-tag-request@w3.org [mailto:www-tag-request@w3.org] On Behalf Of
> Dan Connolly
> Sent: Thursday, September 18, 2008 9:47 AM
> To: www-tag
> Subject: a protocol analog to abstract/concrete identifiers
> This abstract/concrete notion...
> [[
>       * An abstract identifier is an identifier (arc) of a target
>         resource (node) that MUST NOT resolve directly to a
>         representation of that resource, but MAY resolve to a
>         description of that resource (descriptor).
>       * A concrete identifier is an identifier of a target resource that
>         MAY resolve directly to a representation of that resource, but
>         MUST NOT resolve to a descriptor of that resource.
> ]]
>  -- http://wiki.oasis-open.org/xri/AbstractIdentifierArchitecture
> ... has an analog at the protocol level.
> Given I1 which identifies X1,
> when I do an HTTP GET on I1,
>   - if I get a 200 response, the body is a representation of X1
>   - if I get a 3xx or 4xx response, any body I find is closer
>     to a "description" of X1.
> cf the spec for 300 Multiple Choices :
> "the response SHOULD include an entity containing a list of resource
> characteristics and location(s) from which the user or user agent can
> choose the one most appropriate."
>  -- http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.3.1
> I suspect this analog is enough to meet whatever requirements
> motivate the definitions of abstract/concrete identifiers.

Dan, I agree that's a good analogy. It lines up well with the incisive
analysis Eran Hammer-Lahav posted last week of the various options for using
HTTP as a metadata discovery protocol [1].

>From an efficiency standpoint, the key challenge is what Eran calls "Direct
Metadata Access":

	"Direct Metadata Access - enable direct retrieval of metadata
without interaction with the resource itself. Before a resource is accessed,
the consumer should have a way to fetch the resource's metadata without
accessing the resource. This is important for two reasons. First, accessing
an unknown resource may have undesirable consequences. After all, the
information contained in the metadata is supposed to inform the consumer how
to interact with the resource. The second is efficiency - removing the need
to interact with the resource in order to get its metadata (which can reduce
HTTP round-trips, network bandwidth, application latency, and overall

What we're looking for is a solution that provides direct metadata access
without violating the principles of Web architecture.


[1] http://www.hueniverse.com/hueniverse/2008/09/discovery-and-h.html
Received on Monday, 22 September 2008 07:04:54 UTC

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