Archive for February, 2012

When “identification” and “representation” fight, who wins?

Let’s now enter the fantasy world where “resource”, “identification”, and “representation of” have meanings consonant with what is found in the Web architecture documents RFC 3986, the HTTPbis draft (I’m looking at version 18), and AWWW. To make any sense of these documents the words have to be assumed to have something close to their ordinary language meanings (which are rather squishy), since they are otherwise effectively undefined.

Now observe:

1. Web architecture suggests that a URI owner is an authority for what is identified by its URIs (AWWW bullet #2).

2. The HTTP protocol suggests the URI owner is an authority for what is a representation of what is identified (HTTPbis v.18 part 2 section 5.1 bullet 1 taken together with part 1 section 2.7.1).

If both kinds of authority hold, then Jabberwocky is a representation of the Magna Carta, since a URI owner can say both that the URI identifies the Magna Carta and that Jabberwocky is a representation of what is identified. But this is not true. How to resolve this paradox?

There are (at least) three solutions, based on modifying either of the two authority axioms.

1. We can say the URI owner is an authority for what is identified, but not for what is a representation of it. [2/11 I.e. solution = what the URI owner said was a representation is not a representation of what is identified.] In this case a 200 response only says that the payload is a rep, its arrival does not imply that it is a rep.

Accepting this would require modifying the HTTP protocol to say that the payload is only said to be a representation of the resource, not that it is. It is only nominally so.

2. We can say that the URI owner is an authority for representation, but that it is only an authority for identification to the extent that the identified resource is represented by any HTTP 200 responses that have been issued recently. [2/11 I.e. solution = the URI does not identify what the URI owner determined it to. It’s not clear what identity authority would consist of independent of asserting representations, anyhow.]

Accepting solution 2 requires modifying the http: URI scheme to impose this limit on identification when representations are asserted, as this limit is otherwise not entailed by the URI scheme.

This could easily lead to the URI identifying nothing at all, which would be a problem.

2a. We can say that “representation” is redefined as a term of art, not used in an ordinary language sense. The URI owner has authority over representation, but the authority over identification is limited to having a URI identify mysterious sorts of things whose very nature allows some URI owner to have authority over their representations.

Accepting solution 2a also requires modifying the http: URI scheme, to restrict identification to these mysterious things when there are nominal representations. You might call these mysterious things, say, “information resources” (although this would run afoul of AWWW a bit).

Although I haven’t heard from him yet, my suspicion is that Roy Fielding would either insist on option 1, or insist that the problem doesn’t exist, while Tim Berners-Lee would either prefer option 2a or insist that the problem doesn’t exist. I want to get them into a room together to fight this one out, but I need to be there to make sure they don’t decide together that there is no paradox.

Further reading: speech acts

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