Sorry, I
still do not quite follow, I guess, but my feeling is that it is very useful if
one can determine what the metadata really describes. For example, for version,
it is nice to be able to determine that in
=GabeW*resume*($d*2002-10-10T17:00:00Z)*job
*($d*2002-10-10T17:00:00Z) is describing =GabeW*resume and not
job.
The version metadata in
this case actually describes the immediately preceding segment "resume".
Although it just explains "resume", this in turn means that it is describing
"=GabeW*resume".
What is the problem of
this approach?
Nat
From: Drummond Reed
[mailto:]
Sent: Thursday, January 12,
2006 12:30 PM
To: 'Schleiff, Marty'; 'Wachob, Gabe';
Subject: RE: [xri] Feedback requested on
terms used in Metadata spec
Do not listen to that
man behind the giant aluminum curtain at Boeing. Those questions he keeps
asking just happen to have about 30 years of thought and experience behind
them and he has completely resculpted the contour of XRI metadata because he
has asked the hard questions long enough and insisted on clear, clean, even
answers with little (preferably no) ambiguity.
Gabe, here's an
answer your question about the application of the RDF S-P-O pattern (that's
subject-predicate-object for anyone who's not an RDF disciple). As Marty
explained, the solution we realized for making XRI metadata both simple and
unambiguous was by including the subject (the identifier being described) as
part of the cross-reference.
For example, if we
wanted to express that the identifier "pays" was the French word and not the
English word, the S-P-O triple would be:
Subject =
"pays"
Predicate =
"$l"
Object =
"fr"
…with the resulting
XRI being…
$l*fr*pays
…which when used as a
cross-reference in the context of another resource might be something
like…
xri://xri.example.com/dictionary/($l*fr*pays)/defintion.html
What threw off our
analysis of the underlying pattern being the S-P-O pattern was the fact that
the very simple XRI metadata cross-references we have been using for quite a
while actually fit this pattern even though they were only two "levels" deep.
Example:
$v*2.0
…which when used in
context (to choose a particular very real example from the XRI Resolution 2.0
spec) would be…
xri://$res*local*($v*2.0)
In this use of XRI
metadata, the identifier "$v" directly describes the identifier "2.0". In RDF
terms, what we realized was that this was just like the metadata in the
following XML snippet:
<strong>car</strong>
If you analyze this
tag from an RDF perspective, what you get is:
Subject =
"car"
Predicate = fontstyle
(implicit)
Object =
"strong"
In other words, it's
implicit in the semantics of the tag <strong> that it's a font style.
You could make this much more explicit as
follows:
<font style="strong">car</font>
Marty and I realized
that $v and $d both followed this implicit predicate pattern. So "$v*2.0" from
an S-P-O perspective is:
Subject =
"2.0"
Predicate =
$v
Object = version
number (implicit)
And
"$d*2002-10-10T17:00:00Z" from an S-P-O perspective would
be:
Subject =
"2002-10-10T17:00:00Z "
Predicate =
$d
Object = XML datetime
(implicit)
Marty then suggested
that $v and $d, besides their "implicit" numeric version and XML datetime
formats, respectively, could also have other formats defined explicitly
without conflicting with the implicit default.
Examples:
$v*alpha*b.e.c
Subject =
"b.e.c"
Predicate =
$v
Object = "alpha"
(explicit)
$d*year*2005
Subject =
"2005"
Predicate =
$d
Object = "year"
(explicit)
So (I realize this is
a very long answer to your question), the S-P-O pattern applies strictly to
the identifier that is the subject of the cross-reference, i.e., the final
subsegment of the cross-reference (ALWAYS).
What it ducks is the
question of what the entire XRI metadata cross-reference describes – if it
does describe anything explicitly. In other words, under the above use of the
S-P-O pattern, the S-P-O only operates within the XRI metadata
cross-reference. The mere inclusion of an XRI metadata cross-reference in an
XRI does not explicitly tell you what it
describes.
Marty and I did
discuss a way to make that ambiguous, which again would be by using
cross-references. For example, in your question about whether the date
cross-reference in…
=GabeW*resume*($d*2002-10-10T17:00:00Z)*job
...describes your
resume or your job, one easy way to make this unambigous
is…
=GabeW*(resume*($d*2002-10-10T17:00:00Z))*job
=GabeW*resume*(($d*2002-10-10T17:00:00Z)*job)
It sure is
unambiguous to me what describes what in these
identifiers.
However, other than
suggesting/specifying the use of cross-references when you really want to make
it unambiguous what a XRI metadata cross-reference refers to, I'm in favor of
just tightly defining the XRI metadata cross-reference syntax and letting
identifier authorities use these within other XRIs as
needed.
Whatchathink?
=Drummond
From:
Schleiff, Marty [mailto:]
Sent: Wednesday, January 11, 2006 4:57
PM
To: Wachob, Gabe; Drummond
Reed;
Subject: RE: [xri] Feedback requested on
terms used in Metadata spec
Hi Gabe (&
All),
Now you're asking me
really hard questions. I think you need to understand that when Drummond and I
work together, it's mostly him doing the work. I have three unofficial roles
in the collaboration: 1) repeatedly ask stupid questions about a particular
concept until Drummond gets enough practice answering in various
ways, and can figure out the best way to describe it in a spec; 2) think up
bizarre use cases to test out our ideas; and 3) continually pester him and try
to convince him that there are some places where an xref doesn't make sense.
For good answers, I think we'll have to wait for Drummond to chime
in.
I'm apparently guilty
of trying to constrain the specs too much. I don't like the concept of it
being up to the issuer to decide how to interpret the target of the metadata.
In and XRI containing ".../pays/pays/(l*fr)/pays/pays/...", to which part(s)
should the metadata apply? I.e., which part(s) should have French
pronunciation? The ambiguity drove us to putting it into the xref, e.g.,
".../pays/pays/($l*fr*pays)/pays..."
Your "left" / "right"
idea is an interesting approach we hadn't thought of (at least I hadn't
thought of it - maybe Drummond had).
; CISSP
Associate Technical Fellow - Cyber
Identity Specialist
Computing Security
Infrastructure
(425) 957-5667
From: Wachob,
Gabe [mailto:]
Sent: Wednesday, January 11, 2006 4:29
PM
To: Schleiff, Marty;
Drummond Reed;
Subject: RE: [xri] Feedback requested on
terms used in Metadata spec
Marty-
Are you saying that
metadata XREFs should have a way of saying "the thing identified to the left
of me" vs. "the thing identified to the right of me" ? So, in the pattern
of:
xri-metadata
= "$" predicate [ “*” object ] “*” subject
you might
have:
=GabeW*resume*($v*left*1.2)
-- indicating version 1.2 of my resume
vs.
=GabeW*resume*($v*right*2002-10-10T17:00:00Z)*job --
indicating my job as of 10/10/02?
I guess I'm
confused about the concept
of having a *choice* about what the metadata applies to.. I don't quite
understand how/why these metadata xrefs "apply" to parts of the XRI. In that
sense, I guess I'm saying that I don't understand how the subject is *part* of
an XRI. It would seem to me that the subject has to be explicit if you are to
have the Subject-Predicate-Object model.
Can we state
succinctly what it means when such a S-P-O triple appears in an XRI? What
inference do we draw by its mere presence? Isn't it just a convenient way to
qualify everything to the right of the metadata XREF, and nothing more?
-Gabe
From:
Schleiff, Marty [mailto:]
Sent: Wednesday, January 11, 2006 4:19
PM
To: Wachob, Gabe;
Drummond Reed;
Subject: RE: [xri] Feedback requested
on terms used in Metadata spec
Hi Gabe (&
All),
Drummond and I have
discussed/debated at length how to express the portion of an XRI to which
metadata applies. We determined that the only way to clearly associate
metadata with the intended subject of the metadata is to include the subject
and metadata within the same xref. How metadata associates with
portions of the XRI outside the metadata xref is up to the XRI issuer. This
is one of several concepts for which we're interested in reactions from the
rest of the TC. There are other issues, but rather than point them out, I'll
be interested to see if you recognize them on your
own.
;
CISSP
Associate Technical Fellow -
Cyber Identity Specialist
Computing Security
Infrastructure
(425) 957-5667
From:
Wachob, Gabe [mailto:]
Sent: Wednesday, January 11, 2006 12:54
PM
To: Drummond Reed;
Subject: RE: [xri] Feedback requested
on terms used in Metadata spec
I guess I'm partial
to the RDF-mindset, but I'm thinking that it is an odd fit to make that
tight a connection to the S-P-O model since its not clear what the actual
*subject* is in all cases - to wit, isn't the subject the thing outside the
metadata cross-reference? Doesn't the Metadata XREF explicitly
describe/refer to something *outside* the metata XREF?
If we *could* mesh
this with RDF concepts, that'd would be
interesting.
Otherwise, I don't
have a strong preference between the other four items. They all seem pretty
isomorphic to me, without having thought about this very deeply.
-Gabe
From:
Drummond Reed [mailto:]
Sent: Wednesday, January 11, 2006
11:45 AM
To:
Subject: [xri] Feedback requested on
terms used in Metadata spec
I'm happy to report that Marty and I, after a
series of in-depth work sessions (abetted by Dave McAlpin when he's been
in town), believe we have wrestled the metadata dragon most of the way to
the ground.
The "dragon" is a tighter, more consistent ABNF
for all forms of XRI metadata as well as a much cleaner set of rules about
what it describes.
Without going into all the gory detail, what XRI
Metadata 2.0 Working Draft 08 will recommend is a very simple ABNF for
"tagging" an identifier with XRI metadata, which can then be used as a
cross-reference inside another XRI. The important clarification we will be
making is that this metadata only describes the "subject" identifier (the
identifier being described, to borrow term from RDF). For example, in the
XRI…
xri://xri.example.com/some/filename*($v*2.3)
…the metadata "$v" only describes the identifier
"2.3". It tells you it’s a version number. That's it. From the identifier
above, it would be a pretty obvious conclusion that the identifier
authority intended the cross-reference "($v*2.3)" to identify a version
namespace for the resource identifier by "filename". However in the
following XRI…
xri://xri.example.com/some/filename*($v*2.3)*anotherfile
…it may not be quite as clear – does the
identifier "($v*2.3)" create a version namespace for "filename" or for
"anotherfile"?
The answer is: that's up to the identifier
authority and out of scope for the XRI Metadata spec. The scope of the
Metadata spec is limited to defining well-known semantics and syntax for a
limited set of broadly useful identifier metadata (versions, dates,
languages, types, and annotations).
With that explained, here's what we'd like
feedback on: the semantics of the actual production names used in the ABNF
– which (per our experience with XRI syntax) we'd like to match up with
the terms used in the text of the Metadata spec itself.
Following are four "versions" of the ABNF that are
all identical structurally but different in the production names for the
first two of the three productions in the proposed syntax. (Marty and I
both like the term "subject" for the third component -- the identifier
being described -- since this is consistent with RDF and SAML and other
similar uses.)
We'd just like to know which of these (if any) are
preferred by other members of the TC, since semantics is not something
that can be determined by fiat. Please send a reply email to the list with
a comment about your preference if you have one (or a different suggestion
if you feel something else altogether would work
better).
Thanks,
=Drummond and Marty
Example 1 ("tag"
wording)
xri-metadata
= "$" metadata-tag [ “*” metadata-tag-value ] “*” subject
metadata-tag
= ( alpha / xref )
metadata-tag-value
= ( xri-subseg / xref )
subject
= ( xri-subseg / xref )
Example 2 ("attribute"
wording)
xri-metadata = "$" attribute [ “*”
attribute-value ] “*” subject
attribute = ( alpha
/ xref )
attribute-value = ( xri-subseg / xref
)
subject
= ( xri-subseg / xref )
Example 3 ("property"
wording)
xri-metadata = "$" property [ “*”
property-value ] “*” subject
property = (
alpha / xref )
property-value = ( xri-subseg / xref
)
subject
= ( xri-subseg / xref )
Example 4 ("RDF"
wording)
xri-metadata = "$" predicate [ “*”
object ] “*” subject
predicate = ( alpha
/ xref )
object
= ( xri-subseg / xref )
subject
= ( xri-subseg / xref )
Examples of the five types of XRI
metadata:
($l*fr*pays)
($t*ces*
john%20henry%20doe)
($v*2.3)
($d*2002-10-10T17:00:00Z)
($-*some.comment.here)
How these examples parse to the ABNF
above:
First
production
Second
production
Subject
$l
*fr
*pays
$t
*ces
*john%20henry%20doe
$v
*number
(default – can be omitted)
*2.3
$d
*XMLdatetime (default – can be
omitted)
*2002-10-10T17:00:00Z
$-
*string
(default – can be omitted)
*some.comment.here
