Les has a good point. From an XRI resolution
standpoint, resolution is always left to right. So I think it is safe that $d and
$v namespaces *always* create a
datetime or version space for the resource identified by the XRI for their
immediate parent.
=Drummond
From: Chasen, Les
[mailto:]
Sent: Monday, January 16, 2006
12:14 PM
To: Drummond Reed; Schleiff,
Marty; Sakimura, Nat; Wachob, Gabe;
Subject: RE: [xri] Feedback
requested on terms used in Metadata spec
I think a simple rule is missing, normal
left to right resolution rules still apply. The meta-data cross reference
is a sub-segment under the authority to the left. So if presented with
this XRI =sakimura*($v*2.3)*(+email) a
resolver will look for a ($v*2.3) iname under =sakimura. It will then
look for a (+email) iname under
=sakimura*($v*2.3). To me this means that we are looking for version 2.3
under the sakimura authority not look for 2.3 of +email … it is all left
to right.
Note: We have not done any service
end point matching here. (+email) is not a legal service type in the
latest resolution draft. In this example it is just an iname.
- Les
I-Name: =les.chasen
From: Drummond Reed
[mailto:]
Sent: Monday, January 16, 2006
1:54 PM
To: 'Schleiff, Marty'; 'Sakimura,
Nat'; 'Wachob, Gabe';
Subject: RE: [xri] Feedback
requested on terms used in Metadata spec
First,
this is a great thread – I encourage all XRI TC members to read it as it
gets to the heart of the revisions being made to XRI Metadata 2.0 Working Draft
08.
Nat:
Marty already answered the first point I was going to make that under the
proposed new ABNF for XRI metadata, language tags are now completely
unambiguous because the identifier being described is *inside* the cross-reference. So as Marty
illustrates, instead of…
=sakimura*chinese_string*($l*za)*japanese_string*($l*ja)*some_string
…you
would have…
=sakimura*($l*za*chinese_string)*($l*ja*japanese_string)*some_string
The same
is true for identifier types. Whereas under XRI Metadata 2.0 Committee Draft
01, if we'd had $t, we would have had…
@Boeing*($t*oid)*8.4.13.6.72*($t*ces)*some_string
…and
it might have been ambiguous which string was an OID and which was a Case Exact
String, under Working Draft 08 you would have…
@Boeing*($t*oid*8.4.13.6.72)*($t*ces*some_string)
…and
it will not be ambiguous at all.
I was
very happy about that realization, and it produces wonderfully clean ABNF.
However it's true that with $d datetime identifiers and $v version identifiers,
the XRI metadata is only telling you that the cross-reference is a datetime or
a version number. How the datetime or version number cross-reference applies to
the XRI as a whole is up to the identifier authority.
As Marty
points out, XRI authorities do have a clean way to specify in an XRI that a
datetime or version applies to a specific portion of the XRI, and that again
would be to use cross-reference syntax to explicitly group the related
identifiers as he illustrates…
(=sakimura*($d*2002-10-10T11:00:00Z))*(+email)
While
this is certainly valid, I don't think it will prove as useful as simply using
datetime and version cross-references to create namespaces. For example,
with…
=sakimura*($d*2002-10-10T11:00:00Z)*(+email)
…it
doesn't really matter whether "($d*2002-10-10T11:00:00Z)"
is interpreted as "describing" "=sakimura" or "(+email)" because the net effect will be exactly the same: you have an
XRI that includes exactly one datetime space that can be compared with any
other XRI with the same datetime space to determine an ordered relationship.
For example, in the following two XRIs, you will know that the first one comes
before the second one in time:
=sakimura*($d*2002-10-10T11:00:00Z)*(+email)
=sakimura*($d*2002-10-10T11:00:01Z)*(+email)
The same
is true of versions – you know unambiguously that the second XRI below is
a later version than the first:
=sakimura*($v*2.3)*(+email)
=sakimura*($v*3.1)*(+email)
I
believe that with datetimes and versions this is all that is necessary, but of
course I'm too close to the subject at this point to see it objectively, so
please do point out other use cases that this approach may not meet.
Best,
=Drummond
From: Schleiff,
Marty [mailto:]
Sent: Monday, January 16, 2006
9:22 AM
To: Sakimura, Nat; Drummond Reed;
Wachob, Gabe;
Subject: RE: [xri] Feedback
requested on terms used in Metadata spec
Hi All,
Here's
how I look at it: Metadata is data that describes data, not objects.
So,
"$d" (the metadata) describes "2002-10-10T11:00:00Z" (the date/time value). And in "($v*2.3)", the
"$v" metadata describes describes "2.3" to let us know it's
a version number.
Here's a
thought that I think should be an XRI foundational principle: An XRI is an identifier/name
for some object. The purpose of XRI is to NAME objects, not DESCRIBE
objects. I admit that many names have descriptive meaning; however, I think
that's just incidental and up to the whims of the naming authority.
Drummond
and I figured out that we often violate this "principle" when we talk
about metadata. For example if we have a document called "pays", we
often say that "($l*fr*pays)" lets us know that pays is a French
document. However, what the metadata REALLY says is that the letters p-a-y-s
constitute a French word, or a word with French pronunciation.
When we
include metadata in an XRI, it doesn't really describe the object. Instead, it
just becomes part of the XRI and becomes part of the object's name.
So, in
"=sakimura*(+email)*($d*2002-10-10T11:00:00Z)*(+personal)", there is an object (or namespace) called "=sakimura*(+email)*($d*2002-10-10T11:00:00Z)", and within that namespace there is an object called
"(+personal)".
With
language (as with all metadata) Drummond and I claim the intended
"target" of the metadata is within the same cross reference, which
would clarify the multi-lingual example as: "=sakimura*($l*za*chinese_string)*($l*ja*japanese_string)*some_string".
For the
email example where you would like to interprete it as ("me" at
2002-10-10T11:00:00Z)'s email,we think you would use more cross references to
clarify, resulting in an XRI like: "(=sakimura*($d*2002-10-10T11:00:00Z))*(+email)".
I still
feel kinda new to XRI land, so I hope all my examples actually yield valid XRI
syntax.
The
point about metadata that drummond and I still argue about is what to do when
some portion of an XRI is the target for multiple sets of metadata. I don't
think we have to worry about this for $v and $t; it's more of a problem
for $l and $t. For example, if the target of the metadata is both
case-exact-string (a type) and French (a language). Drummond says this is a
case for nesting cross references, as in "($l*fr*($t*ces*pays))". I
don't like that because I can't tell if the "$t" and the
"ces" should have French pronunciation, or just the "pays".
Also, I'm struggling to figure out if "($l*fr*($t*ces*pays))" is
equivalent to "($t*ces*($l*fr*pays))". Instead, I favor adding yetta
'nuther metadata tag, "$m" for "multi", which would somehow
indicate that multiple metadata clauses describe the same target; however, I
can't yet figure out the ABNF for how to do it. Because I'm losing the
arguement with Drummond, I'd like to hear other's comments (if I were winning
that arguement, I might not risk inviting your possibly contentious
views;-).
;
CISSP
Associate
Technical Fellow - Cyber Identity Specialist
Computing
Security Infrastructure
(425)
957-5667
From: Sakimura,
Nat [mailto:]
Sent: Monday, January 16, 2006
6:37 AM
To: Drummond Reed; Schleiff,
Marty; Wachob, Gabe;
Subject: RE: [xri] Feedback
requested on terms used in Metadata spec
I sort of
understand the situation. However, IMHO, having unambiguous rule about what
that version or date metadata points has its merit in a way that by just
looking at it, you will be able to tell what that cross reference is pointing
to. i.e., you do not have to worry about whether the date is describing
(+email) or (+personal) in the following example.
=sakimura*(+email)*($d*2002-10-10T11:00:00Z)*(+personal)
Always needing
to ask the authority about which interpretation is right is an overhead for
implementations. Besides, an authority may start mixing up the uses and may well
screw up.
This pain
point become even more accute when we are talking about multilingual XRI.
Think of this:
=sakimura*chinese_string*($l*za)*japanese_string*($l*ja)*some_string
If I had to
ask the authority (in this case =sakimura?) whether the child or parent that
the language metadata describes, then I will not be able even to show the XRI
correctly on screen when I am offline. If I know 'a priori' that the
metadata describes the immediately preceding, then I will be able to show it without
making a query to the authority. Moreover, on the way of resolving the next
authorities from left to right, if I got stuck with the permission problems, I
will be out of luck - I will never know how to represent the above string on
screen.
Besides, is there
a syntax to know which way the metadata is describing?
The reason I
think that the metadata should describe something on the left is because it
will be restricting what precedes. This follows the pattern of restriction from
left to right. e.g.,
=sakimura*(+email)
actually will
be the vector of email addresses that I had during my life.
*($d*2002-10-10T11:00:00Z) constrains it to this particular date. You could
argue that it could have been
=sakimura*($d*2002-10-10T11:00:00Z)*(+email)
and the date
is describing the email. However, I would like to interprete it as
("me" at 2002-10-10T11:00:00Z)'s email. Let me do one more: If the
metadata were to describe something on its right, then to describe
"me" on a particular date, I would have to do:
=*($d*2002-10-10T11:00:00Z)*sakimura
I think this
is not quite right.
Nat
From: Drummond
Reed [mailto:]
Sent: Monday, January 16, 2006
5:26 PM
To: Sakimura, Nat; 'Schleiff, Marty';
'Wachob, Gabe';
Subject: RE: [xri] Feedback
requested on terms used in Metadata spec
Nat,
there is no inherent "problem" with this approach if that is what the
XRI authority intended. What Marty and Dave and I found, however, was that for
every use of a date cross-reference describing its parent node, another example
can be created in which it described its child node.
So we
concluded that the only thing that can be stated unambiguously is that:
1) A $d
cross-reference describes an identifier representing a date (which is contained
in the cross-reference)
2) The
XRI authority that assigns the $d cross-reference is responsible for
interpretation of what it "describes" outside of the cross-reference.
In other
words, we don't need to define in the Metadata spec that a metadata
cross-reference describes the node before it or the node after it, only that it
defines a date (or version, or language, or type) and can be compared with
other XRIs that are otherwise identical except for the same cross-reference.
So let
me turn the question around: do you see anything wrong with this approach?
=Drummond
From: Sakimura,
Nat [mailto:]
Sent: Sunday, January 15, 2006
6:37 PM
To: Drummond Reed; Schleiff,
Marty; Wachob, Gabe;
Subject: RE: [xri] Feedback
requested on terms used in Metadata spec
Sorry, I still do not
quite follow, I guess, but my feeling is that it is very useful if one can
determine what the metadata really describes. For example, for version, it is
nice to be able to determine that in
=GabeW*resume*($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
