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Re: deterministic naming of blank nodes

From: Alan Ruttenberg <alanruttenberg@gmail.com>
Date: Wed, 1 Oct 2014 18:03:25 -0400
Message-ID: <CAFKQJ8mvMCiYOjhsiEub5C0sUS7PP1miPkc57V=4vecz21vNQg@mail.gmail.com>
To: David Booth <david@dbooth.org>
Cc: Tim Berners-Lee <timbl@w3.org>, Sandro Hawke <sandro@w3.org>, SW-forum Web <semantic-web@w3.org>, Pat Hayes <phayes@ihmc.us>
On Wed, Oct 1, 2014 at 5:09 PM, David Booth <david@dbooth.org> wrote:

> Hi Alan,
> Yes, in the general case of *unrestricted* blank node use, deterministic
> naming of blank nodes would be as difficult as the graph isomorphism
> problem.  That is exactly the motivation behind "Well Behaved RDF"[1]: to
> modestly restrict the use of blank nodes to avoid that complexity problem,
> while retaining the most important and common uses of blank nodes.  Doing
> so would change the complexity from a graph isomorphism problem to a tree
> problem, enabling deterministic naming of blank nodes to be practical.

That may be true, but it is hard for me to see how any benefit this could
bring would outweigh the absolute pain in the ass it would be for everyone
to change their RDF stacks. I'll scan back through the message to see why
you think solving this problem is so important, but I can't come up with a
reason off the top of my head.


> David
> 1. Well Behaved RDF:
> http://dbooth.org/2013/well-behaved-rdf/Booth-well-behaved-rdf.pdf
> On 10/01/2014 11:03 AM, Alan Ruttenberg wrote:
>> Wouldn't deterministic naming of blank nodes mean there was an effective
>> solution to the graph isomorphism problem, which is known to at least be
>> outside P?
>> http://mathworld.wolfram.com/GraphIsomorphism.html
>> -Alan
>> On Fri, Sep 26, 2014 at 6:48 PM, David Booth <david@dbooth.org
>> <mailto:david@dbooth.org>> wrote:
>>     On 09/26/2014 06:20 PM, Tim Berners-Lee wrote:
>>         On 2014-09 -24, at 00:37, Sandro Hawke <sandro@w3.org
>>         <mailto:sandro@w3.org>
>>         <mailto:sandro@w3.org <mailto:sandro@w3.org>>> wrote:
>>             On 09/23/2014 05:34 PM, David Booth wrote:
>>                 BTW, I want to draw attention to the fact that the need
>>                 for defining
>>                 an RDF-specific PATCH operation is *entirely* a
>>                 consequence of RDF's
>>                 allowance of unrestricted blank nodes.  I do not think
>>                 that blank
>>                 nodes should be eliminated from RDF, but I am convinced
>>                 that RDF's
>>                 current treatment of blank nodes is a significant design
>>                 flaw that
>>                 has *many* downstream effects that are ultimately
>>                 detrimental to
>>                 RDF's adoption. The need for RDF PATCH is another example.
>>                 Unix/linux diff and patch utilities have been used
>>                 successfully for
>>                 *decades*, with many other information representations.
>>                 Imagine how
>>                 simple and easy it would be if we could just generate
>>                 canonical
>>                 N-Triples and use standard diff and patch against that!
>>                 But we can't,
>>                 because blank nodes are unstable across RDF
>>                 serializations and no
>>                 canonical way to generate them has been standardized.
>>                 This, in turn
>>                 is because generating a canonical form of unrestricted
>>                 RDF is a hard
>>                 problem (NP-complete), because of blank nodes.  The
>>                 problem is *much*
>>                 easier if the use of blank nodes is limited to
>>                 *implicit* blank nodes
>>                 -- those that are generated implicitly by the use of
>>                 square brackets
>>                 "[]" or parentheses "()" for lists in Turtle -- and
>>                 indeed this is
>>                 the vast majority of blank node use.  (See "Everything
>>                 You Always
>>                 Wanted to Know About Blank Nodes", by Hogan, Arenas,
>>                 Mallea and
>>                 Polleres:
>>                 http://www.__websemanticsjourn
>> al.org/index.__php/ps/article/viewFile/365/__387
>>                 <http://www.websemanticsjournal.org/index.
>> php/ps/article/viewFile/365/387>
>>                 )
>>                 For this reason the use of "Well Behaved RDF" was
>>                 proposed, which
>>                 limits the use of blank nodes to implicit blank nodes:
>>                 http://dbooth.org/2013/well-__behaved-rdf/Booth-well-__
>> behaved-rdf.pdf
>>                 <http://dbooth.org/2013/well-behaved-rdf/Booth-well-
>> behaved-rdf.pdf>
>>                 I don't know if Well Behaved RDF is the best solution to
>>                 this
>>                 problem. Maybe someone will come along with a better
>>                 idea.  But I am
>>                 convinced that the current treatment of blank nodes in
>>                 RDF is a
>>                 serious problem that we should fix in order to make RDF
>>                 simpler to
>>                 use, understand and adopt.
>>                 I really don't like having to make excuses for RDF when
>>                 it cannot be
>>                 used in a similar way as nearly every other information
>>                 representation -- such as being able to easily compare
>>                 two RDF
>>                 documents for "equality" (which in RDF becomes a complex
>>                 graph
>>                 isomorphism problem) or generate a simple diff and patch
>>                 -- all
>>                 because of RDF's unrestricted treatment of blank nodes.
>>                 Clearly this is not something that the Linked Data
>>                 Platform working
>>                 group can fix.  But I think it is important to bring it
>>                 to people's
>>                 attention, in the hope that we will someday soon have
>>                 the creativity
>>                 and gumption to fix it.
>>                 I should also acknowledge that there are some who do not
>>                 feel that
>>                 RDF's treatment of blank nodes is a problem.  Fine.  It
>>                 may not be a
>>                 problem to an elite few who are well steeped in the
>>                 subtleties of
>>                 description logic, model theory and RDF Semantics, and
>>                 who don't mind
>>                 having to use RDF-specific tools instead of generic
>>                 tools.  But
>>                 having tried for over 10 years to explain RDF to a wider
>>                 audience of
>>                 regular software developers, I am convinced that
>>                 subtleties like
>>                 RDF's treatment of blank nodes *are* a problem to a much
>>                 wider
>>                 audience of *potential* RDF users who would be more
>>                 inclined to adopt
>>                 RDF if it didn't have complexities like this. As it is
>>                 they are more
>>                 likely to stick with JSON or XML, whose complexities
>>                 they already
>>                 know, rather than venturing into the obscure and
>>                 esoteric world of RDF.
>>                 RDF tools are not as mature as those for XML or even
>>                 JSON, which is
>>                 much younger than RDF.  I believe blank nodes are one
>>                 specific reason
>>                 they're not.  The fact that we still don't even have a
>>                 simple,
>>                 standard way to compare RDF documents and compute diffs
>>                 and patches,
>>                 is a perfect example.
>>                 David
>>             I agree that it makes sense to have good terminology for
>>             graphs that
>>             can be serialized in Turtle without blank node labels, and
>>             perhaps to
>>             focus diff on these nice graphs.    (When I clicked on your
>>             Well-Behaved RDF link, my PDF viewer remembered I was on
>>             page six. :-)   )
>>             How would you name the blank nodes when serializing this in
>>             n-triples
>>             or n-quads?   I guess given triple S P O, where O is a blank
>>             node,
>>             you'd name O based on the hash of the names of S and P?
>>               Something like
>>                  o.asNTriplesTerm = "_:a"+sha1hex(s.__asNTriplesTerm+"
>>                  "+p.asNTriplesTerm)?
>>             Or maybe leave off the NTriples baggage, the "<"...">" and
>>             "_:".    Maybe:
>>             o.id <http://o.id> = sha1hex(s.id <http://s.id>+" "+p.id
>>             <http://p.id>)
>>             You'd have to evaluate these in the right order, but that
>>             would be the
>>             order you parsed them from Turtle, so it should be fast and
>>             deterministic.   Hm.   Can we avoid the repeated hashing?
>>               I suppose
>>             so, with some kind of path expression:
>>                  Given:  <s1> <p1> [ <p2> ( 1 2 3 [ <p3> ( 11 12 [ <p4>
>>             444 ] ) ] ) ]
>>                  The subject of <p4> 444 would have its id determined by
>>             hashing
>>                  (s1, p1, p2, 4, p3, 3).
>>             (I'm treating integer list positions as if they were
>>             properties.) With
>>             a little cleverness, caching those intermediate hashes, I
>>             think that
>>             could run almost as fast as computing a hash of the turtle
>>             file (ie
>>             very fast).
>>             Wow, this might work.    Very good idea, David.    Beyond
>>             the fact
>>             that it only works on this kind of graph, does anyone see
>>             any problem
>>             with it?
>>         Hmmm , suppose a blank node can be identified by more than one
>> path,
>>         then the hashed path would not be unique and so you couldn't use
>>         them to
>>         compare blank nodes
>>     If the algorithm deterministically chose which path to use in
>>     generating the hash -- say, the first alphabetically -- then it
>>     would always generate the same hashes for the same (isomorphic)
>>     graphs.  Even if the graph changed slightly, such an algorithm could
>>     often choose the same path, so it would be fairly robust to minor
>>     changes in the graph, which is a very desirable property.  And in
>>     the cases where it did choose a different path, you would end up
>>     with two URIs for what should have been considered the same node,
>>     and owl:sameAs could be used to indicate their equivalence, once
>>     their equivalence is detected through application-specific means.
>>     David
Received on Wednesday, 1 October 2014 22:04:24 UTC

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