Re: Turtle Patch simplification (N3 Patch?)

Hi Henry,

On Mon, Sep 22, 2014 at 4:06 PM, henry.story@bblfish.net <
henry.story@bblfish.net> wrote:

>
>  On 22 Sep 2014, at 09:47, Pierre-Antoine Champin <
> pierre-antoine.champin@liris.cnrs.fr> wrote:
>
>  Hi Henry,
>
> On Sat, Sep 20, 2014 at 12:57 PM, henry.story@bblfish.net <
> henry.story@bblfish.net> wrote:
>
>> Turtle Patch [1] makes it obvious that being able to name the bnode
>> URIs in a patch request makes patching exceedingly easy to
>> implement as well as very easy to create patches. All that is needed for
>> a client implementation is to know what triples it needs to remove:
>> there is no need to find a pattern that would identify those triples
>> among all others. The main problem with Turtle Patch is that it
>> requires one to then make a new HTTP request with genid blank nodes,
>> to get the patchable format of the resource, or to always request them
>> and then sadly turn all bnodes de facto into URIs.
>>
>
>  Agreed, also I'm not sure I'm coming to the same conclusions as you do
> below...
>
>>
>> Where there is usually a valid concern about naming bnodes - the
>> point of bnodes is that they the server publishing them should not have to
>> maintain references to them add eternam
>
>
>  Who says I should maintain them *ad eternam*??
>  In my understanding, skolemization does not require the genid URIs to be
> the same across all successives states (versions) of a given resource. The
> only have to persist as long as the resource does not change, so that one
> can safely patch it?
>
>
>  I find genids pretty hackish part of the rdf1.1 spec frankly. Genids are
> recognised apparently by analysing the schema
> of the URI, which is pretty much against web architecture.
> http://www.w3.org/TR/rdf11-concepts/#section-skolemization
>
>  So now every RDF linked data client would need to look at each URI to
> see if it contains a ".wellknown/genid" string to know if it should follow
> it
> or not. That's pretty un linked-data-ish. Frankly I am quite surprised it
> made its way through to the spec. The people supporting it
> must have made a lot of noise.
>

Not everything is about your particular use case, Henry ;-)

RDF does not equate linked data. It does not mandate URIs to be
derefenceable. In that regard, genid URIs are no special case, so they do
not need the special treatment that you suggest above. If you try to
dereference them, you will get a 404, that's all. It's not ideal in a
Linked Data perspective (though not lethal either), but it is perfectly
acceptable from the point of view of RDF.


>
>
>
>> - in the case of a PATCH
>> the action is directly on the graph in question, and so there is in this
>> case no problem of cross reference with other resources.
>>
>
>> Given this I think one should be able to have a simpler version of Turtle
>> patch without the need for genids, that keep the bnodes local to the
>> resource and that also don't require the extra request to be made to the
>> server ( the one that is required to GET the graphs with the genids using
>> the "Prefer: return=representation blank-nodes=use-genid" header ).
>>
>> It could simply be decided that a resource that advertises the given
>> Patch Format - lets call it N3-Patch - understands there to be an
>> automatic
>> mapping from the order of bnodes in their representation to a set
>> of explicit bnodes such as _:bn1 to _:bnN . One could then have something
>> like
>> the following document at /asterix
>>
>
>  Well, in my opinion, we are back to problem #1:
>  you ask the server to maintain an *order*, while the underlying data
> model (RDF abstract syntax) has no such notion.
>  So you still put an extra burden on the server.
>
>
>  No the ordering can be agreed to as part of the protocol. The client or
> the server would only need to
> work on the ordering if the patch had blank nodes. If it did then the
> nodes could be ordered. The server
> could cache the order.
>

In my view, it *has* to cache the order (see argument above). I'm not
saying this is impossible to achieve (I agree with your arguments below as
to how to do it), I'm only saying that it is not a standard functionality
of triple stores, so you have to change them (even if slightly). And I do
not consider that as something that should become a standard feature, since
this notion of order is not part of the RDF model.

 best


> The nice thing is that there is no need for an extra GET to skolemise the
> nodes,
> and on the internet it is http connections that are the slowest of all.
>
>
>  Granted, it is not a huge deal, and is probably a good way to keep the
> extra information that allows you to skolemize bnodes, but this is still
> something that you get out of the box for standard triple stores.
>
>
>  I think you could calculate it from a graph without needing to change
> your triple store.
> One would just need to start from something like
>   http://www.hpl.hp.com/techreports/2003/HPL-2003-142.pdf
> where the alogrithm could be followed by just using triples.
>

>  And in our case we are just interested in an ordering of blank nodes.
>
>  There are more recent works on this too.
>
>
>    pa
>
>
>> GET /asterix HTTP/1.1
>>
>>
>> HTTP/1.1 200 Ok
>> E-Tag: "slab v2"
>>
>> [] foaf:name "Asterix".
>>    foaf:knows [ foaf:name "Julius Caesar";
>>                 foaf:homePage <http://palace.rome/> ].
>>
>> [] foaf:name "Obelix" .
>>
>> And patch it with the following PATCH Request
>>
>> PATCH /asterix HTTP/1.1
>> Content-Type: text/n3
>> If-Match: "slab v2"
>>
>> @prefix foaf: <http://xmlns.com/foaf/0.1/>
>>
>> { _:bnx1 foaf:knows _:bnx2 } patch:replaceWith { _:bnx1 foaf:knows _:bnx3
>> }
>>
>> Which would result in a following request
>>
>> GET /asterix HTTP/1.1
>> Accept: text/turtle
>>
>> HTTP/1.1 200 Ok
>> Content-Type: text/turtle
>>
>> [] foaf:name "Asterix".
>>    foaf:knows [ foaf:name "Obelix" ].
>>
>> [] foaf:name "Julius Caesar";
>>    foaf:homePage <http://palace.rome/>
>>
>>
>> Henry
>>
>>
>> [1] http://www.w3.org/2001/sw/wiki/TurtlePatch
>>
>>
>> Social Web Architect
>> http://bblfish.net/
>>
>>
>>
>
>        Social Web Architect
> http://bblfish.net/
>
>