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Loosely-coupled (modular) LDP, was Re: Recharter scope

From: Sandro Hawke <sandro@w3.org>
Date: Tue, 18 Nov 2014 07:27:39 -0500
Message-ID: <546B3B3B.5080402@w3.org>
To: "henry.story@bblfish.net" <henry.story@bblfish.net>, Arnaud LeHors <lehors@us.ibm.com>
CC: public-ldp-wg@w3.org
On 11/18/2014 06:36 AM, henry.story@bblfish.net wrote:
> During the teleconf there was a discussion about a twitter thread criticising the RESTfulness
> of LDP. I think that it is the most important value that LDP bring to the table, when
> compared to all the other RDF standards that exist. We should think of this group as tying
> loose ends together.  Could someone please point us to that discussion? Note that I don't
> think that RESTafarians have the final say  of what REST is, because they tend
> to miss the semantic part of what it is that a Representation transfers: the state of
> a Resource. So we should not allow us to be buillied by bad RESTafarian arguments into
> abandoning the core value that REST represents for the LDP group.

There is this: 

in which Mark Baker say, "I consider the design overly complex, tightly 
coupled (and therefore unRESTful)".

I had a private conversation with him after that, trying to understand, 
and came to the conclusion that he was right.  In the simplest terms, we 
shouldn't have any notion of an "LDP Server" or "LDP Client".   In 
general, we should just have certain kinds of resources that behave 
certain ways.    An LDP Server could be defined as a web server that 
happens to support one or more of the ldp container types, or something, 
for market simplicity.

For example, I think it's possible (and preferable) to express 
ldp:BasicContainer without talking about server or client conformance, 
and without any RDF dependency, in a way that would make them more 
generally useful.

Actually, even better, I'd probably go with:

ldp:Creator -- a resource which responds to POST by making the POSTed 
entity available as a new Web Resource.   Relative URLs inside the 
entity or in Link headers are understood to be relative to the new 
location.  ldp:Creators may have an associated ldp:Container in which 
created resources automatically appear (or may, in fact, also be a an 

ldp:Container (or ldp:PageSet or ldp:Enumeration or something) -- a 
resource which is is said to "contain" other resources.  Its state 
includes these containment relations, so a GET returns an enumeration of 
its contained resources, possibly along with other data, in an 
appropriate media type.  In RDF serializations, the container 
ldp:contains each of its contained resources.  Variations on (subclasses 
of) ldp:Container control what happens when the container or a contained 
resources is DELETEd, whether subcontainers are allowed, whether 
resource in a subcontainer are considered elements of the parent 
container, etc.

These are much more general concepts, not really even tied to each 
other, not tied to RDF, etc.  They are so general they are already in 
widespread use -- they're just not named, so clients can't detect them 
to allow for interoperability.

       -- Sandro

>> On 17 Nov 2014, at 21:24, Arnaud Le Hors <lehors@us.ibm.com> wrote:
>> Per our discussion on today's call I invite everyone to help develop the list of questions that defines the scope of the new WG we would propose to charter. We could change the format if we want to but that's secondary. Here is the updated list based on today's discussion:
>> 1.        How can retrieval of a container and its contained resources be combined so that fewer HTTP operations are required than it is necessary with LDP 1.0?
> I think this needs to be formulated more clearly.
> As I understand it the aim would be to pass some information about the contents of the ldp:contain-ed resources
> in the LDPContainer. In some way Atom does this using the <content> and <summary> elements. What is desired
> is that this be part of a GET operation ( and not part of a query )
> => 1. define a way to allow  a container to respond in GET requests with full or partial content of the contained
>     resources.
>   ( clearly this either requirs literals or quads )
>> 2.        How can multiple resources be created with a single HTTP request?
> Robert Sanderson wrote:
> "+0.  This seems like going firmly against the current specification that the entity body is a single resource, even if it refers to other non-existing resources on the same server, either by absolute or relative URIs."
> The really simple way to do this would be to POST a tar of a directory to an LDPContainer.
> What the working group should consider immediately is: does this contradict our current containers?
> Ideally POSTing a tar should create a LDPC with each of the tared files in the created LDPC.
>> 3.        How can a client request filtering what part of a resource or container the server is to return?
> Robert Sanderson wrote:
> "+1.  And to keeping it separate from 1 and not using a query syntax"
> That inevitably will end up being some form of query syntax, if we have something
> general enough that it is worth specifying. What I think is desired is that there
> be a very simple query syntax available, that anyone can implement, that has very
> very low complexity properties, but that is extensible over time.  Most importantly
> it has to be semantically explicit, so that a client can know what types of queries
> he is asking as he jumps across servers by following links. ( That is property/value
> queries such as those found in current html forms are not good enough ).
> (btw, this is another reason for QUERY/SEARCH methods, as that builds in the variablitiy
> of syntaxes )
>> 4.        How can a client be notified when something, such as a resource or set of resources, changes?
> Robert Sanderson wrote:
> "+1. And to working with SocialWeb to see if ActivityStreams will solve or already solves the problem for us."
> That requires polling. We should definitively see how a polling solution can help.
> But sometimes active notifications are much more desireable.
>> 5.        How can a client find out whether a service endpoint, such as a SPARQL endpoint, is associated with a resource or set of resources?
>> 6.        How can access to a resource be controlled?
>> 7.        How can a client have greater control of how paging is done (size, sorting, etc.)?"
>> 8.        How can a client learn what property constraints there are when creating or updating a resource?"
> +1 to all the above
>> 9.        How can we do efficient transfer of LDP resources, either some initial set or rolling updates (feed) of changes?
> One wonders wether a feed of PATCHes that would allow someone to reconstitute the changes would be useful.
> This is what people are calling Event Sourcing
> ( just found this reference )
>    http://ookami86.github.io/event-sourcing-in-practice/
>> Please, propose changes and additions as you see fit, as well as deletions if you think that's appropriate.
>> Thanks.
>> --
>> Arnaud  Le Hors - Senior Technical Staff Member, Open Web Standards - IBM Software Group
> Social Web Architect
> http://bblfish.net/
Received on Tuesday, 18 November 2014 12:27:50 UTC

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