notes from meeting in Nice, Jan 29 2009

[Markus Sabadello <markus.sabadello@xdi.org>]

Hi Giovanni (and others),

I'm trying to summarize in a few words what we talked about. Please correct me if I made a mistake or forgot anything.

- We had a look at the xdi-rdf-global-graph-proposal-v1 which Giovanni posted about a week ago:
http://www.oasis-open.org/committees/download.php/30802/xdi-rdf-global-graph-proposal-v1.pdf

- The basic graph model described in that document is essentially the same as the one we are all familiar with (i.e. subject/predicate/object triples), but the document proposes a new methodology for drawing the graph. The main innovation is to draw EVERYTHING that has an XRI as a node (including predicates!). Arcs are drawn between the nodes, but arcs themselves don't have XRIs. A typical statement consisting of subject, predicate and object would be drawn as three nodes and three arcs.

- Arcs are colored to indicate which statement they belong to.

- A node with a given XRI never appears twice in the graph, but it can of course be part of more than one statement. For example if you have the following two statements:
=markus/+email/'markus.sabadello@gmail.com'
+email/$is$a/+contactdata

Then only a single node with the XRI +email would be drawn, but it would be part of both statements.

- Giovanni distinguishes between Syntactic Correctness and Semantic Consistency

- Syntactic Correctness simply means that the graph properly fits into the RDF graph model (subject/predicate/object). This is a similar concept to a "well-formed" XML document.

- Semantic Consistency means that the statements in the graph are consistent with accompanying dictionary statements. E.g. if you have a statement =markus/+friend/=giovanni, then for the graph to be "semantically consistent", there would also have to be the dictionary statement =markus/$has$a/+friend. These dictionary statements could be mixed into the same graph, or they could exist in a separate document or XDI endpoint (such as a Community Dictionary Service). This is a similar concept to a "valid" XML document.

- The XDI Validator application could be extended to support validating this Semantic Consistency.

- If you want you can work with XDI without caring about "semantic consistency" / dictionary entries, just like you can work with XML without caring about schemas / DTDs.

- Giovanni's document also introduces the concept of "roots", which are drawn as a slash in a box. Those roots and the arcs originating from them do not actually appear as statements in the graph, but they indicate who is authoritative for an XRI. For example, all + XRIs (+friend, +email, etc) come from the same "root" (the Community Dictionary Service). Similarly, all $ XRIs ($is, $has, etc) come from the same "root" (the XDI specfications). =markus and =drummond would come from different "roots", since different XDI endpoints are authoritative for them.

- We talked a bit about equivalence. Giovanni thinks that if there is a $is statement between two XRIs, then those two XRIs are drawn as only a single node in the graph. So the node would have two XRIs that identify it. Markus mentioned that is not how his implementation currently works.

- A question that follows is how would XDI messaging be affected by the above. For example, if you have the following XDI graph:

=markus
    $is
        =markus.sabadello
    +name
        "Markus Sabadello
=markus.sabadello
    +nationality
        "AT"

And if you then send this XDI message:

=giovanni
    $get
        /
            =markus
                +nationality

Would there be a reply ("AT"), or would there be no reply?

- A similar example would be the effect of a $del message. If equivalent XRIs are drawn as only a single node in the graph, and you remove the $is statement via a $del message, would the node then "split up" into two nodes?

- Giovanni gave a demo of a project his team has been working on ("SMS" - "Simple Mobile Services" - www.ist-sms.org). It allows users to send small notes about people, locations, services, web sites, etc from their mobile phone to a server. These notes (so-called MEMs) are then shared with an online community. It's possible to advertise and discover new services. Phone security features such as encryption via the SIM card are used. Various authoring tools are available for creating new Simple Mobile Services that communicate via MEMs. A web portal makes it possible to collect and analyze the data that was collected by the system.

- Giovanni explained that many of his XDI inputs were motivated by use cases in the SMS system, e.g. XDI Queries.

- We both think it was a good idea to have a chatroom during the last TC call. We should always use that from now on.

Markus