[Xesam] Iteration two, metadata api

[Philip Van Hoof]

The current metadata storage API has a few problems that we discovered
while implementing the Turtle file support for Tracker.

In a way is inserting metadata via a Turtle file the same thing, with a
difference being that instead of over IPC the data comes from libraptor.

Nonetheless, we are inserting RDF triples, which is the same as what the
metadata storage API is promising to service ... over IPC (the feature
of letting you as an external app insert triples about an existing or
about a new resource).

For this it was needed that per group of triple that we insert, we know
the resource's URI, the rdf:type and the File:Modified predicate.

The API as proposed at http://xesam.org/main/XesamMetadataAPI does not
require that rdf:type and File:Modified are passed as fields.

This means that it would be unimplementable for many Xesam implementers.
Especially the Xesam implementers using a (decomposed) triple store.
They'll need the rdf:type to know which table to elect for the insertion
of the triple. 

The File:Modified is needed for collision handling: what if a record
already exists? How do you know that what is being proposed by the user
of the XesamMetadataAPI for insertion is more recent than what you
already had?

For example:

Set (<maildir://folder/UID001>, ["Message:IsRead"], ["True"])

How do you know that the caller of Set is the most recent?

What you need instead is (something like) this:

Set (<maildir://folder/UID001>, 
     "rdf:type", "Message",
     "Resource:Modified", time(), 
     ["Message:IsRead"], ["True"])
or

Set (<maildir://folder/UID001>, ["rdf:type", "Message:IsRead"], 
     [time(), "True"]);

But for the last one rdf:type and Message:IsRead would be required
fields (predicates).


Some pointers:

http://live.gnome.org/MetadataOnRemovableDevices
http://svn.gnome.org/svn/tracker/branches/turtle/



-- 
Philip Van Hoof, freelance software developer
home: me at pvanhoof dot be 
gnome: pvanhoof at gnome dot org 
http://pvanhoof.be/blog
http://codeminded.be