[Nepomuk] [RFC] Simplify Nepomuk Graph handling

[Vishesh Handa]

Hey everyone

This is another one of those big changes that I have been thinking about
for quite some time. This email has a number of different proposals, all of
which add up to create this really simple system, with the same
functionality.

Graph Introduction
---------------------------

For those of you who don't know about graphs in Nepomuk. Please read [1].
It serves as a decent introduction to where Graphs are used. Currently, we
create a new graph for each data-management command.

What does this provide?
----------------------------------

We currently use graphs for 2 features -

1. Remove Data By Application
2. Backup

What all information do we store?
------------------------------------------------

1. Creation date of each graph
2. Modification date of each graph ( Always the same as creation date )
3. Type of the graph - Normal or Discardable
4. Maintained by which application

(1) and (2) currently serve us no purpose. They never have. They are just
things that are nice to have. I cannot even name a single use case for it.
Except for they let us see when a statement was added.

(3) is what powers Nepomuk Backup. We do not backup everything but only
backup the data that is not discardable. So, stuff like indexing
information is not saved. Currently this system is slightly broken as one
cannot just filter on the basis of not Discardable Data, as that includes
stuff like the Ontologies. So the queries get quite complicated. Plus, one
still needs to save certain information from the Discardable Data such as
the rdf:type, nao:creation, and nao:lastModified. Hence, the query becomes
even more complex. For my machine with some 10 million triples, creating a
backup takes a sizeable amount of time ( Over 5 minutes ), with a lot of
cpu execution.

Current query -

select distinct ?r ?p ?o ?g where {
graph ?g { ?r ?p ?o. }
?g a nrl:InstanceBase .
FILTER( REGEX(STR(?r), '^nepomuk:/(res/|me)') ) .
FILTER NOT EXISTS { ?g a nrl:DiscardableInstanceBase . }
} ORDER BY ?r ?p

+ Requires additional queries to backup the type, nao:lastModified, and
nao:created.

Maybe it would be simpler if we did not make this distinction? Instead we
backup everything (really fast), and just discard the data for files that
no longer exist during restoration? It would save users the trouble of
re-indexing their files as well. More importantly, it (might) save them the
trouble of re-indexing their email, which is a very slow process.

Also, right now one can only set the graph via StoreResources, and not via
any other Data Management command.

----

(4) is the most important reason for graphs. It allows us to know which
application added the data. Stuff starts to get a little messy, when two
application add the same data. In that case those statements need to be
split out of their existing graph and a new graph needs to be created which
will be maintained by the both the applications. This is expensive.

I'm proposing that instead of splitting the statement out of the existing
graph, we just create a duplicate of the statement with a new graph,
containing the other application.

Eg -

Before -

graph <G1> { <resA> a nco:Contact . }
<G1> nao:maintainedBy <App1> .
<G1> nao:maintainedBy <App2> .

After -

graph <G1> { <resA> a nco:Contact . }
graph <G2> { <resA> a nco:Contact . }
<G1> nao:maintainedBy <App1>
<G2> nao:maintainedBy <App2> .

The advantage of this approach is that it would simplify some of the
extremely complex queries in the DataManagementModel. That would result in
a direct performance upgrade. It would also solve some of the ugly
transaction problems we have 2 commands are accessing the same statement,
and one command removes the data in order to move it to another graph. This
has happened to me a couple of times.

---

My third proposal is that considering that the modification and creation
date of a graph do not serve any benefit. Perhaps we shouldn't store them
at all? Unless there is a proper use case, why go through the added effort?
Normally, storing a couple of extra properties isn't a big deal, but if we
do not store them, then we can effectively kill the need to create new
graph for each data management command.

With this one would just need 1 graph per application, in which all of its
data would reside. We wouldn't need to check for empty graphs or anything.
It would also reduce the number of triples in a database, which can get
alarmingly high.

This seems like a pretty good system to me, which provides all the benefits
and none of the losses.

What do you guys think?

[1] http://techbase.kde.org/Projects/Nepomuk/GraphConcepts

-- 
Vishesh Handa
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