Re: [Tracker] tracker-sparql dbus sparql results

From: Philip Van Hoof <philip codeminded be>
To: Jürgen Jakobitsch <j jakobitsch semantic-web at>, Tracker mailing list <tracker-list gnome org>
Subject: Re: [Tracker] tracker-sparql dbus sparql results
Date: Mon, 21 Apr 2014 21:28:55 +0200

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Hi Jürgen,

I recall writing a blog article about the feature when we implemented
it. Maybe you can extract from the date/time of the article and its
content the commits related to the implementation of the feature?:

http://pvanhoof.be/blog/index.php/2010/09/09/less-exciting-features-also-need-to-be-done-return-types

I'm not sure whether column-types (or rather return-types would indeed
be a more suitable name for rdf-world) was done 'correctly' (the way
you describe). I remember that it was done for a limited use-case and
made to work up until it worked for that use-case.

You might have to improve the current situation before it's usable for
your use-case.

Kind regards,

Philip


Jürgen Jakobitsch schreef op 21/04/2014 17:38:

hi, after struggling with UNIX_FD (dbus type 'h') not really being 
supported i swithed to JNA using libtracker-sparql (it's much
easier anyway...).

i however realized that the rdf datatypes do not work as expected
(as far as i understand tracker-cursor simply uses the first
value_type for a column and returns it for every row returned).

how to verify :

1. i added the following test to
tests/libtracker-sparql/tracker-test.c

static void test_tracker_sparql_cursor_types (void) { GError *error
= NULL;

TrackerSparqlCursor *cursor1; TrackerSparqlConnection *connection;

//const gchar* query = "SELECT ?o ?p WHERE { 
<file:///home/turnguard/.local/share/applications/netbeans-7.4.desktop>
?p ?o  . }"; const gchar* query = "SELECT ?date WHERE { 
<file:///home/turnguard/.local/share/applications/netbeans-7.4.desktop>
< http://www.tracker-project.org/ontologies/tracker#added> ?date .
} ORDER BY ?date"; connection = tracker_sparql_connection_get
(NULL, &error); g_assert_no_error (error);

cursor1 = tracker_sparql_connection_query (connection, query, 0,
&error); g_assert_no_error (error); gint n_columns =
tracker_sparql_cursor_get_n_columns (cursor1); g_print("query %s",
query); while (tracker_sparql_cursor_next (cursor1, NULL, NULL)) { 
int i = 0; for (i = 0; i < n_columns ; i++) { g_print ("=>type of
%s is %d ", 
tracker_sparql_cursor_get_string(cursor1,i,NULL),(int)tracker_sparql_cursor_get_value_type

(cursor1, i));

} g_print("\n"); } g_object_unref(cursor1); }

.... g_test_add_func 
("/libtracker-sparql/tracker/test_tracker_sparql_cursor_types", 
test_tracker_sparql_cursor_types);

2. replace <file://home/turnguard....> with a known entity that has
a tracker#added predicate

3. sudo make => ./tracker-test

4. using the first query i get 2 or 1 (depending on ?o ?p or ?p ?o)
for the date (tracker#added)

5. using the second query i get 5 for the value_type (correct)

i assume that the cursor takes the value_types of the first row it 
encounters and takes these value_types for every following row.
such behaviour is suitable for the sql world but not for the sparql
world. is it really the "g_once_init_leave", can i simply delete
g_once_init_leave without sideeffects?

any pointer really appreciated.

wkr turnguard


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2014-04-18 22:17 GMT+02:00 Jürgen Jakobitsch
<j jakobitsch semantic-web at>:

philip, thanks for information.. i'll take a closer look at
steroids1 dbus...

wkr turnguard

| Jürgen Jakobitsch, | Software Developer | Semantic Web Company
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Wien, Austria | Mob +43 676 62 12 710 | Fax +43.1.402 12 35 - 22

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2014-04-18 22:04 GMT+02:00 Philip Van Hoof
<philip codeminded be>:

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Hi Jürgen,

In that case you should use the Steroids1 DBus object which
works by passing file descriptors and pipe(). I'm guessing that
in Java you can use the type FileDescriptor to turn that to a
InputStream.


https://git.gnome.org/browse/tracker/tree/src/tracker-store/tracker-steroids.vala

Steroid1's protocol that goes over pipe()/vmsplice() already passes

the variable-names and column-types over.

Usage of Resources1 is very slow and on top it's not really a 
supported interface. Resources1 is actually only for the
GraphUpdated signal, which is the only officially supported
interface that the DBus object exports.

The Steroids1 DBus object is going to stay and is used for
write queries and fallback for libtracker-sparql in case WAL
direct access mode can't be used.

Kind regards,

Philip


Jürgen Jakobitsch schreef op 18/04/2014 15:53:

hi philipp,

i'm currently using the dbus resource because i'm writing
the openrdf repository in java and didn't want to got down
the jni, jna road...

i already have built in a switch in tracker-resource.vala,
about here [1], testing the cursor.get_value_type, but the
returned value_types are not totally correct. for some uris
the value is 1 for other uris the value type is 2... i'm
gonna issue a bug report, as soon as i have verified, that
i'm not doing something wrong.. also the returned value_types
are only 1 or 2.. there are not other values (like bnode,
double, date...) although the returned string suggest they
should be used..

wkr turnguard

[1]

https://git.gnome.org/browse/tracker/tree/src/tracker-store/tracker-resources.vala#n102

| Jürgen Jakobitsch, | Software Developer | Semantic Web Company

GmbH | Mariahilfer Straße 70 / Neubaugasse 1, Top 8 | A -
1070 Wien, Austria | Mob +43 676 62 12 710 | Fax +43.1.402 12
35 - 22

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2014-04-18 11:32 GMT+02:00 Philip Van Hoof
<philip codeminded be>:


get_value_type and get_variable_name:

https://git.gnome.org/browse/tracker/tree/src/libtracker-sparql/tracker-cursor.vala#n126

https://git.gnome.org/browse/tracker/tree/src/libtracker-sparql/tracker-cursor.vala#n139


You btw shouldn't use the DBus Resources1 object, but use 
libtracker-sparql instead.


Ivan Frade schreef op 17/04/2014 20:14:

Hi Jürgen,

Yes, tracker does not return bindingNames or types of
the resulting nodes. It assumes the client takes care
of interpreting the results of its query.

A workaround would be to parse the SparQL in the
wrapper and add that information when translating the
DBus result set to the openrdf format. The binding
Names can be added straight forward from the SELECT of
the query. The type information is more complicated to
deduce, based on the position in the query and/or the
rdfs:range of the properties (defined in the 
ontology).

Regards,

Ivan


On Thu, Apr 17, 2014 at 10:11 AM, Jürgen Jakobitsch < 
j jakobitsch semantic-web at> wrote:

hi,

i'm currently developing an openrdf [1] repository 
implementation with a tracker-sparql backend via
dbus. this will enable java developers to include
tracker-sparql results in any application (or create
a sparql endpoint) extremly easily. openrdf's api is
quite common in the semweb community..

everything is working very nice already  the only
trouble i'm having is that i get a vector of vectors
containing simple strings as a result object, meaning
i need to find out types (literals, uris, bnodes,
typed literals)..

is there any known way to retrieve some more
information in the results given via dbus? also
bindingNames are missing...

any pointer really appreciated wkr turnguard..


[1] http://www.openrdf.org/

| Jürgen Jakobitsch, | Software Developer | Semantic
Web Company GmbH | Mariahilfer Straße 70 /
Neubaugasse 1, Top 8 | A - 1070 Wien, Austria | Mob
+43 676 62 12 710 | Fax +43.1.402 12 35 - 22

COMPANY INFORMATION | web       : 
http://www.semantic-web.at/ | foaf      : 
http://company.semantic-web.at/person/juergen_jakobitsch

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Follow-Ups:
- Re: [Tracker] tracker-sparql dbus sparql results
  - From: Jürgen Jakobitsch

References:
- [Tracker] tracker-sparql dbus sparql results
  - From: Jürgen Jakobitsch
- Re: [Tracker] tracker-sparql dbus sparql results
  - From: Ivan Frade
- Re: [Tracker] tracker-sparql dbus sparql results
  - From: Philip Van Hoof
- Re: [Tracker] tracker-sparql dbus sparql results
  - From: Jürgen Jakobitsch
- Re: [Tracker] tracker-sparql dbus sparql results
  - From: Philip Van Hoof
- Re: [Tracker] tracker-sparql dbus sparql results
  - From: Jürgen Jakobitsch
- Re: [Tracker] tracker-sparql dbus sparql results
  - From: Jürgen Jakobitsch

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