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{{TutorialBrowser|
 
series=[[../|Nepomuk]]|
 
name=Nepmuk Tips and Tricks|
 
reading=[[../Resources|Resource Handling with Nepomuk]], 
 
[[../AdvancedQueries|Advanced Queries with SPARQL]],
 
[[../RDFIntroduction|RDF and Ontologies in Nepomuk]]
 
}}
 
 
== Always initialize Nepomuk  ==
 
 
Make sure that somewhere in the initialization code of your application or library Nepomuk is initialized via:
 
 
<syntaxhighlight lang="cpp-qt">
 
Nepomuk::ResourceManager::instance()->init();
 
</syntaxhighlight>
 
 
<br>
 
  
 
== Using ontology URIs in your code  ==
 
== Using ontology URIs in your code  ==
Line 41: Line 23:
 
== Debugging the created data  ==
 
== Debugging the created data  ==
  
=== Using sopranocmd ===
+
Soprano provides a command line client to connect to the storage service. It's called <code>sopranocmd</code>. It provides all the features one needs to debug data. It is recommended that you only use sopranocmd for running queries.
  
When using Nepomuk one creates a lot of RDF statements in the Nepomuk RDF storage. It is often of interest to check which data has been created, if statements have been correctly created or simply look at existing data.  
+
Running sopranocmd is cumbersome because of the large number of arguments it requires. This can be made simpler by adding the following alias -
 +
<syntaxhighlight lang="bash">alias nepomukcmd="sopranocmd --socket `kde4-config --path socket`nepomuk-socket --model main --nrl"</syntaxhighlight>
  
Soprano provides a nice command line client to do all this called ''sopranocmd''. It provides all the features one needs to debug data: it can add and remove statements, list and query them, import and export whole RDF files, and even monitor for ''[http://soprano.sourceforge.net/apidox/trunk/classSoprano_1_1Model.html#3e2595166caac3621fd4268e46049adf statementAdded]'' and ''[http://soprano.sourceforge.net/apidox/trunk/classSoprano_1_1Model.html#8fa85bfce2f83e89f83ef602cd818991 statementRemoved]'' events.
 
 
To access the Nepomuk storage one would typically use the D-Bus interface:
 
 
<syntaxhighlight lang="text">
 
# sopranocmd --dbus org.kde.NepomukStorage --model main <command> \
 
    <parameters>
 
</syntaxhighlight>
 
 
If one wanted to list all the resources that have been tagged with the tag whose resource URI is nepomuk:/foobar one would use the following command:
 
 
<syntaxhighlight lang="text">
 
# sopranocmd --dbus org.kde.NepomukStorage --model main list \
 
    "" "" "<nepomuk:/foobar>"
 
</syntaxhighlight>
 
 
or one would use a SPARQL query ('''sopranocmd supports the standard URI prefixes out of the box'''):
 
  
 +
For example -
 
<syntaxhighlight lang="text">
 
<syntaxhighlight lang="text">
# sopranocmd --dbus org.kde.NepomukStorage --model main query \
+
# nepomukcmd query \
 
     "select ?r where { ?r nao:hasTag ?tag . \
 
     "select ?r where { ?r nao:hasTag ?tag . \
 
                       ?tag nao:prefLabel 'foobar'^^xsd:string . }"
 
                       ?tag nao:prefLabel 'foobar'^^xsd:string . }"
 
</syntaxhighlight>  
 
</syntaxhighlight>  
 
To monitor all statements that are added and removed from the Nepomuk storage one would simply use the following command (as with ''list'' one can specify a filter to only list the added and removed statements one is interested in): <syntaxhighlight lang="text">
 
# sopranocmd --dbus org.kde.NepomukStorage --model main monitor
 
</syntaxhighlight>
 
 
<syntaxhighlight lang="text"># sopranocmd --help</syntaxhighlight> is your friend for all details.
 
 
==== nepomukcmd ====
 
 
As a shortcut add the following to your .bashrc to avoid having to type in the dbus and model parameters all the time:
 
 
<syntaxhighlight lang="text">alias nepomukcmd="sopranocmd --socket `kde4-config --path socket`nepomuk-socket --model main --nrl"</syntaxhighlight>
 
  
 
=== Using Konqueror  ===
 
=== Using Konqueror  ===
Line 121: Line 76:
  
 
This will create the same query we used above only using no hard-coded components whatsoever.  
 
This will create the same query we used above only using no hard-coded components whatsoever.  
 
== Restarting Nepomuk and its Services  ==
 
 
The Nepomuk services are controlled by the ''[[../NepomukServer|nepomukserver]]'' application which is started on KDE login. The nepomukserver will take care of starting and stopping all services.
 
 
It is possible to stop the server and all services alltogether by simply calling a D-Bus method:
 
 
<syntaxhighlight lang="text">
 
# qdbus org.kde.NepomukServer /nepomukserver \
 
    org.kde.NepomukServer.quit
 
</syntaxhighlight>
 
 
It can then be restarted by simply calling ''nepomukserver'' again. In many debugging situations it might be of interest to pipe the output of the server (and all services) to a file:
 
 
<syntaxhighlight lang="text">
 
# nepomukserver 2> /tmp/nepomuk.stderr
 
</syntaxhighlight>
 
 
Also interesting to know is that Nepomuk defines a set of debugging areas for the services and the server itself. Use ''kdebugdialog'' to enable or disable them.
 
 
Or one can stop and start single services. In most cases this is sufficient since each service is run in its own process. Thus, changes to a service plugins will be picked up directly:
 
 
<syntaxhighlight lang="text">
 
# qdbus org.kde.NepomukServer /servicemanager \ 
 
    org.kde.nepomuk.ServiceManager.stopService <servicename>
 
 
# qdbus org.kde.NepomukServer /servicemanager \ 
 
    org.kde.nepomuk.ServiceManager.startService <servicename>
 
</syntaxhighlight>
 
 
<br>
 
 
== Listening to changes in the database  ==
 
 
Write about the Resource Watcher.
 
 
== Remove all Strigi-indexed data  ==
 
 
Strigi produces a lot of data in Nepomuk. There might be times where one wants to remove all that data manually.
 
 
The little command below removes all data created by Strigi (caution: this could take a long time):
 
 
DOESN'T WORK - Update it
 
 
<syntaxhighlight lang="text">
 
for a in `nepomukcmd --foo query "select distinct ?g where { \
 
  ?g <http://www.strigi.org/fields#indexGraphFor> ?r . }"`;
 
  do nepomukcmd rmgraph "$a"; done
 
</syntaxhighlight>
 
 
 
== Starting Nepomuk Sever from the Trunk in Ubuntu  ==
 
 
Note: Starting with (K)ubuntu 10.10 (Maverick Meerkat), virtuoso-t is in /usr/bin. So the work around described below is no longer necessary.
 
 
Ubuntu packages virtuoso slightly differently. It provides a package called ''virtuoso-nepomuk'' which installs the executable ''virtuoso-t'' in the ''/usr/lib/virtuoso/'' directory for security purposes.
 
 
When running Nepomuk from the trunk, the nepomukserver is unable to find the ''virtuoso-t'' executable, and therefore the NepomukStorage Service fails to initialize. One way to fix this is to adjust the PATH environment variable.
 
<syntaxhighlight lang="text">
 
PATH=/usr/lib/virtuoso:$PATH
 
export PATH
 
</syntaxhighlight>
 
 
<br>
 
  
 
== Debugging virtuoso-t  ==
 
== Debugging virtuoso-t  ==

Revision as of 22:54, 25 August 2012


Contents

Using ontology URIs in your code

One often needs the URI of a specific class or a specific property in ones code. And not all ontologies are provided by the very convenient Soprano::Vocabulary namespace.

The solution is rather simple: create your own vocabulary namespaces by using Soprano's own onto2vocabularyclass command line tool. It can generate convenient vocabulary namespaces for you. The Soprano documentation shows how to use it manually or even simpler with a simple CMake macro.


Mind the Difference between QString and QUrl

Nepomuk::Resource provides two constructors: one taking a QString as identifier or URI and one taking a QUrl.

The latter one is really simple: the given URI is used as the resource URI. If the resource exists, its data is used, otherwise it will be created with exactly that URI.

The QString one is a bit trickier. It will try to be clever about the parameter and see if it is a URI. If no resource with that URI (if it is a URI) exists, it is interpreted as an identifier (nao:identifier). Resource checks if a resource with that identifier exists. If so, its data is loaded, if not, a new resource with a random URI and that string as identifier is created.

However, be aware that nothing is written to Nepomuk until the first writing call to Resource such as setProperty or addType.


Debugging the created data

Soprano provides a command line client to connect to the storage service. It's called sopranocmd. It provides all the features one needs to debug data. It is recommended that you only use sopranocmd for running queries.

Running sopranocmd is cumbersome because of the large number of arguments it requires. This can be made simpler by adding the following alias -

alias nepomukcmd="sopranocmd --socket `kde4-config --path socket`nepomuk-socket --model main --nrl"


For example -

# nepomukcmd query \
    "select ?r where { ?r nao:hasTag ?tag . \
                       ?tag nao:prefLabel 'foobar'^^xsd:string . }"

Using Konqueror

In the Nepomuk playground repository lives a KIO slave which can handle the nepomuk:/ protocol. It will display all properties of a Nepomuk resource including its links to other resources and the backlinks. This is a convenient way of looking at the Nepomuk data. The KIO slave even support removal of resources.

Nepomuk kio slave.png


Using NepomukShell

NepomukShell is a maintenance and debugging tool, which lives in its own git repository at nepomukshell. It is a simple tool that let's one browse all resources in Nepomuk. Additionally it allows to create subclasses and properties (Caution: do only create subclasses and properties from PIMO classes and properties!) and remove resources.

Pimoshell.png

Constructing SPARQL queries

Hint: In most cases the Nepomuk Query API should be enough and prevent you from writing your own SPARQL which is hard to debug.

Whenever doing something a bit fancier with Nepomuk one has to use SPARQL queries via
Nepomuk::ResourceManager::instance()->mainModel()
    ->executeQuery( myQueryString, 
                    Soprano::Query::QueryLanguageSparql );
Constructing these queries can be a bit cumbersome since one has to use a lot of class and property URIs from different ontologies. Also literals have to be formatted according to the N3 syntax used in SPARQL. Luckily Soprano provides the necessary tools to do exactly that: Soprano::Node::toN3, Soprano::Node::resourceToN3, and Soprano::Node::literalToN3 take care of all formatting and percent-encoding you need. Using those methods the code to create queries might look ugly but the resulting queries are more likely to be correctly encoded and introduce less code duplication.

Typically one would use QString::arg like so (be aware that the standard prefixes are NOT supported out-of-the-box as with sopranocmd):

using namespace Soprano;
 
QString myQuery
     = QString("select ?r where { "
               "?r %1 ?v . "
               "?v %2 %3 . }")
       .arg(Node::resourceToN3(Vocabulary::NAO::hasTag()))
       .arg(Node::resourceToN3(Vocabulary::NAO::prefLabel()))
       .arg(Node::literalToN3("foobar")));

This will create the same query we used above only using no hard-coded components whatsoever.

Debugging virtuoso-t

If virtuoso-t consumes a lot of CPU resources but there are no active queries analysis has to go a bit deeper. Virtuoso is started through Soprano with certain parameters which are set in a temporary ini-file (/tmp/virtuoso_XXXX.ini). Soprano needs to be modified manually to start Virtuoso with different parameters in the ini-file, e.g. to improve virtuoso-t's behaviour by modifying backends/virtuoso/virtuosocontroller.cpp (Soprano) and setting NumberOfBuffers to 40000 (line 344) and SchedulerInterval to 0 (line 350).

After re-compiling soprano one has to attach gdb to virtuoso-t as soon as it starts consuming CPU and create a full threaded backtrace:

set logging file /tmp/virtuoso-t.out
set logging on
thread apply all bt full

Note: The above settings should only be used for debugging!


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