Scope of this page

This page serves as a very simple overview to Nepomuk. It focuses on getting started with Nepomuk and tries not to deal with internal Nepomuk concepts. Please keep in the mind that the processes described here are targeted to the general use case. If you have a more specialized use case in mind, this might not be the best way to use the Nepomuk libraries.

Libraries and Headers

Most of the non graphic parts of Nepomuk are present in the NepomukCore repository. It is shipped with KDE 4.9, and should be provided by your distribution. When starting with Nepomuk development, it would be better to stick with your distribution version.

With KDE 4.9, the Nepomuk libraries moved to the Nepomuk2 namespace. The headers also follow a similar convention.


All the Nepomuk headers in are preceded by Nepomuk2. Here are some common Nepomuk includes that are frequently used -

#include <Nepomuk2/Tag>
#include <Nepomuk2/Resource>
#include <Nepomuk2/ResourceManager>
#include <Nepomuk2/File>


KDE Projects generally use cmake as a build tool. Nepomuk is no different. In order to use Nepomuk you need to add the relevant macros for Nepomuk in your CMakeLists.txt.

find_package(NepomukCore REQUIRED)
target_link_libraries(myfile ${NEPOMUK_CORE_LIBRARY} )

Basic Terminology

At the lowest level Nepomuk is just a database which many applications use to store data. The main difference is that the data is not stored in rows and columns like traditional relational databases. Instead Nepomuk can be viewed as something similar to an object or document store. We store a large number of objects, and each of these objects have (key, value) pairs attached to them.

Each object in our database is called a Resource. Everything in Nepomuk revolves around resources. All the higher level concepts such as files, tags, contacts, music albums, actors, etc. they are all Resources. Each of these resources has a number of (key, value) pairs associated with it. In the Nepomuk world, the keys are called properties or predicates, and have already been defined.

Nepomuk is also based around a concept called the Semantic Web. However, understanding the ideas and details behind the Semantic Web is not necessary.

Tags and Ratings

Every tag in Nepomuk is a Resource. In fact the Tag class is also derived from the Resource class.

Setting Tags

Nepomuk2::Tag tag( "Awesome-Tag-Name" );
Nepomuk2::Resource res( url );
res.addTag( tag );

The Nepomuk2::Tag class will automatically look for a tag called "Awesome-Tag-Name", if it finds it then Tag::exists() will return true. Otherwise, the tag will be saved the first time the tag is used. In our case, when we add the tag to the Resource via Resource::addTag(), the tag will be saved.

Retrieving Tags

using namespace Nepomuk2;

Resource res( url );
QList<Tag> tags = res.tags();
foreach(const Tag& tag, tags)
    kDebug() << tag.genericLabel();

Every resource has a predefined function for retrieving all the Tags any resource is tagged with - Resource::tags(). The Resource::genericLabel() function tries to find a presentable name for any resource. In the case of tags, it returns the tags name.

Listing Tags

The simplest way to list all the tags that are present in Nepomuk is via a static function - Tag::allTags(). You, however, need to be careful using this in a production environment as it executes a blocking query in the background. Depending on the number of tags on the system, it could take some time.

using namespace Nepomuk2;

QList<Tag> tags = Tag::allTags();
foreach(const Tag& tag, tags)
    kDebug() << tag.genericLabel();

There are also convenience functions to list all the resources that have been assigned a particular tag - Tag::tagOf

using namespace Nepomuk2;

Tag tag("Awesome-Tag-Name");
QList<Resource> taggedResources = tag.tagOf();

foreach(const Resource& res, taggedResources)
    kDebug() << res.genericLabel();


Every resource in Nepomuk can be given a numeric rating. It is generally done on a scale of 0-10.

using namespace Nepomuk2;

File fileRes( urlOfTheFile );
int rating = fileRes.rating();

rating = rating + 1;
fileRes.setRating( rating );

Dealing with files

Every file in Nepomuk is represented as a resource. Since dealing with files is very common, we provide a special File class which is derived from the Resource class. We also provide convenience functions for checking if a resource is a file.

using namespace Nepomuk2;

File fileRes( urlOfTheFile );
kDebug() << fileRes.url();

Resource res( urlOfTheFile );
if( res.isFile() )
    kDebug() << res.toFile().url();

Internally Nepomuk Files are not very different from other Resources.

Working Example

We understand that using a new framework such as Nepomuk can be quite intimidating at times. Therefore, we though it would be nice to have a working example for you to get started with - kdeexamples/nepomuk/test. This example does nothing but it creates an executable called nepomuk-test. It serves as a simple starting point to Nepomuk where all the boiler plate code has been taken care of.

This page was last edited on 9 February 2018, at 12:35. Content is available under Creative Commons License SA 4.0 unless otherwise noted.