From KDE TechBase

Status of Indexing

File Indexing has gone through a major overhaul in 4.10. We no longer rely on strigi. This means that we need to write our own file indexer from scratch. However writing a file indexer is very simple.

At this time all the indexers live in nepomuk-core/services/fileindexer/indexer. They use a private library that is not exported. Therefore public indexers for custom formats are currently not allowed. All indexers must go in the nepomuk-core repository.

Extractor Plugin

In order to write a file indexer, we have to write a plugin derived from Nepomuk2::ExtractorPlugin. We are required to implement two simple functions -

class NEPOMUK_EXPORT ExtractorPlugin : public QObject
    ExtractorPlugin(QObject* parent);
    virtual ~ExtractorPlugin();

    virtual QStringList mimetypes() = 0;
    virtual SimpleResourceGraph extract(const QUrl& resUri, const QUrl& fileUrl, const QString& mimeType) = 0;

These two functions are mimetypes and extract. Each plugin can act on a certain set of mimetypes. Each plugin simply needs to list out all the mimetypes they support.

The second function extract is the heart of the extractor. You are provided with the mimetype and the url of the file. The file can be read and information can be extracted from it.

Saving the Extracted Data

The Nepomuk Extractors are based around two simple classes SimpleResource and SimpleResourceGraph. The SimpleResourceGraph is just a collection of SimpleResources. A SimpleResource is just a collection of (key, value) pairs which contain the properties of that particular resource.

The main file resource has a resource uri which is passed as a parameter. It can be used as follows -

    SimpleResource fileRes( resUri );
    fileRes.addType( NFO::PlainTextDocument() );
    fileRes.addProperty( NIE::plainTextContent(), contents );
    fileRes.addProperty( NFO::wordCount(), words );
    fileRes.addProperty( NFO::lineCount(), lines );
    fileRes.addProperty( NFO::characterCount(), characters );

This fileRes can then be added to a SimpleResourceGraph and returned. It will then be saved in Nepomuk.

Required Files

Since the plugin interface still isn't public. It would be best to directly contribute to nepomuk-core. The relevant code can be found at nepomuk-core/services/fileindexer/indexer/.

Testing the Indexer

In order to test the indexer, you should call it manually on the specified file by executing nepomukindexer fileUrl. If there were no errors, then the file should have been indexed correctly.

You can view the indexed data by running nepomukshow fileUrl. This 'nepomukshow' tool does not output the plain text content by default. You can print the plain text by calling nepomukshow --plainText fileUrl


It might be common to get errors that a properties range/domain/cardinality is not being followed. These errors occur when the ontologies are not being properly followed. In that case it would be best to look where you're adding that property and if it actually has the correct domain/range/cardinality.

The ontologies can be found over here -