Difference between revisions of "Projects/Nepomuk/DataFeeders"

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Each SimpleResource with a blank uri needs to be translated to a corresponding nepomuk resource uri, if that resource already exists. Otherwise a new resource needs to be created. This translation process is called resource identification. It is performed using the properties specified in the SimpleResource.  
 
Each SimpleResource with a blank uri needs to be translated to a corresponding nepomuk resource uri, if that resource already exists. Otherwise a new resource needs to be created. This translation process is called resource identification. It is performed using the properties specified in the SimpleResource.  
  
Certain properties in the ontologies are marked as [http://oscaf.sourceforge.net/nrl.html#nrl:DefiningProperty| defining properties]. The criteria is decided as follows -
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Certain properties in the ontologies are marked as [http://oscaf.sourceforge.net/nrl.html#nrl:DefiningProperty defining properties]. The criteria is decided as follows -
* Properties with a literal range are always defining, unless explicitly marked as a [http://oscaf.sourceforge.net/nrl.html#nrl:NonDefiningProperty|nrl:NonDefiningProperty]
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* Properties with a literal range are always defining, unless explicitly marked as a [http://oscaf.sourceforge.net/nrl.html#nrl:NonDefiningProperty nrl:NonDefiningProperty]
* Properties with a resource range are always NOT defining, unless explicitly marked with [http://oscaf.sourceforge.net/nrl.html#nrl:DefiningProperty|nrl:DefiningProperty]
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* Properties with a resource range are always NOT defining, unless explicitly marked with [http://oscaf.sourceforge.net/nrl.html#nrl:DefiningProperty nrl:DefiningProperty]
  
 
Two resources are said to match each other if the following criteria are met -
 
Two resources are said to match each other if the following criteria are met -

Revision as of 16:25, 9 August 2012

Some applications need to push large quantities of data into Nepomuk. They are typically called "feeder" applications as they provide Nepomuk with the data it requires. A database is only as powerful as the data it holds.

While one can use the Resource class to push the data. It'll be slow as the Resource class is synchronous and writes back into the database after each command. What one requires is an asynchronous API to push the application can just write all the data, and then Nepomuk can process and merge all of the data provided with its internal database.

Contents

SimpleResources

Applications can use the SimpleResource class to model the data that they want to push. The SimpleResource class is not connected to the Nepomuk database, and is just a convenience wrapper around a QMultiHash. Any changes made to these SimpleResources are not reflected back to the database, unless explicitly specified.

An example -

    Nepomuk2::SimpleResource coldplay;
    coldplay.addType( NCO::Contact() );
    coldplay.addProperty( NCO::fullname(), "Coldplay" );
 
    Nepomuk2::SimpleResource album;
    album.addType( NMM::MusicAlbum() );
    album.addProperty( NIE::title(), "X&Y" );
 
    Nepomuk2::SimpleResource fileRes;
    fileRes.addType( NFO::FileDataObject() );
    fileRes.addType( NMM::MusicPiece() );
    fileRes.addProperty( NMM::performer(), coldplay );
    fileRes.addProperty( NMM::musicAlbum(), album );
    fileRes.addProperty( NIE::url(), fileUrl );
    fileRes.addProperty( NIE::title(), "What If" );


In the above example we wish to push data about a song "What If" by the popular english artist "Coldplay". We create a different SimpleResource for each resource that we want to push into Nepomuk, and then add the relevant metadata. These SimpleResources can reference each other.

All of this data is currently just stored in memory in a hash table. In order to push the data into Nepomuk, we group it all together using a SimpleResourceGraph. After which was can push the data by calling SimpleResourceGraph::save().

Example -

    Nepomuk2::SimpleResourceGraph graph;
    graph << coldplay << album << fileRes;
 
    KJob* job = graph.save();


The save operation returns a KJob which has already begun execution. This operation will continue asynchronously, and on completion will emit the signal completed.

The completed signals also return the respective KJob. This job can then be checked for errors, which may have occurred if we tried to save invalid data. It is up to the programmer to make sure that the data is valid. Invalid valid data is completely ignored and an error is given.

StoreResources

Calling the SimpleResourceGraph::save operational, internally calls the StoreResourcesJob with its default parameters. For more specialized use cases storeResources can directly be called.

The storeResources is a lengthy procedure that has performs multiple operations on the data after which it pushes the data into Nepomuk. The two main parts of the job are outlined below.

Identification

Each SimpleResource contains a uri, which is either an actual uri of the form nepomuk:/res/some-unique-identifier or is a blank uri of the form _:identifier. The SimpleResources which contain resource uris can just directly be pushed into Nepomuk. The blank uris require some additional processing.

Each SimpleResource with a blank uri needs to be translated to a corresponding nepomuk resource uri, if that resource already exists. Otherwise a new resource needs to be created. This translation process is called resource identification. It is performed using the properties specified in the SimpleResource.

Certain properties in the ontologies are marked as defining properties. The criteria is decided as follows -

  • Properties with a literal range are always defining, unless explicitly marked as a nrl:NonDefiningProperty
  • Properties with a resource range are always NOT defining, unless explicitly marked with nrl:DefiningProperty

Two resources are said to match each other if the following criteria are met -

  • Their list of rdf:types matches.
  • The resources do not have any defining properties which do not match.
  • At least one defining property matches.

- Provide an example -

All blank uris are either mapped to existing resources or new resources are created.

Merging

Once the identification process has been completed, each SimpleResource goes through a series of checks which check if the domain, range and carnality of properties is correct, and then pushes the data into the database after merging the graphs for the statements that already exist, and creating a new graph for the new statements.

Who else is using it?

The SimpleResource API is currently the de facto method of pushing data into Nepomuk. It is being heavily utilized by our own file indexer, and KDE PIM. PIM uses the SimpleResource api in order to push emails, contacts and event information into Nepomuk.

For more examples on how to use SimpleResource, we suggest you look at our comprehensive tests present in the datamanagementmodel. Add link!!

Graph Handling

Most developers do not need to worry about graphs present in Nepomuk. However, for the sake of completion we're documenting what happens internally. Hopefully, this will help you better understand the intricacies on Nepomuk.

When a SimpleResourceGraph is saved or passed onto storeResources, each statement in the graph is checked for existance in the database. If that triple already exists, it is set aside and specially handled. All other triples are pushed into this one big graph that is created with each call to storeResources.

That graph contains the following data - <nepomuk:/ctx/some-graph> a nrl:Graph . get some data!!

When ..


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