The Semantic Web - with quadruples
Nepomuk is based on the Semantic Web, and even though the Semantic Web is largely advertised as a triple store, Nepomuk is NOT a triple store. We in fact store quadruples -> Subject - Predicate - Object - Graph. The Graph part of these quadruples is always automatically managed by Nepomuk. Clients rarely (if ever) need to care about the graph.
However, if you still want to know why they are present. Read on.
The graph is also called the context of the statement. It is used to store some information about each triple
Every graph in Nepomuk is given a unique uri which is of the form nepomuk:/ctx/uuid. The ctx over here is an abbreviation for context.
The first obvious question is - When are graphs created? Is everything just added into one big graph?
Nepomuk works by creating new graphs for each atomic operation. These operations are the ones defined by the DataManagement functions -
Each time any of these operations is called and new data is being pushed, a new graph with the current date time is created.
Graphs in Nepomuk store the following information -
- Graph Type
- Creation Date
- Modification Date
- Graph Maintainer
<nepomuk:/ctx/c4d93812-7d8c-4c8f-b8a7-e1d4dbc5fed5> rdf:type nrl:InstanceBase nao:created 2011-07-30T11:34:19.587Z nao:modified 2011-07-30T11:34:19.587Z nao:maintainedBy nepomuk:/res/e2eb2efb-14ee-4038-ac24-698f916289b0
The nao:created and nao:lastModified properties are identical to those used in resources. Except for the fact that graphs are generally never modified. They are only created or destroyed.
Each graph has a type which depends on the type of data it holds. The 3 most commonly used types are -
- nrl:Ontology - Used to hold ontology data.
- nrl:InstanceBase - Used for normal data
- nrl:DiscardableInstanceBase - Used for data which can be reproduced and should not be backed up. This type is generally reserved for data created during indexing of files.
Each graph contains a property called nao:maintainedBy which specifies which application created that data -
<nepomuk:/ctx/c4d93812-7d8c-4c8f-b8a7-e1d4dbc5fed5> rdf:type nrl:DiscardableInstanceBase nao:created 2011-07-30T11:34:19.587Z nao:modified 2011-07-30T11:34:19.587Z nao:maintainedBy nepomuk:/res/e2eb2efb-14ee-4038-ac24-698f916289b0 <nepomuk:/res/e2eb2efb-14ee-4038-ac24-698f916289b0> rdf:type nao:Agent rdf:type rdfs:Resource nao:identifier nepomukindexer
Each application is represented as a nao:Agent with the application name specified as the nao:identifier. This name is automatically determined via KComponentData and can be spoofed. Security was not one of our concerns.
Each triple may be maintained by a number of different applications. This information is valuable for the advanced DataManagement functions such as removeDataByApplication.