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== 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 -> <tt>Subject - Predicate - Object - Graph</tt>. The <tt>Graph</tt> 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 <tt>graph</tt> is also called the context of the statement. It is used to store some information about each triple

== Graph Naming ==

Every graph in '''Nepomuk''' is given a unique uri which is of the form <tt>nepomuk:/ctx/uuid</tt>. The <tt>ctx</tt> over here is an abbreviation for context.

== Graph Creation ==

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 [http://api.kde.org/4.x-api/kdelibs-apidocs/nepomuk-core/html/datamanagement_8h.html DataManagement functions] -

* [http://api.kde.org/4.x-api/kdelibs-apidocs/nepomuk-core/html/group__nepomuk__datamanagement.html#gafb8a64e1fb9e8339ff87be1b7a370a57 addProperty]
* [http://api.kde.org/4.x-api/kdelibs-apidocs/nepomuk-core/html/group__nepomuk__datamanagement.html#gaa86c603d624d55703b0e41bbec77d2ec setProperty]
* [http://api.kde.org/4.x-api/kdelibs-apidocs/nepomuk-core/html/group__nepomuk__datamanagement.html#gad731308a74b75a1a2bb20b1676eda2ed createResource]
* [http://api.kde.org/4.x-api/kdelibs-apidocs/nepomuk-core/html/group__nepomuk__datamanagement.html#gac094511c6157fb1f96f3a3344d48128d storeResources]

Each time any of these operations is called and new data is being pushed, a new graph with the current date time is created.

== Graph contents ==

Graphs in '''Nepomuk''' store the following information -

* Graph Type
* Creation Date
* Modification Date
* Graph Maintainer

Example -
<syntaxhighlight lang="text">
<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
</syntaxhighlight>

The <tt>nao:created</tt> and <tt>nao:lastModified</tt> properties are identical to those used in resources. Except for the fact that graphs are generally never modified. They are only created or destroyed.

=== Graph Type ===

Each graph has a type which depends on the type of data it holds. The 3 most commonly used types are -
* <tt>nrl:Ontology</tt> - Used to hold ontology data.
* <tt>nrl:InstanceBase</tt> - Used for normal data
* <tt>nrl:DiscardableInstanceBase</tt> - 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.

=== Agents ===

Each graph contains a property called <tt>nao:maintainedBy</tt> which specifies which application created that data -

<syntaxhighlight lang="text">
<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
</syntaxhighlight>

Each application is represented as a <tt>nao:Agent</tt> with the application name specified as the nao:identifier. This name is automatically determined via <tt>KComponentData</tt> 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 [http://api.kde.org/4.x-api/kdelibs-apidocs/nepomuk-core/html/group__nepomuk__datamanagement.html#ga0fbf22be4b581afd0b5bcd1f203e934f <tt>removeDataByApplication</tt>].

[[Category:Documentation]]
[[Category:Development]]
[[Category:Tutorials]]

Projects/Nepomuk/GraphConcepts/en - Revision history

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