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# Overview
## Introduction
The core of the NEPOMUK Information Element Ontology and the entire
Ontology Framework revolves around the concepts of [nie:DataObject](nie-ontology.html#nie:DataObject) and
[nie:InformationElement](nie-ontology.html#nie:InformationElement). They express the representation
and content of a piece of data. Their specialized subclasses (defined
in the other ontologies) can be used to classify
a wide array of desktop resources and express them in RDF.
[nie:DataObject](nie-ontology.html#nie:DataObject) class represents a bunch of
bytes somewhere (local or remote), the physical entity that contain
data. The *meaning* (interpretation) of that entity, the
information for the user contained in those bytes (e.g. a music file,
a picture) is represented on the
[nie:InformationElement](nie-ontology.html#nie:InformationElement) side of the
ontology.
Both sides are linked using the
property [nie:interpretedAs](nie-ontology.html#nie:interpretedAs) (and its reverse
[nie:isStoredAs](nie-ontology.html#nie:isStoredAs)), indicating the correspondence
between the physical element and its interpretation. There is also a
property to
link [nie:InformationElement](nie-ontology.html#nie:InformationElement)s,
representing the logical containment between them (like a picture and
its album).
## Common properties
Given that the classes defined in this ontology are the superclasses for almost
everything in the Nepomuk set of ontologies, the
properties defined here will be inherited for a lot of classes. It is
worth to comment few of them with special relevance:
- [nie:title](nie-ontology.html#nie:title): Title or name or short text describing the item
- [nie:description](nie-ontology.html#nie:description): More verbose comment about the element
- [nie:language](nie-ontology.html#nie:language): To specify the language of the item.
- [nie:plainTextContent](nie-ontology.html#nie:plainTextContent): Just the raw content of the file, if it makes sense as text.
- [nie:generator](nie-ontology.html#nie:generator): Software/Agent that set/produced the information.
- [nie:usageCounter](nie-ontology.html#nie:usageCounter): Count number of accesses to the information. It can be an indicator of relevance for advanced searches
## Date and timestamp representations
There are few important dates for the life-cycle of a resource. These dates are properties of the nie:InformationElement class, and inherited for its subclasses:
- [nie:informationElementDate](nie-ontology.html#nie:informationElementDate): This is an ''abstract'' property that act as superproperty of the other dates. Don't use it directly.
- [nie:contentLastModified](nie-ontology.html#nie:contentLastModified): Modification time of a resource. Usually the mtime of a local file, or information from the server for online resources.
- [nie:contentCreated](nie-ontology.html#nie:contentCreated): Creation time of the content. If the contents is created by an application, the same application should set the value of this property. Note that this property can be undefined for resources in the filesystem because the creation time is not available in the most common filesystem formats.
- [nie:contentAccessed](nie-ontology.html#nie:contentAccessed): For resources coming from the filesystem, this is the usual access time to the file. For other kind of resources (online or virtual), the application accessing it should update its value.
- [nie:lastRefreshed](nie-ontology.html#nie:lastRefreshed): The time that the content was last refreshed. Usually for remote resources.
## URIs and full representation of a file
One of the most common resources in a desktop is a file. Given the split between Data Objects and Information Elements, some times it is not clear how a real file is represented into Nepomuk. Here are some indications:
1. Every file (local or remote) should generate one DataObject instance and an InformationElement instance.
2. Even when Data Objects and Information Elements are different entities.
3. The URI of the DataObject is the real location of the item (e.g. ''file://path/to/file.mp3'')
3. The URI of the InformationElement(s) will be autogenerated IDs.
4. Every DataObject must have the property [nie:url](nie-ontology.html#nie:url), that points to the location of the resource, and should be used by any program that wants to access it.
5. The InformationElement and DataObject are related via the nie:isStoredAs / nie:interpretedAs properties.
Here comes an example, for the image file /home/user/a.jpeg:
```
# Properties as nmm:Photo
<urn:uuid:10293801928301293> a nmm:Photo ;
nie:isStoredAs <file:///home/user/a.jpeg> ;
nfo:width 49 ;
nfo:height 36 ;
nmm:flash <nmm:flash-off>;
nmm:whiteBalance <nmm:white-balance-automatic> ;
nfo:equipment [ a nfo:Equipment ; nfo:make 'Nokia'; nfo:model 'N900'; nfo:equipmentSoftware 'Tracknon' ] .
# Properties from nfo:FileDataObject
<file:///home/user/a.jpeg> a nfo:FileDataObject ;
nie:interpretedAs <urn:uuid:10293801928301293> ;
nfo:fileSize 12341234 ;
# Mandatory for any nfo:DataObject
nie:url <file:///home/user/a.jpeg> .
```
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