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== Scope of this page ==

This document describes in depth how URIs are stored in '''Nepomuk''', and their significance.

== What is a URI? ==

A URI or ''Uniform Resource Identifier'' is a string of characters used to identify or locate a resource. URIs can be of two types, or both -
* URN - Uniform Resources Names
* URL - Uniform Resource Locator.

A URN is a unique way of identifying a resource. It does not provide any information about how the resource can be found> Rather it provides a way to uniquely identify a resource. For example - <tt>urn:isbn:037584614X</tt>. This urn tells you nothing about how to find the book that it just mentions a unique identifier. You can take the ISBN and somehow locate [http://www.amazon.com/Nick-Norahs-Infinite-Playlist-Rachel/dp/037584614X the book]

A URL, in contrast, provides a way to access the resource. For example - <tt>file:///home/user/file</tt>. This url clearly dictates that the <tt>file</tt> protocol should be used along with the path <tt>/home/user/file</tt>. It does not give a unique way of identifying the resource. It just provides one way to access the resource.

== So what does Nepomuk use? ==

Nepomuk URIs are in a way URNs, though unlike conventional URNs which provide a globally unique way of identifying a resource, they provide a unique way of identifying a resource '''on one machine'''. This is important - '''Nepomuk''' URIs are NOT machine independent. The same resource can have different URIs on different systems. This is because all URIs in '''Nepomuk''' are randomly generated. They follow a pattern -

<blockquote>nepomuk:/res/uuid</blockquote>

UUID stands for a ''Universally Unique Identifier''. In Nepomuk this UUID is guaranteed to be unique on one system. A resource with the same UUID can exist as completely different resources on different systems.

== Doesn't Nepomuk need the location of the Resource? ==

Yes, we do! But the location of a resource is independent of its name. Its name ( UUID ) never changes. If the resource has a location then the location is stored as a URL using the property [http://oscaf.sourceforge.net/nie.html#nie:url <code>nie:url</code>].

Not all resources have a location. '''Nepomuk''' is also used to store abstract stuff like Contacts, Events and People, they do not have a location. Concrete stuff like pictures, videos and files do have locations.

== Tell me more about these URLS ==

A URL is like the physical representation of an object. In '''Nepomuk''' all URLs must have a proper scheme that KDE supports. Example <tt>smb:/192.168.1.50/Multimedia/</tt> or <tt>akonadi:?item=21</tt>. These all are stored in the nie:url property of the resource.

== You're lying! I see resource URIs like <tt>akonadi:?item=21</tt> ==

This URI nomenclature was decided in June 2011. We're still in the process of making sure all URIs follow our naming convention. With KDE 4.10, there will be a data migration client which will make sure the old data is converted to this new uri format.

== Why go through all this trouble? ==

The rationale in having random UUID based URIs in '''Nepomuk''' is two-fold. One, we have a guarantee that the URI is random, and means nothing. This is important when restoring backups or synchronization or when sharing resource among different systems. If you're restoring a resource and its URI already exists, no problem, just generate another one. It's not like they have any concrete meaning. The URL however does have a concrete meaning, and that we need to respect.

The second reason is that all URIs follow one naming convention.

== What about the Semantic Web? ==

'''Nepomuk''' is the Semantic Desktop, while we acknowledge the existence of the Semantic Web, we currently do not go out of our way to support it. It's a messy situation, and we don't have a solution right now. If you have any ideas on how we can have better interoperability with the Semantic Web, then please come talk to us.

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

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