The aim of this page is not to fully explain the Nepomuk technology and every detail, but to give a short overview, some examples, share the vision behind it and link to relevant information on the web.
As the Glossary mentions, Nepomuk is about classification, organisation and presentation of data. It is not an application, but a component which can be used by developers within applications.
For example, Dolphin makes use of Nepomuk. For the following to hold Nepomuk and Strigi must be enabled from . The information sidebar of Dolphin allows you to assign tags, ratings and comments to files. This information is then stored in Nepomuk and indexed by Strigi. You can then search for metadata using the navigation bar in Dolphin. Write "nepomuksearch:/" followed by search terms.
Nepomuk offers several 'layers' of functionality to applications. The first and most simple of those is manual tagging, rating and commenting, as used in Dolphin. This helps you to find your files faster, but is also a lot of work.
To make finding files containing text easier, Nepomuk offers a second functionality: indexing the text of files. It uses a technology called Strigi for this. You can now also find files by entering some words which you know are in there, or just (part of) their title.
The third layer is a very complex one, and the reason why Nepomuk was conceived as a research project of several companies and universities in the European union. This is where you will find difficult words like 'semantic desktop' and 'ontologies'. Basically, it is about context and relationships.
Let me try to explain what Nepomuk offers using two examples.
Say you received a photo from a friend of yours, 2 weeks ago. You saved it somewhere on your computer. Now how to you find that file? If you don't remember the location, you're out of luck.
Now Nepomuk aims to help you. You know this file came from that friend of yours, your computer does not. Nepomuk, however, can remember this relationship. Searching on the name of your friend will therefor pop up the photo!
Another potential relationship is between a web page you copied text from and the document you pasted it into, or two images showing the same car. Such relations can sometimes be extracted from the files themselves (you could analyze photos and see who or what is on there) or supplied by the applications involved (as in the above email example). This part of Nepomuk is still under heavy development, and needs integration in applications, so you can expect it to take a few more years to really shine.
All in all, this part of Nepomuk is about making search smart. Think about how Google tries to be smart with your searches: when you search for a hotel and a city name, it shows above the website results a google map showing hotels in the city you mentioned! It might even suggest a better name in case you made a spelling mistake. Google also tries to put the most relevant information on top of the list of results, using complex calculations on relationships (links) between websites. Nepomuk will be able to offer such smart results and order them on relevancy using relationship information.
These relationships can not only help you while searching for files, but also have an influence on applications and what information they present. Note that this way of using Nepomuk is still more a vision than reality! Many of the components are in place, but it is not yet integrated in applications and the desktop as a whole.
So here an example of bringing context awareness to your desktop could help you work more efficiently.
Say you are working on finishing some notes you took in a meeting. The phone rings, and somebody asks you to find that spreadsheet with prices, adjust it for a customer. After a few more interruptions you find your desktop full of files and windows...
It'd be nice to be able to organize all that a bit better, right?
Enter 'activities'. These have been introduced in Plasma, and currently offer different 'desktops'. They are a bit like virtual desktops, except that the desktop itself changes, not the set of applications. Different widgets, background, things like that. Of course, since KDE 4.3, each virtual desktop can have it's own activity, bringing the two in sync.
If applications and desktop were aware of activities, you could create an activity for each of the tasks you regularly work on. So if you often have to change a spreadsheet with prices, you create an activity for that: put a Folder View (or several) widget on the desktop, add a calculator and a todo-widget to keep track of what you still have to change. Maybe even an email folder widget showing the mails with questions regarding these prices spreadsheets!
As soon as somebody asks a question about prices, you switch to this activity. Fire up your spreadsheet application. It is aware of your activity so it shows recent price spreadsheets, not the recent list of inventory you were working on in another activity! Kopete, the chat application shows your colleague who knows all about prices, as she is the person you always chat with when working on this activity.
When you are finished, you go back to another activity, and once again all applications adjust their behavior to fit what you are doing.
The benefits of such an activity-based work flow go further than you might at first expect. It not only helps you find files and contact persons, but also helps in switching tasks itself. The human brain isn't very good at multi-tasking - it takes most people several minutes to get up to speed after switching tasks. Changing the 'environment' helps a lot in speeding this up, even if it's just on the screen. Compare it with getting in the mood for your holiday by packing your bag!
Of course, the above is mostly relevant to people working behind their computer in the office or at home. A gamer or a casual user would probably not use these activities much.
Note that the scenario described above is still years away from reality. Much of the basic infrastructure for this in KDE is in place but much is still left.
Frequently Asked Questions
The following is taken from a KDE forums post. Please feel free to add/remove/modify details if you have the time!
Q. What is the Nepomuk Semantic Desktop, and the Nepomuk File Indexer?
A. The Nepomuk Semantic Desktop is the foundation of the all the other modules of the Nepomuk infrastructure. It provides a way to organize, annotate and build relationships among the data (not only file name and content, but for example which applications used a certain file, or how it is tagged). A number of KDE applications and workspaces use this basic infrastructure to deliver features such as email tagging (KMail) or activity setup (Plasma).
On the other hand, the Nepomuk File Indexer is a system to index files so that they can be added to the main Nepomuk repository, a convenient way to use them within Nepomuk without adding any file manually. Also, applications such as Dolphin can then search for files basing on content, name, or other meta-data (e.g. tags) associated to indexed files. Such an indexer can also index non-text files, such as PDFs, by accessing the meta-data contained in these files (author, publication information, etc.). Some KDE components ship additional "analyzers" for more file types. Nepomuk can be fully functional without the use of the File Indexer, which is an additional (and optional) component.
Q. How can I disable the semantic desktop?
A. Most of the times, the easiest way is to disable file indexing, which is usually, among the Nepomuk components, the heavier in resource usage (although many optimizations have been included in the 4.7 release which reduce resource usage). This is done by unchecking "Enable Nepomuk File Indexer" in the "Desktop Search" section of System Settings. In case you want to turn off all semantic features, uncheck "Enable Nepomuk semantic desktop". Notice that this will turn off search in Dolphin as well.
Notice that with the latter option some programs who use Nepomuk for meta-data will offer reduced functionality: for example KMail will not be able to tag mail, or Plasma activities will not offer additional features such as icons, or program data information.
Q. Why do I have nepomukservicestub processes even though I've disabled Nepomuk?
A. It may be a bug. Please file a bug report with a complete description of your problem and the steps to trigger it.
Q. File indexing of PDF/some other file types doesn't work.
A. PDF indexing is a known issue and it's being tracked at this bug report. If you have issues with other files, open a bug, preferably adding a sample file that shows the problem.
Q. The program nepomukservicestub crashes at startup.
A. A large number of fixes for crashes has been fixed for the 4.7.2 release of the KDE Workspaces and Applications. If you encounter more, please file bugs report with detailed instructions on how to reproduce the problem, as sometimes the developers are unable to trigger them in their test setups.
Q. The virtuoso-t process hangs at 100% CPU.
A. Virtuoso-t is a key component of the Nepomuk infrastructure and in some occasions the commands sent by the other components end up taking too much time (hence showing the effect of 100% CPU). Sebastian Trueg (the lead developer of Nepomuk) has fixed most of these problems in 4.7.1 or newer.
Q. Sometimes Nepomuk consumes too much RAM.
A. Many of these problems have been fixed, in other cases however the developers are unable to reproduce the issues correctly. In this case, providing examples and test cases to bug reports increase the chances to get these bugs fixed.
Q. Nepomuk re-indexes files at startup.
A. This bug has been fixed in 4.7.0 versions. Now Nepomuk just "scans" for changes, without indexing anything.
Q. Nepomuk accesses the disk too much on startup.
A. In 4.7 and newer this problem has been lessened thanks to a throttling mechanism implemented in the file indexer.
Q. My Nepomuk database has been corrupted. How do I clean it?
A. In the extreme case your database is really corrupted and all other attempts have failed, you can delete the $KDEHOME/share/apps/nepomuk directory (where $KDEHOME is usually .kde or .kde4) while Nepomuk is not running. The database will be cleared, but you will also lose existing information such as tags, ratings and comments.
Sharing and privacy
There is one thing I need to touch on before pointing to other sources of information: sharing Nepomuk data. It'd be great if your tags, ratings and comments would be shared with others when you send them files. However, if you tagged a contact with a slightly embarrassing tag ('boring in bed') and send that persons contact information to a mutual friend you probably don't want that tag to be send as well...
This issue is of course being considered and an important subject of research by the Nepomuk researchers. For the time being, these privacy concerns, combined with technical challenges, are the reason Nepomuk context is private. Rest assured the Nepomuk team does all it can to make sure your privacy is respected.