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Semantic Search

The aim of this page is not to fully explain KDE's Semantic Search 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.

Baloo is the next generation of semantic search

From KDE Applications 4.13 onwards, the 'Baloo' file indexing and file search framework replaces Nepomuk. Read details on the changes for Applications 4.13 here. Semantic Search no longer uses a single, big database, but separate, specialized databases for each type of data. The new search databases are in $HOME/.local/share/baloo. If you upgraded to KDE Applications 4.13 from an earlier KDE release, you can delete $KDEHOME/share/apps/nepomuk.

簡短說明

As the Glossary mentions, Semantic Search is about classification, organisation and presentation of data. It is not an application, but a component which can be used by developers within applications.

Try out in Dolphin

For example, the Dolphin file manager makes use of Search. In KDE Applications versions prior to 4.13 Semantic Search must be enabled from System settings -> Desktop search. The information sidebar of Dolphin (Control -> Panels -> Information, or press F11) presents information extracted by Search about the selected file, and also allows you to assign tags, ratings and comments to files. This information is then stored and indexed by Search. You can then search for metadata using the navigation bar in Dolphin. Click to Find, or press Ctrl+F, and search for file names or file contents.

功能

KDE's Semantic Search 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, Search offers a second functionality: indexing the text of files. You can 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 the underlying technology, 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.

Indexing files

Search does not index every file on the hard drive. Its default configuration in most Linux distributions excludes some common patterns for backup files and configuration directories. You can change this in System Settings -> Desktop Search. Add folders to be excluded. If you want to turn off indexing of files entirely, just add your home folder there.

In System Settings you can also control whether Search indexes files on removable media such as USB drives and CD-ROMs. In KDE Applications 4.13 this is not available, removable media are not indexed. Future versions are planned to reintroduce this functionality.

例子

Let me try to explain what Semantic Search offers using two examples. These features are not available fully yet - the base is there but application developers need to integrate this in their applications.

關係（Relationship）

假設你2週前從你的某個好友手上收到一張照片。你隨後保存照片到電腦的某個地方。現在你如何找到那個文件哪？如果你不記得保存的位置，你就杯具了（人生是個茶几，你是個杯具）。

Now Semantic Search aims to help you. You know this file came from that friend of yours, your computer does not know. Search, however, can remember this relationship. Searching on the name of your friend will therefore 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 Search 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 Semantic Search 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. Semantic Search will be able to offer such smart results and order them on relevancy using relationship information.

語境（Context）

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 Search 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.

這裡有個例子講把語境認知（context awareness）帶到桌面上助你工作更有效率。

比如說你正在整理會議上摘的筆記。這時手機響了，某人問你要帶報價的電子表格，還要你根據客戶要求進行修改。再多來幾個打擾後，你會發現整個桌面全是文件和窗口...

如果能更好的組織這一切會非常棒，對嗎？

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 Plasma 4.3, each virtual desktop can have its own activity, bringing the two in sync.

如果程序和桌面認識活動，你可以根據經常工作的任務創建活動。所以如果你經常不得不修改帶報價的電子表格，你創建這樣一個活動：擺放一個「文件夾視圖」（或多個）部件到桌面上，添加一個計算器和一個todo 部件來記錄還需要修改的東西。可能需要一個「郵件文件夾」部件來顯示有關報價表問題的郵件！

一旦有人問報價問題，你就切換到這個活動。打開表格程序。表格程序認識你的活動，它的最近打開列表顯示的是報價表格，不是你在另外一個活動工作的存貨清單！ Kopete，這個聊天程序顯示著某個知道有關價格的同事，因為她是你經常在當前這個活動聊天的對象。

等你完成後，你回到另外一個活動，所有程序再一次調整它們的行為來適應你當前的工作。

這樣一個基於活動的工作流程的收益會遠遠超越你最初的期望。它不但能幫你找到文件和聯繫人，還有助於切換任務本身。人腦不擅長多任務-人類切換任務後需要花費幾分鐘來達到正常速度。改變'環境'能大大加快反應，即使只是屏幕上的。把它跟你假日收拾包裹時的情緒對比下！

當然，上面所述很大程度與電腦後頭工作的人是在辦公室還是在家有關。遊戲玩家或臨時用戶可能不太會用到這些活動。

The scenario described above is already partially implemented in the Activities but much work 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!

What is the Nepomuk Semantic Desktop, and the Strigi Desktop File Indexer?

Nepomuk and Strigi are technologies part of what delivers the abilities of the Semantic Search in KDE. Both are not used directly in the latest generation of KDE's Semantic Search (details), however their successors share much of their code and concepts. Semantic Search 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).

The file indexing allows applications such as Dolphin to search for files based 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.

Why do we need both Akonadi and Semantic Search? Aren't they doing the same thing?

In short, Akonadi provides a cache of PIM data like calendar items, contacts and email, which is used by applications like KMail and Korganizer but also the calendar build in Plasma. Semantic Search plugs in Akonadi to provide search functionality. How Baloo offers search is actually up to the application. In case of KDE PIM, Xapian is used to provide indexing and search.

How can I disable the semantic desktop?

File indexing can be disabled by adding the users' home folder to the System Settings -> Desktop Search -> Do not search in these locations list. The other functionality is part of the applications that use it and thus can't be disabled without crippling these applications. For example, to not have any search in KMail you'll have to simply remove KMail...

In versions of the KDE Applications before 4.13, Semantic Search would have components running separate from applications. This functionality could be disabled 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 Semantic Search 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.

Baloo/Semantic Search is eating 100% CPU! What do I do?

Just wait. Certain files are very hard or even impossible to Index. At the moment, this includes for example text files of over 50 megabyte. When Search finds these, it will try for a fixed time. When it fails, it will try to find out what file is broken and disable indexing it in the future. As it indexes files in batches of about 40, it has to find the problematic file by indexing that bunch in parts: first half/second half, index problematic half in pieces again, until the file is found. This can take up to 30 minutes of heavy cpu usage. Unfortunately, while Baloo will not start to index a new batch of 40 files while on battery power, it continues to determine the broken file while on battery. This behaviour has been fixed in in KDE Applications 4.13.1 (it will stop indexing immediately when the power cord is unplugged) and the time the search for each file can take has been reduced to about 10 minutes. The Semantic Search team is working on improving the indexing tools to handle more difficult files.

Why do I have nepomukservicestub processes even though I've disabled Nepomuk?

It may be a bug. Please file a bug report with a complete description of your problem and the steps to trigger it.

File indexing of PDF/some other file types doesn't work.

PDF indexing is a known issue and it's being tracked in bug #231936. If you have issues with other files, open a bug, preferably adding a sample file that shows the problem.

The program nepomukservicestub crashes at startup.

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.

The virtuoso-t process hangs at 100% CPU.

Virtuoso-t is a key component of the old Semantic Search 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).

Virtuoso is no longer used by Semantic Search starting the Applications 4.13 release.

Sometimes Nepomuk consumes too much RAM.

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.

Search accesses the disk too much on startup.

A throttling mechanism implemented in the file indexer, versions after KDE SC 4.8 should no longer have this issue.

My Search database has been corrupted. How do I clean it?

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 in your home directory) while Nepomuk is not running. The database will be cleared, but you will also lose existing information such as tags, ratings and comments.

Advanced troubleshooting

共享和隱私

在給出其它鏈接前我要指出的是：共享Nepomuk 數據。如果你的標記（tags），評分（ratings）和註釋（comments）能夠在你發送文件給其他人時同時共享給他會很棒。但是,要是你給一個聯繫人打上了“麻煩”的標籤('在床上煩人')，然後發送這個聯繫人的信息給一個共同的好友，但你又不希望這個標籤也一併發送過去...

這個問題當然在考慮之中，是Nepomuk 研究的一個重要課題。由於這些隱私顧慮，外加技術挑戰，所以暫時Nepomuk 內容是私有的。放心，Nepomuk 團隊盡其可能尊重你的隱私。

External links

The new Search technology (post KDE Applications 4.13):

The old Search technology: