Nepomuk Indexing Architecture
Over the last 6 months, while working for Blue Systems, Nepomuk has undergone a number of changes. The most public and noticeable change has been the major refactoring of the file indexer. One large part of this has been the migration from Strigi. The other large part is the introduction of 2 phase indexing.
With 4.9 and earlier, the file indexing service used to just have one queue, whose speed could be controlled. This queue was filled on startup by comparing the mtime of the file with the one stored in the database. This would involve scanning through all the indexed folders. Once the scan was complete it would also listen to the file watcher to be notified when a file is modified or created.
This architecture had some shortcomings -
- Indexing each file is a time consuming process, and it involves extracting and pushing large amounts of data in Nepomuk.
- Since this process was slow and we did not want to annoy the users, artificial delays were introduced which were changed based on if the user is idle.
- The entire indexing process was suspended when on battery
- Faulty files which cannot be indexed do not have any information stored, and could not even be searched by filename
With this new release, we have split the indexing into 2 parts - Basic Indexing and File Indexing. The basic indexing just extracts the stat information and mimetype of the file. Whereas, the file indexing actually extracts data from the file.
This basic indexing is always enabled, and is very fast. It can process around 10-20 files per second. Also, it consumes very little cpu. Extracting this basic information first allows us to search on the basis of type, file and enabled the timeline kioslave to work properly.
The file indexing is the relatively heavy process that is only run when the user is idle, by default.
This two phase architecture allows us to still index all the files, while providing a relatively light burden to the user. It also allows us to provide finer control than a simple on/off switch. For example - Now when on battery, file indexing is disabled, but the simple indexing still continues.
This new approach will also allow us to provide more user feedback in future releases, such as an indexing progress bar.
The new architecture is much faster and more resilient to abnormal files and faulty plugins. It tries to save the basic information first, so that one can easily answer simple queries. The full file information is stored later, when the user is idle.