What is Tracker?
Tracker is an efficient search engine and distributed database for desktop, embedded and mobile.
It is a middleware component aimed at desktop application developers who
want their apps to browse and search user content. It's not designed to be
used directly by desktop users, but it provides a commandline tool named
tracker3 for the adventurous.
Tracker allows your application to instantly perform full-text searches across all documents. This feature is used by the 'search' bar in GNOME Files, for example.
This is achieved by indexing the user's home directory in the background.
Tracker also allows your application to query and list content that the user has stored. For example, GNOME Music displays all the music files that are found by Tracker. This means that GNOME Music doesn't need to maintain a database of its own.
Apps can also store data in their own Tracker SPARQL database. If you want, this data can be published as a D-Bus service to enable distributed queries.
Tracker 3 is divided into two subprojects:
- Tracker SPARQL, a lightweight database library built around SQLite.
- Tracker Miners, a set of daemons which crawl the filesystem and extract metadata from many types of file to provide a system-wide search service.
Who uses Tracker?
Tracker is a core dependency of the GNOME desktop.
Gnome's Shell doesn't use Tracker directly. Instead, the search results in the shell are provided by multiple apps on the system, using the SearchProvider API. Some of these apps use Tracker internally, so they return search results provided by Tracker to gnome-shell.
The following GNOME applications use Tracker:
- GNOME Books (uses Tracker Miner FS to find ebooks)
- GNOME Boxes (uses Tracker Miner FS to find VM images)
- GNOME Documents (uses Tracker Miner FS to find documents)
- GNOME Files (uses Tracker Miner FS for full-text search within files)
- GNOME Games (uses Tracker Miner FS to find games)
- GNOME Music (uses Tracker Miner FS to find music and store playlist data)
- GNOME Notes (uses Tracker SPARQL to store notes)
- GNOME Photos (uses Tracker Miner FS to find photos and Tracker SPARQL to store album data)
- GNOME Usage (uses Tracker Miner FS to measure disk usage)
- GNOME Videos (uses Tracker Miner FS to find video content)
- Health (uses Tracker SPARQL to store health data)
Although Tracker is able to store contacts and calendar entries, GNOME uses Evolution Data Server for this.
The file chooser dialog supplied by GTK has a search interface. There's a Tracker backend for this.
Grilo is a library for finding and fetching media content from many different sources. It uses Tracker Miner FS to browse and search local media content.
Rygel is a home media solution that serves content over UPnP. It uses Tracker Miner FS to find your media files.
Xapian provides similar functionality to Tracker Miner FS. It focuses more on scalability and less on having a lightweight footprint. Unlike Tracker, it doesn't support SPARQL or provide a Linked Data endpoint.
Baloo is a metadata and search framework by KDE, implemented using Xapian.
Recoll is a cross-platform desktop search application powered by Xapian.
Apache Lucene + Solr is a search engine which targets very large-scale workloads. It has a much heavier footprint compared to Tracker.
See the Wikipedia article on Desktop search for more information.