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.
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.