148 Personal Content Experience: Managing Digital Life in the Mobile Age be triggered by the system, such as receiving a message or deleting a fi le, while many events depend on the applications being properly triggered by the framework, for instance viewing an object. Events are used to create interaction history and usage logs. The simplest forms of using logs include creating dynamically updated and adapted playlists, such as the most often played songs. The logs can also be used by more advanced harvesters to infer totally new type of metadata, such as when the user is alone she likes to listen to classical music, but with her friends she prefers rock. Events are also useful when making more complex searches. Another way to use usage history is to create procedural metadata. Procedural metadata describes all actions and changes that have been applied to the content. For example, typical actions in digital photog- raphy include colour corrections, cropping, applying fi lters like sharp- ening, etc. All such actions can be stored as procedural metadata, for example, making undoing changes easier. It also makes synchronizing changed content simpler, since it is enough to send only the procedural metadata, i.e., steps needed to recreate the new content from the original content. All events have some information about them, that is, once again, meta-metadata. They all have timestamps to store the date and time when the event occurred, and one or two actors. Actors are used to store the application creating the event, and the person (a contact in the phonebook) that was involved. For example, when sending an e-mail, the messaging application is one actor, and the recipient the other. Obviously, some events only have one actor. Applications can also subscribe to certain events, or to events con- cerning certain objects or object types. For example, an application may subscribe to change notifi cations. When a metadata attribute of the content object is modifi ed, the subscribed applications are informed. This allows a media player application to update its user interface when a song is played, regardless of the application that was used to play the song. 5.4 From Content Management to Metadata Management We have discussed some of the key issues in content management in a distributed mobile environment, and presented a classifi cation scheme for different types of metadata. We pointed out that mobile personal content is by its very nature distributed across many devices.