152 Personal Content Experience: Managing Digital Life in the Mobile Age timestamps are updated, and perhaps also the ratings or some other metadata is modifi ed. If metadata is embedded inside the content binary, that binary is also changed and fl agged as needing backup. Consequently, the whole binary fi le is transferred in the backup process. When metadata is stored in the database, only the modifi ed metadata rows in a table need to be backed up, making the whole process faster and easier. There are also some negative consequences to using a database. In particular, separating metadata from the content objects it describes may result in them getting out of synchronization. Consider, for instance, transferring the latest hit movie from Eddie’s loot to the home PC in order to watch it in your home theatre. What happens to metadata that is stored in the database in your mobile device? Can you move the metadata to the PC, too? One solution could be embedding whatever metadata there is in the database into the movie fi le upon transfer. However, some meta- data, such as relationships, are hard if not impossible to embed. Another option is to export the metadata into a separate metadata fi le (a metadata manifest, see section 5.5), and then also transfer it to a PC along with the content object. In this case, even though all meta- data is transferred, relationships to other content objects in the mobile device may still be broken and, obviously, a PC needs to understand the exported metadata format. Other synchronization problems become evident when you modify some metadata in the PC. For example, the play count of a song should be increased no matter where or how you watch the movie. Another likely scenario is that new metadata is added, and existing metadata updated, due to the easier text input methods that the PC provides. Of course, this new metadata needs to be moved back to the mobile device. Worse problems arise if the metadata in the mobile device is modi- fi ed while it is also being altered in the PC. A typical case is listening to the same song in a PC and a mobile device, when the play count needs to be updated both ways. Solving the metadata synchronization is a diffi cult problem, and there is no automatic solution that is capable of performing fl awlessly in all possible cases. There are always situa- tions where the user needs to choose how to merge confl icting meta- data or should some metadata be discarded. However, synchronization is such a common problem that many functional solutions exist, for example, OMA SyncML13 and other protocols that are designed espe- cially for such problems. 13 http://www.openmobilealliance.org/tech/wg_committees/ds.html