Chapter 5: Realizing a Metadata Framework 133 5.2.2 Synchronization Synchronization will be one of the most prominent challenges in devel- oping robust content management systems, even more so when mobile devices are involved. Synchronization itself is a vast domain, discussed in detail in, e.g., Bernstein et al. (1987); Adjeroh and Nwosu (1997); and Lin (2002). In this section, we do not present any detailed techni- cal solutions, which would be beyond the scope of this book, but confi ne ourselves to discussing the synchronization-related challenges that the developer of a personal content management system will eventually have to face and solve, often on a case-by-case basis. Currently, personal content is stored and used in a myriad of dif- ferent devices that may or may not connect to each other. Consider the implications to the user: how to create a shared family photo album, when each family member has a digital camera of their own and transfers the images to their own laptop computer? How to syn- chronize favourite playlists between a home media server and a por- table music player? Sometimes an automatic synchronization based on, for instance, the play count, can be applied, whereas sometimes the user may prefer to synchronize manually created playlists. Synchronization implies keeping track of changes made in the data store, such as additions, deletions, and modifi cations. Occasional con- fl icts need to be addressed, for instance when the same object has been modifi ed at both ends. In those cases, the system needs to resolve the confl ict, either by asking the user explicitly for instructions or by applying predefi ned rules. A typical example of explicit synchronization, from a consumer point of view, is switching from an old phone to a new one. The need to transfer all locally stored personal content to the new device is obvious and essential. Typically, there are two options available. Either the contents are fi rst transferred to a PC, which is then used to copy the contents further to the other device. Another solution is a direct point-to-point connection, where the content is transferred directly between devices. What is perhaps the most challenging aspect of synchronization related to personal content is keeping the user from having to deal with it. Deciding once or twice which version of a photo to carry in a mobile device could be considered fi ne, especially if the user interface for such an operation is properly designed. However, on a general basis the user should be able to happily forget the problem of synchro- nization altogether. This is a goal not trivial to achieve. One of the desirable features of a connected mobile phone is a synchronized clock, since the time of day is provided by the operator.