23 Efficient User Data Synchronization Stančin, Sara; Đorđević, Nemanja; and Tomažič, Sašo Abstract User data synchronization feature of computing user devices presents an essential role in the vision of pervasive networks. Typical user environment includes several devices - a work desktop computer, home laptop, and mobile device for example. Through each of these devices a user can access different data records. The aim of synchronization is to enable user access to a particular data record through a device other than the one on which the record was whether created whether last modified. Users can read, change, or delete a specific data record by accessing any of its available replications. Such independent access can embrace any type of file or web content in applications like email, file content distribution, and personal information manager. The synchronization environment can include numerous devices with different processing, memory and connection capabilities. Further on, devices involved operate in two possible modes, the online and the offline. As long as a device is not connected to a particular network new data record modifications are not consistent with corresponding data records on other devices in that network. As soon as a device connects to a network, the synchronization process needs to determine and reconcile possible differences between documents replicated on multiple devices. Due to multi device involvement in data updating, different synchronization conflicts may occur. Synchronization protocols in force implement different, more or less complex mechanisms which dictate system behaviour if synchronization conflicts arise. The efficiency of these mechanisms varies. A new paradigm for achieving efficient user data synchronization is presented. Documents are associated with globally unique identification marks while document replication validity is achieved using timestamp values. The proposed paradigm enables reliable synchronization of user data, while the synchronization process remains simple and efficient from the computational point of view. The synchronization procedure according to the new paradigm is efficient even in a dynamic environment which involves numerous devices. KEYWORDS User data synchronization, Data reconciliation, Unique global document identification 1. I NTRODUCTION 1.1. Fundamentals of user data synchronization Users access their data through different computing devices like mobile telephones, laptops, desktops, and handheld computers. Each single user device stores a specific user data subset. Depending on the used device, users can therefore access different subsets of data. Recently, more and more of these devices, belonging to the same user, communicate and collaborate among themselves in such information delivery. Access to user data that has previously been enabled through one particular user device is being enabled through other user devices as well. Users can read, change, or delete a specific data record by accessing any of its available replications. Such independent access can embrace any type of file or web content in applications like email, file content distribution, and personal information manager. Multiple copies of data records that represent the same data on different devices must be the same in content. This leads us to the problem of user data record synchronization. The synchronization process needs to determine the differences between user data replicated on multiple devices and thereafter reconcile these differences as illustrated in Figure 1.1. Manuscript received May, 11, 2009. Sara Stančin is with the University of Lubljana, Slovenia Sašo Tomažič is with the University of Ljubljana, Slovenia Nemanja Đorđević is with the University of Belgrade, Serbia