  Online educational systems running on smart devices have the advantage of
allowing users to learn online regardless of the location of the users. In
particular, data synchronization enables users to cooperate on contents in real
time anywhere by sharing their files via cloud storage. However, users cannot
collaborate by simultaneously modifying files that are shared with each other.
In this paper, we propose a content collaboration method and a history
management technique that are based on distributed system structure and can
synchronize data shared in the cloud for multiple users and multiple devices.
