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As more and more individuals have multiple computing devices, how to maintain the consistency of user files stored on multiple devices becomes a challenge. Cloud-based file synchronization services (FSS), e.g., Drop box and iFolder, are promising approaches. However, we haven't seen any public documents discussing the consistency protocol, which plays a vital role in any FSS. This paper focuses on maintaining the consistency of user files in FSS. We define a new file abstraction layer called user view, which describes all the files and their metadata in a sync-folder. We propose a user view based file consistency protocol with a conflicts resolution mechanism for FSS. The consistency protocol automatically keeps user files synchronized among the clients and server. We implement our consistency protocol into a prototype called Sync Views and set up a LAN-based environment to evaluate its feasibility and performance. We measured the time consumption of each synchronous operation on Sync Views and compared with iFolder, an open source implementation. The results show that our protocol works well during the file synchronization processes and outperforms iFolder when the synchronized file becomes very large.