Practical web-based delta synchronization for cloud storage services

Delta synchronization (sync) is known to be crucial for network-level efficiency of cloud storage services (e.g., Dropbox). Practical delta sync techniques are, however, only available for PC clients and mobile apps, but not web browsers—the most pervasive and OS-independent access method. To understand obstacles of web-based delta sync, we implemented a traditional delta sync solution (named WebRsync) for web browsers using JavaScript, and find that WebRsync severely suffers from the inefficiency of JavaScript execution inside web browsers, thus leading to frequent stagnation and even crashing. Given that the computation burden on the web browser mainly stems from data chunk search and comparison, we reverse the traditional delta sync approach by lifting all chunk search and comparison operations from the client side into the server side. Inevitably, this brings enormous computation overhead to the servers. Hence, we further leverage locality matching and a more efficient checksum to reduce the overhead. The resulting solution (called WebR2sync+) outpaces WebRsync by an order of magnitude, and it is able to simultaneously support ∼7300 web clients’ delta sync using an ordinary VM server based on a Dropbox-like system architecture.