The Mysteries of Dropbox

File synchronization services like Dropbox, Google Drive and OneDrive are increasingly popular—hundreds of millions of people entrust their precious data to them every day. But they are complex distributed systems, and their behaviour is subtler than you might think, especially when files are changed on more than one machine. What can we rely on when we use these applications—and how can they be tested effectively? I’ll describe how we used QuickCheck to characterize their behaviour and generate tests, using Dropbox as the main example. And there may be a few surprises…