…and how we rewrote the heart of sync with confidence. Executing a full rewrite of the Dropbox sync engine was pretty daunting. (Read more about our goals and how we made the decision in our previous post here.) Doing so meant taking the engine that powers Dropbox on hundreds of millions of user’s machines and swapping it out mid-flight. To pull this off, we knew we would need a serious investment in automated testing. Our testing strategy gave us confidence that we were on the right track throu...