More and more web and mobile developers are using local storage to ensure good response times and that applications continue to work in the absence of a network connection. But how do you sync local storage with the backend? How do you resolve conflicts? In this talk, I present to you an open source framework (Amplify DataStore) that allows you to develop modern web and mobile applications that answer these questions with a minimum of code. I will demo the mechanisms with two Swift-based application : one on iOS and one on macOS.