Modern mobile application development requires a well thought-out plan for keeping user data in sync across various devices. This is a thorny problem with many gotchas and pitfalls, but users expect the feature and expect it to work well. For iOS and macOS, Apple provides a robust toolkit, called CloudKit API, which allows developers targeting Apple platforms to solve this synchronization problem. In this article, Toptal Software Engineer Paul Young demonstrate how to use CloudKit to keep a user’s data in sync between multiple clients.