Realm Sync allows apps to have hundreds of thousands of concurrent or offline users operating on copies of the same data on their mobile devices and ensures that all devices will converge in a consistent state. Only one year old, Realm Sync is already supporting over 2MM end users. This talk will explore the MongoDB features that allowed us to build a performant scalable system, including flexible document models, change streams, and multi-document transactional operations. We will conclude with the learnings from the original Partition-Based Sync, and present Flexible Sync, a new model released in Jan 2022 that iterates on these learnings. And we’ll discuss the benefits of the new approach and the features of MDB 5.0, such as snapshotting, that enable it.