CRDTs let replicas diverge and still converge without locks

Multi-user editing faces a fundamental problem: when two people edit the same document offline or on different connections, both copies change, and someone must reconcile them. Traditional approaches either lock parts of the document — blocking collaboration — or use operational transformation, whose state combinations explode and are famously hard to prove correct.

In 2011, Marc Shapiro and colleagues proposed conflict-free replicated data types (CRDTs): data structures engineered so that concurrent updates commute. If every update commutes and merges are associative, replicas that diverged can be merged in any order and still converge to one identical state, with no central coordinator and no conflict dialog. Registers, counters, sets and text editors can all be built this way.

The theory hardened over the next years: the 2018 OpSets paper by Martin Kleppmann and colleagues built an executable framework that mechanically verifies replicated-datatype algorithms, exposed a correctness property for collaborative text that several published algorithms violated, and specified an atomic move operation previously thought impossible without locking.

Production systems adopted the idea. Figma's multiplayer engine is explicitly inspired by CRDT research: its server-authoritative, per-property design works like a last-writer-wins register, chosen after the company rejected operational transformation as unnecessarily complex. Redis, Riak and Cosmos DB ship CRDT-based data types, and Apple's Notes uses CRDTs to sync offline edits between devices.