Practical Demystification of CRDTs

Practical Demystification of CRDTs

This talk was presented at Amsterdam.Scala meetup ( on 27th August 2015 by Nami Nasserazad (, Didier Liauw and Dmitry Ivanov (

In a connected world, synchronising mutable information between different devices with different clock precision can be a difficult problem. A piece of data may have many out-of-sync replicas but all of those should eventually be in a consistent state. For example, TomTom ( users, having personal navigation devices, smartphones, MyDrive ( website accounts, expect their navigation information be synchronised properly even in the occasional absence of network connection.
Conflict-free Replicated Data Types (CRDTs) provide robust data structures to achieve proper synchronisation in an unreliable network of devices. They enable the conflict resolution being done locally at the data type level while guaranteeing the eventual consistency between replicas.
In this talk, in addition to an introduction to CRDT, our main focus is on a special type of CRDT-set called OUR-set (Observed, Updated, Removed) which we created to extend known CRDT-sets with update functionality. We will explain the advantages of this data structure to solve many synchronisation problems as well as its limitations. We also show how a basic implementation of OUR-set CRDT in Scala and its counterpart in Java looks like and enumerate a set of subtle considerations which should be taken into account.


Dmitry Ivanov

August 27, 2015