Resilient Conflict-free Replicated Data Types without Atomic Broadcast

In a distributed system, applications can perform both reads and updates without costly synchronous network round-trips by using Conflict-free Replicated Data Types (CRDTs). Most CRDTs are based on some variant of atomic broadcast, as that enables them to support causal dependencies between updates of multiple objects. However, the overhead of this atomic broadcast is unnecessary in systems handling only independent CRDT objects. We identified a set of use cases for tracking resource usage where there is a need for a replication mechanism with less complexity and network usage as compared to using atomic broadcast. In this paper, we present the design of such a replication protocol that efficiently leverages the commutativity of CRDTs. The proposed protocol CReDiT (CRDT enhanced with intelligence) uses up to four communication steps per update, but these steps can be batched as needed. It uses network resources only when updates need to be communicated. Furthermore, it is less sensit ive to server failures than current state-of-the-art solutions as other nodes can use new values already after the first communication step, instead of after two or more.