Keep CALM and CRDT On

TL;DR

This paper proposes an extension to CRDTs that incorporates a query model based on the CALM Theorem, enabling safe, coordination-free interactions with replicated data for more reliable distributed applications.

Contribution

It introduces a query model for CRDTs that ensures safe, coordination-free queries, enhancing the reliability and usability of CRDT-based distributed systems.

Findings

Extended CRDTs with a query model for safe interactions

Applied CALM Theorem to reason about query safety

Laid out a development agenda for CRDT data stores

Abstract

Despite decades of research and practical experience, developers have few tools for programming reliable distributed applications without resorting to expensive coordination techniques. Conflict-free replicated datatypes (CRDTs) are a promising line of work that enable coordination-free replication and offer certain eventual consistency guarantees in a relatively simple object-oriented API. Yet CRDT guarantees extend only to data updates; observations of CRDT state are unconstrained and unsafe. We propose an agenda that embraces the simplicity of CRDTs, but provides richer, more uniform guarantees. We extend CRDTs with a query model that reasons about which queries are safe without coordination by applying monotonicity results from the CALM Theorem, and lay out a larger agenda for developing CRDT data stores that let developers safely and efficiently interact with replicated application state.