Process-Commutative Distributed Objects: From Cryptocurrencies to Byzantine-Fault-Tolerant CRDTs

TL;DR

This paper introduces a new class of distributed objects that require only FIFO order per process, balancing between BFT CRDTs and blockchain ledgers, with formal guarantees and practical implementations.

Contribution

It formally characterizes process-commutative distributed objects using Mazurkiewicz traces and provides a generic algorithm for crash- and Byzantine-tolerant implementations.

Findings

Formalization of FIFO-based distributed objects with SEC and PC guarantees

A generic algorithm for crash- and Byzantine-tolerant implementations

Practical instances including money transfer and Petri nets

Abstract

This paper explores the territory that lies between best-effort Byzantine-Fault-Tolerant Conflict-free Replicated Data Types (BFT CRDTs) and totally ordered distributed ledgers, such as those implemented by Blockchains. It formally characterizes a novel class of distributed objects that only requires a First In First Out (FIFO) order on the object operations from each process (taken individually). The formalization leverages Mazurkiewicz traces to define legal sequences of operations and ensure both Strong Eventual Consistency (SEC) and Pipleline Consistency (PC). The paper presents a generic algorithm that implements this novel class of distributed objects both in a crash- and Byzantine setting. It also illustrates the practical interest of the proposed approach using four instances of this class of objects, namely money transfer, Petri nets, multi-sets, and concurrent work stealing dequeues.