Arbitration-Free Consistency is Available (and Vice Versa)

2025-10-24

AI Summary

Fundamental trade-offs between availability and consistency in distributed storage are often oversimplified in existing work—particularly through extreme models like CAP—lacking precise characterization of how object semantics interact with diverse consistency models. Method: We propose a unified semantic framework that integrates operational semantics with multi-level consistency constraints—including causal consistency, sequential consistency, snapshot isolation, and SQL transactional semantics—covering key data objects such as key-value stores, counters, sets, CRDTs, and transactional databases. Contribution/Results: We introduce the Arbitration-Free Consistency (AFC) theorem, establishing that an object admits highly available implementations if and only if its visibility or read dependencies do not require total-order arbitration. This theorem unifies and generalizes classical results including CAP, providing the first formal, object-centric criterion for designing highly available distributed systems.

Technology Category

• Constraint Satisfaction and Optimization: Distributed CSP/Optimization
• Search and Optimization: Distributed Search
• Multiagent Systems: Agreement, Argumentation & Negotiation

Application Category

• Security and Privacy: Data transparency and provenance
• Semantics and Knowledge: Provenance, trust, security and privacy, and ethical issues in managing semantic data
• Systems and Infrastructure for Web, Mobile and WoT: Web performance, measurement, and characterization

Abstract

The fundamental tension between emph{availability} and emph{consistency} shapes the design of distributed storage systems. Classical results capture extreme points of this trade-off: the CAP theorem shows that strong models like linearizability preclude availability under partitions, while weak models like causal consistency remain implementable without coordination. These theorems apply to simple read-write interfaces, leaving open a precise explanation of the combinations of object semantics and consistency models that admit available implementations. This paper develops a general semantic framework in which storage specifications combine operation semantics and consistency models. The framework encompasses a broad range of objects (key-value stores, counters, sets, CRDTs, and transactional databases) and consistency models (from causal consistency and sequential consistency to snapshot isolation and transactional and non-transactional SQL). Within this framework, we prove the emph{Arbitration-Free Consistency} (AFC) theorem, showing that an object specification within a consistency model admits an available implementation if and only if it is emph{arbitration-free}, that is, it does not require a total arbitration order to resolve visibility or read dependencies. The AFC theorem unifies and generalizes previous results, revealing arbitration-freedom as the fundamental property that delineates coordination-free consistency from inherently synchronized behavior.

Problem

• Identifies arbitration-free consistency as enabling available distributed storage implementations
• Develops semantic framework combining object semantics with consistency models
• Proves arbitration-freedom determines coordination-free consistency across storage systems

Innovation

• Framework combines operation semantics with consistency models
• Arbitration-free consistency enables available implementations
• Theorem generalizes coordination-free distributed storage design