Light Cone Consistency: Toward a Unified Theory of Consistency in Message-Passing Systems

TL;DR

The paper introduces Light Cone Consistency, a unified framework describing all known distributed system consistency models through three core constraints, revealing their interdependencies and fundamental limitations.

Contribution

It formalizes a comprehensive framework mapping 85 configurations of consistency models and uncovers the intrinsic entanglement of core distributed computing constraints.

Findings

Mapped 85 configurations covering all known models

Proved CAP, FLP, AFC constraints are minimal and independent

Identified the fundamental entanglement of consistency constraints

Abstract

Every distributed system -- databases, networks, postal services, CPU caches -- is a message-passing system. Every message-passing system is a growing causal log observed by a set of observers. We present Light Cone Consistency (LCC), a framework that describes every known consistency model as a configuration of three constraints on each observer's visible sub-DAG: causal closure $C(\mathrm{deps})$, fork resolution $O(\pi)$, and timeliness $R(\delta)$, plus an orthogonal return-value function $F$. We map 85 configurations, covering all 50+ named models from Viotti and Vukolic's taxonomy, with caveats for fork-based and probabilistic models. We show that three impossibility results of distributed computing -- CAP, FLP, and AFC -- each constrain exactly one pair of parameters, and prove they are minimal and independent. Our central result is the observation that these three constraints are fully entangled: violation of any one surface cascades to the other two, because restoring any parameter requires messages -- and those messages are subject to all three constraints. The three parameters and their pairwise impossibility surfaces form a fully connected triangle. Every distributed system must exit the triangle by relaxing at least one parameter. The triangle activates only when the system is in use: $C \neq \mathrm{none}$, $O \neq \mathrm{trivial}$, or $R \neq \mathrm{absent}$ each introduces a constraint that exposes the system to the surfaces. A system that demands nothing -- or writes far slower than its propagation delay -- is trivially linearizable. We identify open problems including a conjectured fourth surface (log locality), undiscovered constraints, and the universality of the safety-liveness fork as the consequence of crossing any boundary.