Light Cone Consistency: Toward a Unified Theory of Consistency in Message-Passing Systems
For distributed systems researchers and practitioners, this provides a unified theoretical framework that explains and relates all consistency models and fundamental impossibility results.
The paper introduces Light Cone Consistency (LCC), a framework that unifies all known consistency models in message-passing systems using three parameters (causal closure, fork resolution, timeliness) and an orthogonal return-value function. It maps 85 configurations covering 50+ named models and shows that three classic impossibility results (CAP, FLP, AFC) each constrain exactly one pair of these parameters, forming a fully connected triangle that every distributed system must exit by relaxing at least one parameter.
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 $O(π)$, and timeliness $R(δ)$, 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.