Consensus as Collapse Policy: Communication Evidence, Horizons, and Prefix Decisions

Abstract

Consensus protocols are usually specified by their terminal artifact: a decided value, replicated log, or finalized prefix. This output-first view hides the communication-derived evidence that makes such artifacts safe. This paper makes that carrier explicit: distributed execution is read as an order-2 evidence state induced by communication, while classical consensus outputs are order-1 projections of that state. Under this view, consensus protocols can be compared as collapse policies. A protocol specifies which evidence is legitimate, which finite horizon it inspects, when it projects communication evidence into a value or prefix, and how it repairs or defers collapse when the visible evidence is insufficient. The impossibility lineage supports the same distinction. FLP is not a statement that communication cannot accumulate structure; it constrains deterministic guaranteed collapse to a terminal decision under full asynchrony with one crash failure. Set agreement then exposes the width of the output carrier, and topological distributed computing asks when a history/view carrier admits a structure-preserving map to an output carrier. The contribution is not a new impossibility theorem or a replacement for protocol-specific proofs, but a denotational specification framework: consensus is collapse under evidence.