Consensus as Collapse Policy: Communication Evidence, Horizons, and Prefix Decisions
2026-07-11 • Distributed, Parallel, and Cluster Computing
Distributed, Parallel, and Cluster Computing
AI summaryⓘ
The authors explain consensus protocols by focusing on the communication that produces evidence supporting decisions, rather than just the final decided value or log. They describe this evidence as a structured state from which typical consensus outputs are simplified views. Using this perspective, they compare protocols by how they handle and interpret such evidence, clarifying classical impossibility results like FLP in terms of when and how decisions can reliably form. Their work provides a new way to understand consensus as a process of 'collapsing' communication evidence into a decision, rather than introducing new impossibility results or protocol proofs.
consensus protocolsdistributed executioncommunication evidenceFLP impossibilityset agreementdenotational semanticscollapse policiesasynchronycrash failurestopological distributed computing
Authors
Tomislav Grospić
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.