The read/write protocol complex is collapsible

TL;DR

This paper proves that the complex structure of read/write protocols in distributed systems is collapsible, linking the topological properties of these protocols to their computational capabilities.

Contribution

It demonstrates that the read/write iterated protocol complex is collapsible, extending the topological understanding of distributed protocols beyond snapshot models.

Findings

Read/write protocol complex is collapsible.

Collapse operations correspond to wait-free snapshot implementations.

Supports the topological approach to distributed computability.

Abstract

The celebrated \emph{asynchronous computability theorem} provides a characterization of the class of decision tasks that can be solved in a wait-free manner by asynchronous processes that communicate by writing and taking atomic snapshots of a shared memory. Several variations of the model have been proposed (immediate snapshots and iterated immediate snapshots), all equivalent for wait-free solution of decision tasks, in spite of the fact that the protocol complexes that arise from the different models are structurally distinct. The topological and combinatorial properties of these snapshot protocol complexes have been studied in detail, providing explanations for why the asynchronous computability theorem holds in all the models. In reality concurrent systems do not provide processes with snapshot operations. Instead, snapshots are implemented (by a wait-free protocol) using operations that write and read individual shared memory locations. Thus, read/write protocols are also computationally equivalent to snapshot protocols. However, the structure of the read/write protocol complex has not been studied. In this paper we show that the read/write iterated protocol complex is collapsible (and hence contractible). Furthermore, we show that a distributed protocol that wait-free implements atomic snapshots in effect is performing the collapses.