When Is Recoverable Consensus Harder Than Consensus?

TL;DR

This paper investigates the complexity of solving recoverable consensus with shared objects in crash-recovery systems, comparing it to standard consensus, and characterizes which object types can solve these problems.

Contribution

It provides a characterization of readable object types that can solve recoverable consensus and compares the solvability bounds to standard consensus.

Findings

Readable object types can solve consensus for more processes than recoverable consensus, but only slightly more.

The paper offers a formal characterization of object types capable of solving recoverable consensus.

Recoverable consensus is generally harder than standard consensus in crash-recovery models.

Abstract

We study the ability of different shared object types to solve recoverable consensus using non-volatile shared memory in a system with crashes and recoveries. In particular, we compare the difficulty of solving recoverable consensus to the difficulty of solving the standard wait-free consensus problem in a system with halting failures. We focus on the model where individual processes may crash and recover and the large class of object types that are equipped with a read operation. We characterize the readable object types that can solve recoverable consensus among a given number of processes. Using this characterization, we show that the number of processes that can solve consensus using a readable type can be larger than the number of processes that can solve recoverable consensus using that type, but only slightly larger.