When Is Recoverable Consensus Harder Than Consensus?

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. CCS CONCEPTS• Theory of computation → Concurrent algorithms; Shared memory algorithms; Distributed computing models; • Computer systems organization → Dependable and fault-tolerant systems and networks. KEYWORDSRecoverable consensus; consensus hierarchy; shared memory; readable objects; asynchronous; wait-free; non-volatile memory; crash and recovery 1 Alternatively, it could execute a recovery function. Our results hold either way. We use the simpler assumption of re-starting upon recovery to prove our results.