Que Sera Consensus: Simple Asynchronous Agreement with Private Coins and Threshold Logical Clocks

Abstract

It is commonly held that asynchronous consensus is much more complex, difficult, and costly than partially-synchronous algorithms, especially without using common coins. This paper challenges that conventional wisdom with que sera consensus QSC, an approach to consensus that cleanly decomposes the agreement problem from that of network asynchrony. QSC uses only private coins and reaches consensus in $O(1)$ expected communication rounds. It relies on "lock-step" synchronous broadcast, but can run atop a threshold logical clock (TLC) algorithm to time and pace partially-reliable communication atop an underlying asynchronous network. This combination is arguably simpler than partially-synchronous consensus approaches like (Multi-)Paxos or Raft with leader election, and is more robust to slow leaders or targeted network denial-of-service attacks. The simplest formulations of QSC atop TLC incur expected $O(n^2)$ messages and $O(n^4)$ bits per agreement, or $O(n^3)$ bits with straightforward optimizations. An on-demand implementation, in which clients act as "natural leaders" to execute the protocol atop stateful servers that merely implement passive key-value stores, can achieve $O(n^2)$ expected communication bits per client-driven agreement.