RMWPaxos: Fault-Tolerant In-Place Consensus Sequences

Building consensus sequences based on distributed, fault-tolerant consensus, as used for replicated state machines, typically requires a separate distributed state for every new consensus instance. Allocating and maintaining this state causes significant overhead. In particular, freeing the distributed, outdated states in a fault-tolerant way is not trivial and adds further complexity and cost to the system. In this article, we propose an extension to the single-decree Paxos protocol that can learn a sequence of consensus decisions `in-place', i.e., with a single set of distributed states. Our protocol does not require dynamic log structures and hence has no need for distributed log pruning, snapshotting, compaction, or dynamic resource allocation. The protocol builds a fault-tolerant atomic register that supports arbitrary read-modify-write operations. We use the concept of consistent quorums to detect whether the previous consensus still needs to be consolidated or is already finished so that the next consensus value can be safely proposed. Reading a consolidated consensus is done without state modifications and is thereby free of concurrency control and demand for serialisation. A proposer that is not interrupted reaches agreement on consecutive consensus decisions within a single message round-trip per decision by preparing the acceptors eagerly with the previous request.