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jabley : resilience   51

Recovering Shared Objects Without Stable Storage
This paper considers the problem of building fault-tolerant shared objects when processes can crash
and recover but lose their persistent state on recovery. This Diskless Crash-Recovery (DCR) model
matches the way many long-lived systems are built. We show that it presents new challenges, as
operations that are recorded at a quorum may not persist after some of the processes in that quorum
crash and then recover.
To address this problem, we introduce the notion of crash-consistent quorums, where no recoveries
happen during the quorum responses. We show that relying on crash-consistent quorums enables
a recovery procedure that can recover all operations that successfully finished. Crash-consistent quorums
can be easily identified using a mechanism we term the crash vector, which tracks the causal
relationship between crashes, recoveries, and other operations.
We apply crash-consistent quorums and crash vectors to build two storage primitives. We give
a new algorithm for multi-writer, multi-reader atomic registers in the DCR model that guarantees
safety under all conditions and termination under a natural condition. It improves on the best prior
protocol for this problem by requiring fewer rounds, fewer nodes to participate in the quorum, and
a less restrictive liveness condition. We also present a more efficient single-writer, single-reader
atomic set—a virtual stable storage abstraction. It can be used to lift any existing algorithm from
the traditional Crash-Recovery model to the DCR model. We examine a specific application, state
machine replication, and show that existing diskless protocols can violate their correctness guarantees,
while ours offers a general and correct solution.
paper  filetype:pdf  comp-sci  distributedsystems  consensus  resilience  crash 
october 2017 by jabley

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