Practically-Self-Stabilizing Vector Clocks in the Absence of Execution Fairness

TL;DR

This paper introduces the first stabilizing vector clock algorithm for asynchronous crash-prone systems that can recover wait-free after transient faults, ensuring safety and bounded recovery time even without execution fairness.

Contribution

It presents a novel stabilizing vector clock algorithm capable of wait-free recovery from transient faults in asynchronous crash-prone systems, addressing safety, size bounds, and counter overflow issues.

Findings

Never violates safety in absence of transient faults

Provides bounded time recovery during fair executions

Guarantees bounded violations without execution fairness

Abstract

Vector clock algorithms are basic wait-free building blocks that facilitate causal ordering of events. As wait-free algorithms, they are guaranteed to complete their operations within a finite number of steps. Stabilizing algorithms allow the system to recover after the occurrence of transient faults, such as soft errors and arbitrary violations of the assumptions according to which the system was designed to behave. We present the first, to the best of our knowledge, stabilizing vector clock algorithm for asynchronous crash-prone message-passing systems that can recover in a wait-free manner after the occurrence of transient faults. In these settings, it is challenging to demonstrate a finite and wait-free recovery from (communication and crash failures as well as) transient faults, bound the message and storage sizes, deal with the removal of all stale information without blocking, and deal with counter overflow events (which occur at different network nodes concurrently). We present an algorithm that never violates safety in the absence of transient faults and provides bounded time recovery during fair executions that follow the last transient fault. The novelty is that in the absence of execution fairness, the algorithm guarantees a bound on the number of times in which the system might violate safety (while existing algorithms might block forever due to the presence of both transient faults and crash failures). Since vector clocks facilitate a number of elementary synchronization building blocks (without requiring remote replica synchronization) in asynchronous systems, we believe that our analytical insights are useful for the design of other systems that cannot guarantee execution fairness.