Logical Physical Clocks and Consistent Snapshots in Globally Distributed Databases 中文译文

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混合逻辑时钟和全局分布式数据库中的分布式快照

There is a gap between the theory and practice of distributed systems in terms of the use of time. The theory of distributed systems shunned the notion of time, and introduced “causality tracking” as a clean abstraction to reason about concurrency. The practical systems employed physical time (NTP) information but in a best effort manner due to the difficulty of achieving tight clock synchronization. In an effort to bridge this gap and reconcile the theory and practice of distributed systems on the topic of time, we propose a hybrid logical clock, HLC, that combines the best of logical clocks and physical clocks. HLC captures the causality relationship like logical clocks, and enables easy identification of consistent snapshots in distributed systems. Dually, HLC can be used in lieu of physical/NTP clocks since it maintains its logical clock to be always close to the NTP clock. Moreover HLC fits in to 64 bits NTP timestamp format, and is masking tolerant to NTP kinks and uncertainties. We show that HLC has many benefits for wait-free transaction ordering and performing snapshot reads

对于时间的理解，在理论和实际中的分布式系统中有一个差异。理论避开了分布式系统的时间误差，而是用了因果序的逻辑时钟来解释并发。在实际的系统中使用了物理时间（NTP）信息，但难以实现紧密的时钟同步（分布式系统中个各个机器物理时钟有差异）。为了弥补分布式系统中理论与实际时钟的差异，我们提出了混合逻辑时钟的概念——HLC。HLC捕获了类似于逻辑时钟的因果关系，并且简化了分布式系统中snapshot。HLC可以用来替换物理时钟，因为HLP的逻辑时钟很接近物理时钟，而且对于NTP纽结和不确定性，HLC有更强的健壮度。HLC在多版本全局分布的数据库中具有无等待事务排序和快照读取的优点。

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