wikipedia Logical clock

A logical clock is a mechanism for capturing chronological and causal relationships in a distributed system. Distributed systems may have no physically synchronous global clock, so a logical clock allows global ordering on events from different processes in such systems. The first implementation, the Lamport timestamps, was proposed by Leslie Lamport in 1978 (Turing Award in 2013).

NOTE:

一、

chronological relationship 时间关系

causal relationship 因果关系

二、"Distributed systems may have no physically synchronous global clock"

这是分布式系统的挑战

Algorithms

Some noteworthy logical clock algorithms are:

1、Lamport timestamps, which are monotonically increasing software counters.

NOTE:

非常简单，但是具有重要的意义

2、Vector clocks, that allow for partial ordering of events in a distributed system.

3、Version vectors, order replicas, according to updates, in an optimistic replicated system.

4、Matrix clocks, an extension of vector clocks that also contains information about other processes' views of the system.

NOTE:

从名称就可以看出是vector clock的升级版