post: add snowflake time go back

content/posts/2023/09/雪花id-时钟回退问题.md

@@ -0,0 +1,75 @@
+---
+author: "jdlau"
+date: 2023-09-27
+linktitle: 雪花id的时钟回退问题
+menu:
+next:
+prev:
+title: 雪花id的时钟回退问题
+weight: 10
+categories: ['Snowflake', 'Time']
+tags: ['mark']
+---
+
+雪花id由64位二进制组成，转成字符串则长为19. 它依赖于系统时钟，如果出现时钟回退，会导致已经在用的id再次被生成。
+
+怎么办呢？
+
+0. 记录上次生成时间，在本次生成时比较时间，如果当前时间比上次生成时间要小，则认为时钟回拨，直接报错。也可以一直重试，直到当前时间不小于上次生成时间。
+
+1. 采用历史时间则天然的不存在时间回拨问题。但是在超高并发情况下，历史的时间很快用完，时间一直保持在最新时间的话，这个时候还是会出现时间回拨。
+
+2. Go1.9开始，使用单调时钟: time.Now(), time.Since(), time.Until().
+
+> // # Monotonic Clocks
+>
+> //
+>
+> // Operating systems provide both a “wall clock,” which is subject to
+>
+> // changes for clock synchronization, and a “monotonic clock,” which is
+>
+> // not. The general rule is that the wall clock is for telling time and
+>
+> // the monotonic clock is for measuring time. Rather than split the API,
+>
+> // in this package the Time returned by time.Now contains both a wall
+>
+> // clock reading and a monotonic clock reading; later time-telling
+>
+> // operations use the wall clock reading, but later time-measuring
+>
+> // operations, specifically comparisons and subtractions, use the
+>
+> // monotonic clock reading.
+>
+> //
+>
+> // For example, this code always computes a positive elapsed time of
+>
+> // approximately 20 milliseconds, `even if the wall clock is changed` during
+>
+> // the operation being timed:
+>
+> //
+>
+> //	start := time.Now()
+>
+> //	... operation that takes 20 milliseconds ...
+>
+> //	t := time.Now()
+>
+> //	elapsed := t.Sub(start)
+>
+> //
+>
+> // Other idioms, such as time.Since(start), time.Until(deadline), and
+>
+> // time.Now().Before(deadline), are similarly robust against wall clock
+>
+> // resets.
+>
+
+[R1](https://go.dev/src/time/time.go?s=17583:17610)
+
+[R2](https://github.com/bwmarrin/snowflake/pull/18/files)