Scalable Global ID Generator Design

此为某国外大公司系统设计面试题目 :)

参考： http://stackoverflow.com/questions/2671858/distributed-sequence-number-generation

这里将其中关于 Twitter Snowflake 的部分摘出来：

 

Snowflake is a:

• Networked service, i.e. you make a network call to get a unique ID;
• which produces 64 bit unique IDs that are ordered by generation time;
• and the service is highly scalable and (potentially) highly available; each instance can generate many thousand IDs per second, and you can run multiple instances on your LAN/WAN;
• written in Scala, runs on the JVM. 

The main ideas are:

• The most significant 40 or so bits: A timestamp; the generation time of the ID. (We're using the most significant bits for the timestamp to make IDs sort-able by generation time.)

• The next 14 or so bits: A per-generator counter, which each generator increments by one for each new ID generated. This ensures that IDs generated at the same moment (same timestamps) do not overlap.

• The last 10 or so bits: A unique value for each generator. Using this, we don't need to do any synchronization between generators (which is extremely hard), as all generators produce non-overlapping IDs because of this value.