分布式系统是由一组通过网络进行通信、为了完成共同的任务而协调工作的计算机节点组成的系统。
分布式系统的发展，体现的也是人类在对更高性能、更低成本的不断追求，
LESLIE LAMPORT在其1977年的论文《Proving the Correctness of Multiprocess Programs》中第一次描述了安全性和活性，后来成为构建分布式系统的标准。
Safety
Only a value that has been proposed may be chosen.
Only a single value is chosen.
A node never learns that a value has been chosen unless it actually has been.
Liveness
Some proposed value is eventually chosen.
If a value has been chosen, a node can eventually learn the value.
简单解释就是：
safety ：something “bad” will never happen
liveness ：something “good” will must happen (but we don’t know when)