条件操作
CompareAndSwap (CAS) 其实就是条件操作，或是判断执行
先进行比较，然后根据比较结果来决定是否进行后面的操作，可以用于构建分布式锁机制(协调服务)
Note: CompareAndSwap 不能用于目录，如果测试用于目录，会获得 102 “Not a file” 的错误
Item | Comment |
|---|---|
prevValue | 比较当前的值 |
prevIndex | 比较当前的modifiedIndex |
prevExist | 检查key是否存在，如果是 true 就是一个更新操作; 如果是 false 就是一个创建操作 |
[root@docker ~]# curl http://127.0.0.1:2379/v2/keys/foo -XPUT -d value=one
{"action":"set","node":{"key":"/foo","value":"one","modifiedIndex":89,"createdIndex":89},"prevNode":{"key":"/foo","value":"abc","moddex":54}}
[root@docker ~]# curl http://127.0.0.1:2379/v2/keys/foo?prevExist=false -XPUT -d value=three
{"errorCode":105,"message":"Key already exists","cause":"/foo","index":89}
[root@docker ~]# curl http://127.0.0.1:2379/v2/keys/foo?prevValue=two -XPUT -d value=three
{"errorCode":101,"message":"Compare failed","cause":"[two != one]","index":89}
[root@docker ~]# curl http://127.0.0.1:2379/v2/keys/foo?prevValue=one -XPUT -d value=five
{"action":"compareAndSwap","node":{"key":"/foo","value":"five","modifiedIndex":90,"createdIndex":89},"prevNode":{"key":"/foo","value9,"createdIndex":89}}
[root@docker ~]# curl http://127.0.0.1:2379/v2/keys/foo?prevIndex=99 -XPUT -d value=xxx
{"errorCode":101,"message":"Compare failed","cause":"[99 != 90]","index":90}
[root@docker ~]# curl http://127.0.0.1:2379/v2/keys/foo?prevIndex=90 -XPUT -d value=xxx
{"action":"compareAndSwap","node":{"key":"/foo","value":"xxx","modifiedIndex":91,"createdIndex":89},"prevNode":{"key":"/foo","value"0,"createdIndex":89}}
[root@docker ~]# 
