zookeeper_action

连接串

从节点列表本地缓存
主节点对未分配的任务，随机分配给从节点(不合理？？)
从节点保存一个本地待执行任务列表
单独的线程对节点已分配任务进行循环

进程p为了获锁——＞创建节点znode_/lock且其为临时节点：/lock 临时节点 避免该znode崩溃而锁无法释放

https://zookeeper.apache.org/doc/trunk/zookeeperOver.html

ZooKeeper is a distributed, open-source coordination service for distributed applications. It exposes a simple set of primitives that distributed applications can build upon to implement higher level services for synchronization, configuration maintenance, and groups and naming. It is designed to be easy to program to, and uses a data model styled after the familiar directory tree structure of file systems. It runs in Java and has bindings for both Java and C.

【要解决的问题：race conditions and deadlock】

Coordination services are notoriously hard to get right. They are especially prone to errors such as race conditions and deadlock. The motivation behind ZooKeeper is to relieve distributed applications the responsibility of implementing coordination services from scratch.

【core --- They maintain an in-memory image of state, along with a transaction logs and snapshots in a persistent store.】

The servers that make up the ZooKeeper service must all know about each other. They maintain an in-memory image of state, along with a transaction logs and snapshots in a persistent store. As long as a majority of the servers are available, the ZooKeeper service will be available.