ZooKeeper Performance

Different Correctness Definition

• Write: linearizable.
• Read: observe last write from the same client (maintain log index returned by the last write).

Different Programming Model

Watch

Version

def inc():
    while True:
        val, version = getData("cnt")
        if setData("cnt", val + 1, version):
            break

If the version is the same, setData will success.

This rule prevents you from interleaving getData and setData.

分布式锁

def acquire():
    lock = create(directory + "lock-", ephemeral|sequential)
    while True:
        children = getChildren(directory)
        if isLowest(lock, children):
            break
        # 监听上一个节点, 防止惊群效应.
        if exist(prevPath(lock, children), watch=True):
            waitForNotification()

def release():
    delete("lock")

If lock holder fail, system will enter intermediate state (ephemeral flag prevent deadlock).

So system must clean up the intermediate state or accept retry.

注册中心

ZooKeeper作为注册中心提供的功能:

• 订阅: 当ZK数据节点变更时, 客户端能够收到通知.
• 心跳: 当ZK临时节点心跳丢失后, 会被自动删除.

ZooKeeper作为注册中心的不足:

• 当出现网络分区时, 少数派无法提供服务.
• 写请求只能由Leader完成, 在服务同时上下线时, Leader压力会非常大.