# zookeeper学习系列：四、Paxos算法和zookeeper的关系

Paxos 这个算法是Leslie Lamport在1990年提出的一种基于消息传递的一致性算法.Paxos 算法解决的问题是一个分布式系统如何就某个值（决议）达成一致。

1. prepare 阶段：
1. proposer 选择一个提案编号 n 并将 prepare 请求发送给 acceptors 中的一个多数派；
2. acceptor 收到 prepare 消息后，如果提案的编号大于它已经回复的所有 prepare 消息，则 acceptor 将自己上次接受的提案回复给 proposer，并承诺不再回复小于 n 的提案；
2. 批准阶段：
1. 当一个 proposer 收到了多数 acceptors 对 prepare 的回复后，就进入批准阶段。它要向回复 prepare 请求的 acceptors 发送 accept 请求，包括编号 n 和根据 P2c 决定的 value（如果根据 P2c 没有已经接受的 value，那么它可以自由决定 value）。
2. 在不违背自己向其他 proposer 的承诺的前提下，acceptor 收到 accept 请求后即接受这个请求。

3. Learn阶段:

• 当各个Acceptor达到一致之后，需要将达到一致的结果通知给所有的Learner.

二、资料证实

There is a very common misunderstanding that the leader election algorithm in zookeeper is paxos or fast paxos. The leader election algorithm is not paxos or fast paxos, please consider the following facts:

1. There is no the concept of proposal number in the leader election in zookeeper. the proposal number is a key concept to paxos. Some one think the epoch is the proposal number, but different followers may produce proposal with the same epoch which is not allowed in paxos.

2. Fast paxos requires at least 3t + 1 acceptors, where t is the number of servers which are allowed to fail [3]. This is conflict with the fact that a zookeeper cluster with 3 servers works well even if one server failed.

3. The leader election algorithm must make sure P1. However paxos does provide such guarantee.

4. In paxos, a leader is also required to achieve progress. There are some similarities between the leader in paxos and the leader in zookeeper. Even if more than one servers believe they are the leader, the consistency is preserved both in zookeeper and in paxos. this is very clearly discussed in [1] and [2].

1）邮件列表

Our protocol instead, has only two phases, just like a two-phase
commit protocol. Of course, for Paxos, we can ignore the first phase in runs in
which we have a single proposer as we can run phase 1 for multiple instances at
a time, which is what Ben called previously Multi-Paxos, I believe. The trick
with skipping phase 1 is to deal with leader switching. 

2）出书的访谈

We made a few interesting observations about Paxos when contrasting it to Zab, like problems you could run into if you just implemented Paxos alone. Not that Paxos is broken or anything, just that in our setting, there were some properties it was not giving us. Some people still like to map Zab to Paxos, and they are not completely off, but the way we see it, Zab matches a service like ZooKeeper well.

zk的分布式一致性算法有了个名称叫Zab

3）论文

We use an algorithm that shares some of the character- istics of Paxos, but that combines transaction logging needed for consensus with write-ahead logging needed for data tree recovery to enable an efficient implementa- tion.

zk的选主策略：

there can be at most one leader (proposer) at any time, and we guarantee this by making sure
that a quorum of replicas recognize the leader as a leader by committing to an
epoch change. This change in epoch also allows us to get unique zxids since the
epoch forms part of the zxid. 

1. The leader sends a PROPOSAL message, p, to all followers.

2. Upon receiving p, a follower responds to the leader with an ACK, informing the

leader that it has accepted the proposal.

itself), the leader sends a message informing the followers to COMMIT it.

[1] Reed, B., & Junqueira, F. P. (2008). A simple totally ordered broadcast protocol. Second
Workshop on Large-Scale Distributed Systems and Middleware (LADIS 2008). Yorktown
Heights, NY: ACM. ISBN: 978-1-60558-296-2.
[2] Lamport, L. Paxos made simple. ACM SIGACT News 32, 4 (Dec. 2001), 1825.
[3] F. Junqueira, Y. Mao, and K. Marzullo. Classic paxos vs. fast paxos: caveat emptor. In
Proceedings of the 3rd USENIX/IEEE/IFIP Workshop on Hot Topics in System Dependability
(HotDep.07). Citeseer, 2007.

[4]O'Reilly.ZooKeeper.Distributed process coordination.2013

[5] http://agapple.iteye.com/blog/1184023  zookeeper项目使用几点小结

posted on 2014-10-29 12:50 坚毅的刀刀 阅读(...) 评论(...) 编辑 收藏