paxos协议_一步一步理解paxos算法_从paxos到zookeeper

Paxos协议/算法是分布式系统中比较重要的协议，它有多重要呢？

<分布式系统的事务处理>：

Google Chubby的作者Mike Burrows说过这个世界上只有一种一致性算法，那就是Paxos，其它的算法都是残次品。

<分布式存储系统>：

理解了这两个分布式协议之后(Paxos/2PC)，学习其他分布式协议会变得相当容易。

学习Paxos算法有两部分：a) 算法的原理/证明；b) 算法的理解/运作。

理解这个算法的运作过程其实基本就可以用于工程实践。而且理解这个过程相对来说也容易得多。

网上我觉得讲Paxos讲的好的属于这篇：paxos图解及Paxos算法详解，我这里就结合进一步阐述。一些paxos基础通过这里提到的两篇文章，以及wiki上的内容基本可以理解。

Paxos在原作者的《Paxos Made Simple》中内容是比较精简的：

Phase 1

(a) A proposer selects a proposal number n and sends a prepare request with number n to a majority of acceptors.

(b) If an acceptor receives a prepare request with number n greater than that of any prepare request to which it has already responded, then it responds to the request with a promise not to accept any more proposals numbered less than n and with the highest-numbered pro-posal (if any) that it has accepted.

Phase 2

(a) If the proposer receives a response to its prepare requests (numbered n) from a majority of acceptors, then it sends an accept request to each of those acceptors for a proposal numbered n with a value v , where v is the value of the highest-numbered proposal among the responses, or is any value if the responses reported no proposals.

(b) If an acceptor receives an accept request for a proposal numbered n, it accepts the proposal unless it has already responded to a prepare request having a number greater than n.

借用paxos图解文中的流程图可概括为：

Paxos中有三类角色Proposer、Acceptor及Learner，主要交互过程在Proposer和Acceptor之间。

Proposer与Acceptor之间的交互主要有4类消息通信，如下图：

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