一个常见的paxos疑问

原文在 http://*.com/questions/14435646/paxos-value-choice/14472334#14472334

引用

Now, let's take a hypothetical example that people often give, and which they think breaks Paxos. Suppose we have three Acceptors A1, A2, and A3. A1 and A2 have both accepted value ABC at round 1 and A3 has chosen XYZ at round 2 (ie. from a different proposer). We can see that A1 and A2 form a majority and that ABC has been "chosen."

Continuing along this hypothetical example, a proposer sends Prepare(3) and receives back responses from A2 and A3, viz Promise(ABC @ 1) and Promise(XYZ @ 2). The Proposer sees XYZ has the highest round, and sends that along in the Accept phase, overwriting ABC on the other hosts. And viola, Paxos is broken, Right?

引用

No. The problem is with the start state, which is impossible. Let me show you why.

First, some propositions, which are key to Paxos running correctly:

Proposition A: For A1 and A2 to have the value ABC @ 1, a proposer must have sent Accept(ABC @ 1) which means it must have received a majority of Promises in response to sending Prepare(1).

Proposition B: For A3 to have the value XYZ @ 2, a proposer must have sent Accept(XYZ @ 2) which means it must have received a majority of Promises in response to sending Prepare(2).