same question as:
why is PAXOS necessary?
1, what if >1 nodes become leaders simultaneously?
that‘s why we need phase#1 (prepare) to select a leader.
2, what if there is a netwrok partition?
that‘s why we need a majority to make some nodes progress. if less than majority, just fails.
3, what if a leader crashes in the middle of solicitation?
4, what if a leader crashes after deciding but before announcing results?
5, what if the new leader proposes different values than already decided value?
that‘s why we need ballot number (made up of a unique increasing number and the processor id) so that any processor is likely to be a new leader.
why is agreement hard in a distributed system?
原文：http://www.cnblogs.com/miaoz/p/4100961.html