C100DBA MongoDB 讲解 replica set vote

C100DBA MongoDB 讲解 replica set vote

题目中文：

Given a replica set with five data-bearing members, suppose the primary goes down with operations in its oplog that have been copied to only one secondary .

Assuming no other problems occur, which of the following describes what is most likely to happen?

1. The primary will rollback the operations
2. The secondary with the most recent oplog will be elected primary
3. The most recent secondary will roll back the operations following the election
4. Reads will stale until primary comes backup up

翻译：

对于一个有五个承载数据的成员的replica set 而言，假设primary服务器宕机了，且其oplog中最新的操作信息只成功拷贝到一个secondary上了。

假设没有其他问题发生，以下哪个选项是正确的？

1. Primary将在恢复时回滚部分操作
2. 拥有最新的oplog的这个secondary服务器将赢得election并成为primary
3. 在election后最新的这个secondary会回滚部分操作
4. 直到primary重新可用，之前读操作将读到过时的数据

关于此问题 官方文档有较明确解释：

A rollback does not occur if the write operations replicate to another member of the replica set before the primary steps down and if that member remains available and accessible to a majority of the replica set.     http://docs.mongodb.org/manual/core/replica-set-rollbacks/

只要primary上的写操作已经复制到某个secondary成员上，secondary成员可用，就可以保证rollback不会发生。