C100DBA MongoDB 讲解-advantage of having delayed replica set member?

C100DBA MongoDB 讲解-advantage of having delayed replica set member?

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What is the principal advantage of having a delayed replica set member?

1. It allows the load on the secondary severs to be more evenly spread
2. It allows you to perform queries against historical versions of data
3. It increases write speed to the primary
4. It makes it easier to upgrade the system without downtime
5. It provides a window of time to recover from operator error

翻译为：

部署一个有延迟的replica set member复制集成员的主要好处是什么？

1. 其允许二级服务器上的负载能够被均衡覆盖
2. 其允许用户查询历史版本的数据 ， (注解：其实就相当于是延迟的dataguard standby ，可以查到过去某个时间点的数据）
3. 其帮助primary节点提供写入的速度
4. 其帮助系统升级而无需宕机时间更简单方便
5. 其为操作错误后的恢复提供了一个时间窗口

显然答案是B 其允许用户查询历史版本的数据。

delayed replica set member的配置方法：

z

cfg = rs.conf()
cfg.members[0].priority = 0
cfg.members[0].hidden = true
cfg.members[0].slaveDelay = 3600
rs.reconfig(cfg)

详见 官方文档http://docs.mongodb.org/manual/tutorial/configure-a-delayed-replica-set-member/

主要参数为slaveDelay ，注意slaveDelay的长度必须小于oplog的窗口，如果oplog要比slaveDelay时间短，则延迟的成员将无法成功复制操作。

Because delayed members are a “rolling backup” or a running “historical” snapshot of the data set, they may help you recover from various kinds of human error. For example, a delayed member can make it possible to recover from unsuccessful application upgrades and operator errors including dropped databases and collections.

由于延迟节点是一个 “滚动的备份” 或一个保持着历史快照的数据集， 所以他们可以帮助你从各种人为故障中恢复出数据。 例如如果一个不成功的应用升级或其他操作失误（例如误drop database或collection)时，你可以直接从延迟节点中抽取数据来恢复。