MongoDB Replica Set是MongoDB官方推荐的主从复制和高可用方案，用于替代原有的Master-Slave主从复制方案。Replicat Set具有自动
MongoDB Replica Set是MongoDB官方推荐的主从复制和高可用方案，用于替代原有的Master-Slave主从复制方案。Replicat Set具有自动切换功能，当Primary挂掉之后，可以自动由Replica Set中的某一个Secondary来切换到Primary，以实现高可用的目的，不像MySQL那样需要使用第三方软件。
目前很多游戏公司都开始使用MongoDB作为数据库，我们公司线上使用的版本是2.4.6.
一 MongoDB Replica Set的原理
复制主要用于备份、灾难恢复和读写分离。一个Replica Set就是一组mongod实例。Replica Set中的Primary接收所有的写操作，Secondaries从Primary复制操作然后应用到自己的data set。
一个Replica Set中的成员角色有三种：Primary，Secondary和Arbiter。
Primary 接收来自客户端的所有的写操作，一个Replica Set中有且只有一个Primary。Primary如果宕掉，Replica Set会自动选举一个Secondary成为Primary。Primary将它data sets的所有操作都记录到oplog中。
Secondary Secondary从Primary复制oplog，然后将oplog中的操作应用到自己的data sets。Secondary和Primary之间是异步复制，也就是Secondary中的数据可能不是最新的。默认情况下，Secondary不可读不可写，但是可以通过设置运行客户端从Secondary读。
Arbiter Arbiter不需要维护自己的data sets，，只是当Primary挂掉之后参与投票选择哪个Secondary可以升级为Primary。当Replica Set中的成员个数为偶数个时，就需要添加一个Arbiter用于投票选举哪个可以升级为Primary。Arbiter对硬件的要求很低。不能在Primary或者Secondary主机上运行Arbiter。
一个Replica Set可以最多拥有12个成员，但是只有7个成员可以同时参与投票选举成为Primary，如果成员数量超过12，就需要使用Master-Slave主从复制方式。
部署一个Replica Set至少需要三个成员，一个Arbiter，一个Secondary和一个Primary或者一个Primary，两个Secondary。
可以将Secondary配置为以下几种特殊用途：
A.在选举中阻止其成为Primary，只用作备份数据。通过设置优先级priority为0来实现。
B.阻止应用程序从它读，通过设置优先级priority为0和设置hidden为true来实现。
一个隐藏的成员同样复制Primary的数据，但是对于客户端应用程序来讲，它不可见。
C.保留历史镜像数据用于数据回档，比如如果误删除数据，可以使用Delayed Replica Set成员中的数据恢复。
Delayed members即延时成员会延时从Primary复制oplog
二 MongoDB Replica Set部署架构
Replica Set Elections 复制集选举
Replica Set通过投票选举的方式来决定哪个成员可以升级为Primary。初始化一个Replica Set后就会产生选举出现，或者任何时候当Primary不可用时也会有选举出现。需要注意的是，投票选举Primary会花费一定的时间来完成，在这段时间内，整个Replica Set无法进行写操作，所以尽量避免重新投票选举的情况出现。
影响选举的因素和条件有：
Heartbeats 心跳检测
Replica Set中成员每2秒向其他成员发送心跳，如果在10秒内无回应，这个成员就被其他成员标记为不可用。
Priortiy Comparisons 对比优先级
设定成员的优先级priority会影响投票选举，优先级越高，越容易被选举成为Primary。如果优先级设置为0，那么这个成员不会永远不会被选举成为Primary，它也不需要其他成员为它投票。
Optime
Optime是Replica Set的成员上一次从oplog中将操作应用到data sets的时间戳。一个成员不会成为Primary，除非它比其他可见的成员拥有最新的时间戳。
Connections
一个Replica Set中的成员如果想要成为Primary，它必须能够连接这个Replica Set中具有投票权利的大部分成员。
Network Partitions
为了避免当Primary宕掉后，整个Replica Set无法选举出新的Primary，Replica Set变成只读，需要在一个数据中心放置大部分Replica Set中的成员实例。
参考文档
CentOS 编译安装 MongoDB与mongoDB的php扩展
CentOS 6 使用 yum 安装MongoDB及服务器端配置
Ubuntu 13.04下安装MongoDB2.4.3
MongoDB入门必读(概念与实战并重)
Ubunu 14.04下MongoDB的安装指南
《MongoDB 权威指南》(MongoDB: The Definitive Guide)英文文字版[PDF]
Nagios监控MongoDB分片集群服务实战
基于CentOS 6.5操作系统搭建MongoDB服务
MongoDB 的详细介绍：请点这里
MongoDB 的下载地址：请点这里
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