Redis 作为高性能的key-value存储，一直在单实例上表现良好，但是长期以来一直缺乏一种官方的 高可用 方案支持。于是 Redis-sentinel 应运而生，提供了对客户端透明的高可用支持。下面文章对Redis- sentinel 的原理进行了系统的讲解。希望对大家有用。 文章
Redis作为高性能的key-value存储，一直在单实例上表现良好，但是长期以来一直缺乏一种官方的高可用方案支持。于是Redis-sentinel应运而生，提供了对客户端透明的高可用支持。下面文章对Redis-sentinel的原理进行了系统的讲解。希望对大家有用。
文章来源：www.wzxue.com
Redis-sentinel是Redis实例的监控管理、通知和实例失效备援服务，是Redis集群的管理工具。在一般的分布式中心节点数据库中，Redis-sentinel的作用是中心节点的工作，监控各个其他节点的工作情况并且进行故障恢复，来提高集群的高可用性。
Redis-sentinel是Redis的作者antirez在今年6月份完成的，因为Redis实例在各个大公司的应用，每个公司都需要一个Redis集群的管理工具，被迫都自己写管理工具来管理Redis集群，antirez考虑到社区的急迫需要(详情)，花了几个星期写出了Redis-sentinel。
Redis-sentinel的三大功能: * 监测 * 通知 * 自动故障恢复 首先Redis-sentinel要建立一个监控的master列表，然后针对master列表的每个master获取监控其的sentinels和slaves供以后故障恢复使用。
一个sentinel实例的监控master在配置文件中指定，如sentinel A在配置文件中指定管理的master ip:port列表，并且指定连接失败超时的时间和是否可以故障恢复等参数。这样，sentinel A就建立了监控的master列表。
sentinel A作为客户端建立对其监控master 1的pub/sub连接和命令传输连接，pub/sub连接的目的是通过消息传递来获取监控master 1的所有sentinels，具体方式是订阅”sentinel:hello”频道，并这个频道publish 自己的host信息，这样，所有监控master 1的sentinels都通过订阅master 1的“sentinel:hello”频道来获取其他sentinels的信息。sentinel A就建立了master 1的sentinels列表。 sentinel A还通过间断的info命令获取master 1的slaves列表，如果连接超时或者失败，就会进入故障处理的例程。
Redis-sentinel定义了故障的master两种状态，O_DOWN(objectively)和S_DOWN(subobjectively)。当sentinel A连接master 1失败后，sentinel A认定master 1为subobjectively状态，然后sentinel A查看监控master 1的sentinels的认定情况。因为sentinel之间会建立连接并且不断的发起info命令询问和回应，因此在sentinel A认定master 1 S_DOWN后，sentinel A通过SENTINEL IS-MASTER-DOWN-BY-ADDR命令来不断获得其他sentinel的认定情况，如果有超过quorum(配置中指定)个sentinel认定master 1已经DOWN，那么就确定master 1为O_DOWN。然后sentinel
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A必须有master 1的can failover权限，这也是配置文件指定的。接着，sentinel A需要得知master 1的leader sentinel，由leader来进行master 1的错误恢复。
master 1的leader sentinel由选举产生，每个监控master 1的，并且具有错误恢复权限的sentinel都需要进行一次叫做subjective leader判断，也就是sentinel自己认定的leader，然后sentinel之间通过SENTINEL IS-MASTER-DOWN-BY-ADDR命令交流leader的认定情况，然后最终得到一个共识。
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然后由这个master 1的leader sentinel来启动错误恢复例程。
sentinel A作为master 1的leader，会选取一个master 1的slave作为新的master。slave的选取是根据一个判断DNS情况的优先级来得到，优先级相同通过runid的排序得到，但目前优先级设定还没实现，所以直接获取runid排序得到slave 1。
然后发送命令slaveof no one来取消slave 1的slave状态来转换为master。当其他sentinel观察到该slave成为master后,就知道错误处理例程启动了。sentinel A然后发送给其他slave slaveof
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new-slave-ip-port 命令，当所有slave都配置完后，sentinel A从监测的masters列表中删除故障master，然后通知其他sentinels。
Redis-sentinel作为一个集群管理工具，基本满足了要求。但是Redis集群的需求却仍然存在。Redis集群作为分区结果，分区的配置仍然需要客户端或者代理协议实现，Redis集群作为一个整体还需要更长的路。Redis-cluster的草案早于出炉，但是实现却已经停留了很久。antriez承诺在Redis 3.0推出Redis-cluster功能。
原文地址：Redis核心解读–集群管理工具(Redis-sentinel), 感谢原作者分享。
