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超全面分布式缓存高可用方案：哨兵机制
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不断的检查master和slave是否正常运行（master存活检测、master与slave运行情况检测）"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"消息通知"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 当被监控的服务器出现问题时，向其他哨兵、客户端发送通知"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"自动故障转移"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 断开故障master与slave的连接，选取一个slave作为新master，将其他slave连接到新的master并告知客户端新的服务器地址。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注意：哨兵也是一台Redis服务器，只是不提供数据服务；通常哨兵配置的数量为单数。"}]}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"哨兵的工作原理"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"下面主要针对哨兵在进行主从切换过程中经历的三个阶段分别进行阐述。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"1、集群监控"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"step1：哨兵1连接到Redis集群"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"发送info命令到master，并建立cmd连接；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"哨兵端保存哨兵状态（SentinelStatus），保存所有哨兵状态，主节点和从节点的信息；master端会记录 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publish sentinel：hello"}]}]}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"直到主节点故障，哨兵报出 sdown，同时此哨兵还会向其他哨兵发布消息说这个主节点挂了。发送的指令是 sentinel is-master-down-by-address-port。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其余的哨兵接收到指令后，主节点挂了吗？让我去看看到底挂没挂。发送的信息也是 hello。"}]}]}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"其余的哨兵也会发送他们收到的信息并且发送指令 sentinel is-master-down-by-address-port 到自己的内网，确认一下第一个发送 sentinel is-master-down-by-address-port 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此时选举方式应用的是Raft协议，这个之前有过介绍，感兴趣的同学可以移步了解："},{"type":"link","attrs":{"href":"https://xie.infoq.cn/article/266bb921802ee3c3fb37c8c5d","title":""},"content":[{"type":"text","text":"https://xie.infoq.cn/article/266bb921802ee3c3fb37c8c5d"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"2）其次，哨兵Leader从所有的slave节点找出一个作为master节点"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"主要的规则："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"选择在线的节点，pass掉已下线的节点；"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"选择响应速度快的，pass掉响应慢的节点"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"选择与原master断开时间短的，pass掉断开时间较长的；"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"假如以上优先级均一致，会考虑其他优先原则："}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"偏移量较大"}]}]}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"假如说 slave1 的 offset 为 50，slave2 偏移量为 55，则哨兵就会选择 slave2 为新的主节点。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"runid偏大的"}]}]}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这点类似于职场中的论资排辈，也就说根据 runid 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主从复制的作用中有这么一句话“主从复制是高可用的基石”，那实现高可用必不可少的就是哨兵和集群。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"1、Sentinel的作用"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"集群监控"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"不断的检查master和slave是否正常运行（master存活检测、master与slave运行情况检测）"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"消息通知"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当被监控的服务器出现问题时，向其他哨兵、客户端发送通知"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"自动故障转移"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"断开故障master与slave的连接，选取一个slave作为新master，将其他slave连接到新的master并告知客户端新的服务器地址。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2、Sentinel的工作方式"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"每个Sentinel以每秒钟一次的频率向它所知的Master，Slave以及其他 Sentinel 实例发送一个 PING 命令 "}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果一个实例（Instance）距离最后一次有效回复 PING 命令的时间超过 down-after-milliseconds 选项所指定的值， 则这个实例会被 Sentinel 标记为主观下线。"}]}]}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 若 Master 重新向 Sentinel 的 PING 命令返回有效回复， Master 的主观下线状态就会被移除。"}]}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果一个Master被标记为主观下线，则正在监视这个Master的所有 Sentinel 要以每秒一次的频率确认Master的确进入了主观下线状态。 "}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当有足够数量的 Sentinel（>=配置文件指定的值）在指定的时间范围内确认Master的确进入了主观下线状态， 则Master会被标记为客观下线 "}]}]}]},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"若没有足够数量的 Sentinel 同意 Master 已经下线， Master 的客观下线状态就会被移除。"}]}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在一般情况下， 每个 Sentinel 会以每 10 秒一次的频率向它已知的所有Master，Slave发送 INFO 命令 "}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"当Master被 Sentinel 标记为客观下线时，Sentinel 向下线的 Master 的所有 Slave 发送 INFO 命令的频率会从 10 秒一次改为每秒一次"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}
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