Redis-sentinel是Redis實例的監控管理、通知和實例失效備援服務，是Redis集群的管理工具。在一般的分布式中心節點數據庫中，Redis-sentinel的作用是中心節點的工作，監控各個其他節點的工作情況并且進行故障恢復，來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，因為Redis實例在各個大公司的應用，每個公司都需要一個Redis集群的管理工具，被迫都自己寫管理工具來管理Redis集群，antirez考慮到社區的急迫需要(詳情)，花了幾個星期寫出了Redis-sentinel。

Redis-sentinel的三大功能: * 監測 * 通知 * 自動故障恢復 首先Redis-sentinel要建立一個監控的master列表，然后針對master列表的每個master獲取監控其的sentinels和slaves供以后故障恢復使用。

自動發現sentinel和slave

一個sentinel實例的監控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，并且指定連接失敗超時的時間和是否可以故障恢復等參數。這樣，sentinel A就建立了監控的master列表。

sentinel A作為客戶端建立對其監控master 1的pub/sub連接和命令傳輸連接，pub/sub連接的目的是通過消息傳遞來獲取監控master 1的所有sentinels，具體方式是訂閱”sentinel:hello”頻道，并這個頻道publish 自己的host信息，這樣，所有監控master 1的sentinels都通過訂閱master 1的“sentinel:hello”頻道來獲取其他sentinels的信息。sentinel A就建立了master 1的sentinels列表。 sentinel A還通過間斷的info命令獲取master 1的slaves列表，如果連接超時或者失敗，就會進入故障處理的例程。

故障恢復準備

Redis-sentinel定義了故障的master兩種狀態，O_DOWN(objectively)和S_DOWN(subobjectively)。當sentinel A連接master 1失敗后，sentinel A認定master 1為subobjectively狀態，然后sentinel A查看監控master 1的sentinels的認定情況。因為sentinel之間會建立連接并且不斷的發起info命令詢問和回應，因此在sentinel A認定master 1 S_DOWN后，sentinel A通過SENTINEL IS-MASTER-DOWN-BY-ADDR命令來不斷獲得其他sentinel的認定情況，如果有超過quorum(配置中指定)個sentinel認定master 1已經DOWN，那么就確定master 1為O_DOWN。然后sentinel

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A必須有master 1的can failover權限，這也是配置文件指定的。接著，sentinel A需要得知master 1的leader sentinel，由leader來進行master 1的錯誤恢復。

master 1的leader sentinel由選舉產生，每個監控master 1的，并且具有錯誤恢復權限的sentinel都需要進行一次叫做subjective leader判斷，也就是sentinel自己認定的leader，然后sentinel之間通過SENTINEL IS-MASTER-DOWN-BY-ADDR命令交流leader的認定情況，然后最終得到一個共識。

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然后由這個master 1的leader sentinel來啟動錯誤恢復例程。

故障處理

sentinel A作為master 1的leader，會選取一個master 1的slave作為新的master。slave的選取是根據一個判斷DNS情況的優先級來得到，優先級相同通過runid的排序得到，但目前優先級設定還沒實現，所以直接獲取runid排序得到slave 1。

然后發送命令slaveof no one來取消slave 1的slave狀態來轉換為master。當其他sentinel觀察到該slave成為master后,就知道錯誤處理例程啟動了。sentinel A然后發送給其他slave slaveof

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new-slave-ip-port 命令，當所有slave都配置完后，sentinel A從監測的masters列表中刪除故障master，然后通知其他sentinels。

總結

Redis-sentinel作為一個集群管理工具，基本滿足了要求。但是Redis集群的需求卻仍然存在。Redis集群作為分區結果，分區的配置仍然需要客戶端或者代理協議實現，Redis集群作為一個整體還需要更長的路。Redis-cluster的草案早于出爐，但是實現卻已經停留了很久。antriez承諾在Redis 3.0推出Redis-cluster功能。

總結

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