Redis Sentinel是Redis的高可用方案。是Redis 2.8中正式引入的。
在之前的主从复制方案中，如果主节点出现问题，需要手动将一个从节点升级为主节点，然后将其它从节点指向新的主节点，并且需要修改应用方主节点的地址。整个过程都需要人工干预。
下面通过日志具体看看Sentinel的切换流程。
Sentinel的切换流程
集群拓扑图如下。
角色 IP 端口 runID
主节点 127.0.0.1 6379
从节点-1 127.0.0.1 6380
从节点-2 127.0.0.1 6381
Sentinel-1 127.0.0.1 26379 d4424b8684977767be4f5abd1e364153fbb0adbd
Sentinel-2 127.0.0.1 26380 18311edfbfb7bf89fe4b67d08ef432053db62fff
Sentinel-3 127.0.0.1 26381 3e9eb1aa9378d89cfe04fe21bf4a05a901747fa8
kill -9 将主节点进程杀死。
1. 最先反应的是从节点。
其会马上输出如下信息。
28244:S 08 Oct 16:03:34.184 # Connection with master lost.
28244:S 08 Oct 16:03:34.184 * Caching the disconnected master state.
28244:S 08 Oct 16:03:34.548 * Connecting to MASTER 127.0.0.1:6379
28244:S 08 Oct 16:03:34.548 * MASTER <-> SLAVE sync started
28244:S 08 Oct 16:03:34.548 # Error condition on socket for SYNC: Connection refused
28244:S 08 Oct 16:03:35.556 * Connecting to MASTER 127.0.0.1:6379
28244:S 08 Oct 16:03:35.556 * MASTER <-> SLAVE sync started
...
2. Sentinel的日志30s后才有输出，这个与“sentinel down-after-milliseconds mymaster 30000”的设置有关。
下面，依次贴出哨兵各个节点及slave的日志输出。
Sentinel-1
28087:X 08 Oct 16:04:04.277 # +sdown master mymaster 127.0.0.1 6379
28087:X 08 Oct 16:04:04.379 # +new-epoch 1
28087:X 08 Oct 16:04:04.385 # +vote-for-leader 18311edfbfb7bf89fe4b67d08ef432053db62fff 1
28087:X 08 Oct 16:04:05.388 # +odown master mymaster 127.0.0.1 6379 #quorum 3/2
28087:X 08 Oct 16:04:05.388 # Next failover delay: I will not start a failover before Mon Oct 8 16:10:04 2018
28087:X 08 Oct 16:04:05.631 # +config-update-from sentinel 18311edfbfb7bf89fe4b67d08ef432053db62fff 127.0.0.1 26380 @ mymaster 127.0.0.1 6379
28087:X 08 Oct 16:04:05.631 # +switch-master mymaster 127.0.0.1 6379 127.0.0.1 6381
28087:X 08 Oct 16:04:05.631 * +slave slave 127.0.0.1:6380 127.0.0.1 6380 @ mymaster 127.0.0.1 6381
28087:X 08 Oct 16:04:05.631 * +slave slave 127.0.0.1:6379 127.0.0.1 6379 @ mymaster 127.0.0.1 6381
28087:X 08 Oct 16:04:35.656 # +sdown slave 127.0.0.1:6379 127.0.0.1 6379 @ mymaster 127.0.0.1 6381
Sentinel-2
28163:X 08 Oct 16:04:04.289 # +sdown master mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:04.366 # +odown master mymaster 127.0.0.1 6379 #quorum 3/2
28163:X 08 Oct 16:04:04.366 # +new-epoch 1
28163:X 08 Oct 16:04:04.366 # +try-failover master mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:04.373 # +vote-for-leader 18311edfbfb7bf89fe4b67d08ef432053db62fff 1
28163:X 08 Oct 16:04:04.385 # 3e9eb1aa9378d89cfe04fe21bf4a05a901747fa8 voted for 18311edfbfb7bf89fe4b67d08ef432053db62fff 1
28163:X 08 Oct 16:04:04.385 # d4424b8684977767be4f5abd1e364153fbb0adbd voted for 18311edfbfb7bf89fe4b67d08ef432053db62fff 1
28163:X 08 Oct 16:04:04.450 # +elected-leader master mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:04.450 # +failover-state-select-slave master mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:04.528 # +selected-slave slave 127.0.0.1:6381 127.0.0.1 6381 @ mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:04.528 * +failover-state-send-slaveof-noone slave 127.0.0.1:6381 127.0.0.1 6381 @ mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:04.586 * +failover-state-wait-promotion slave 127.0.0.1:6381 127.0.0.1 6381 @ mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:05.543 # +promoted-slave slave 127.0.0.1:6381 127.0.0.1 6381 @ mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:05.543 # +failover-state-reconf-slaves master mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:05.629 * +slave-reconf-sent slave 127.0.0.1:6380 127.0.0.1 6380 @ mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:06.554 # -odown master mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:06.555 * +slave-reconf-inprog slave 127.0.0.1:6380 127.0.0.1 6380 @ mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:06.555 * +slave-reconf-done slave 127.0.0.1:6380 127.0.0.1 6380 @ mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:06.606 # +failover-end master mymaster 127.0.0.1 6379
28163:X 08 Oct 16:04:06.606 # +switch-master mymaster 127.0.0.1 6379 127.0.0.1 6381
28163:X 08 Oct 16:04:06.606 * +slave slave 127.0.0.1:6380 127.0.0.1 6380 @ mymaster 127.0.0.1 6381
28163:X 08 Oct 16:04:06.606 * +slave slave 127.0.0.1:6379 127.0.0.1 6379 @ mymaster 127.0.0.1 6381
28163:X 08 Oct 16:04:36.687 # +sdown slave 127.0.0.1:6379 127.0.0.1 6379 @ mymaster 127.0.0.1 6381
Sentinel-3