官网：
Redis的主从只能实现数据热备份的功能，主宕机后从无法自动接管服务，因此Redis推出了Sentinel的主从监控模式。
Sentinel实质上是类似于MHA的一个监控主从并自动切换的monitor，实现的功能类似MongoDB自动切换的replica set，其命令行自带于redis的安装包中，
即：redis-sentinel。
而针对超大的数据量redis 3.0后还推出了Redis Cluster，实质上是分片，类似于MongoDB的auto sharding。
一、Sentinel.conf配置文件
Sentinel.conf实例：
daemonize yes logfile "/redis/sentinel1/sentinel.log" port 26379 dir "/redis/sentinel1" protected-mode no --必须设置为no，否则无法自动故障转移。 sentinel monitor mymaster 127.0.0.1 6379 2 sentinel auth-pass mymaster leo --必须写在"sentinel monitor mymaster 127.0.0.1 6379 2"之后，否则会报找不到master name的错误。 sentinel down-after-milliseconds mymaster 60000 sentinel config-epoch mymaster 0
Sentinel配置文件官网网址：http://download.redis.io/redis-stable/sentinel.conf
很多配置项都可以在这里找到详细的解释，以下只翻译了最重要的几个配置项。
############################################################################## # 需要注意的是虽然本说明只列出了以下几种参数，但其实一些redis.conf的参数也是可以在sentinel.conf中设置的，例如 # daemonize、logfile等参数。 # 此外如果主从设置了auth验证，那么这里还需要配置：sentinel auth-pass <master-name> <password> ############################################################################## # Example sentinel.conf # *** IMPORTANT *** # # By default Sentinel will not be reachable from interfaces different than # localhost, either use the 'bind' directive to bind to a list of network # interfaces, or disable protected mode with "protected-mode no" by # adding it to this configuration file. # 默认Sentinel是不允许除localhost以外的服务器连接的，因此要么在bind中指明网段，要么设置protected-mode no # Before doing that MAKE SURE the instance is protected from the outside # world via firewalling or other means. # # For example you may use one of the following: # # bind 127.0.0.1 192.168.1.1 # protected-mode no --必须设置为no，或者bind 127.0.0.1以及本地IP，否则sentinel之间无法通信不能进行自动failover # port <sentinel-port> # The port that this sentinel instance will run on port 26379 # sentinel announce-ip <ip> # sentinel announce-port <port> # # The above two configuration directives are useful in environments where, # because of NAT, Sentinel is reachable from outside via a non-local address. # # When announce-ip is provided, the Sentinel will claim the specified IP address # in HELLO messages used to gossip its presence, instead of auto-detecting the # local address as it usually does. # # Similarly when announce-port is provided and is valid and non-zero, Sentinel # will announce the specified TCP port. # # The two options don't need to be used together, if only announce-ip is # provided, the Sentinel will announce the specified IP and the server port # as specified by the "port" option. If only announce-port is provided, the # Sentinel will announce the auto-detected local IP and the specified port. # # Example: # # sentinel announce-ip 1.2.3.4 # dir <working-directory> # Every long running process should have a well-defined working directory. # For Redis Sentinel to chdir to /tmp at startup is the simplest thing # for the process to don't interfere with administrative tasks such as # unmounting filesystems. dir /tmp # sentinel monitor <master-name> <ip> <redis-port> <quorum> # # Tells Sentinel to monitor this master, and to consider it in O_DOWN # (Objectively Down) state only if at least <quorum> sentinels agree. # 此命令告诉哨兵去监控主节点，如果至少有<quorum>个setinel检测到主S_DOWN，那么就将主设置为O_DOWN状态，然后就可以failover。 # O_DOWN S_DOWN Objectively/Subjectively：意思分别是客观下线和主观下线，前者表示多个sentinel实例共同作出了master已下线的判断， # 后者表示单个sentinel实例做出了master已下线的判断。只有至少<quorum>个sentinel进程检测到主S_DOWN，才会做出主O_DOWN的判断，然后其中一个sentinel就会开始进行failover。 # Note that whatever is the ODOWN quorum, a Sentinel will require to # be elected by the majority of the known Sentinels in order to # start a failover, so no failover can be performed in minority. # 这段说明setinel也是需要多数存活才能实现故障转移投票，因此为防止脑裂建议配置奇数个sentinel。 # Slaves are auto-discovered, so you don't need to specify slaves in # any way. Sentinel itself will rewrite this configuration file adding # the slaves using additional configuration options. # Also note that the configuration file is rewritten when a # slave is promoted to master. # # Note: master name should not include special characters or spaces. # The valid charset is A-z 0-9 and the three characters ".-_". sentinel monitor mymaster 127.0.0.1 6379 2 # sentinel auth-pass <master-name> <password> # # Set the password to use to authenticate with the master and slaves. # Useful if there is a password set in the Redis instances to monitor. # # Note that the master password is also used for slaves, so it is not # possible to set a different password in masters and slaves instances # if you want to be able to monitor these instances with Sentinel. # # However you can have Redis instances without the authentication enabled # mixed with Redis instances requiring the authentication (as long as the # password set is the same for all the instances requiring the password) as # the AUTH command will have no effect in Redis instances with authentication # switched off. # # Example: # # sentinel auth-pass mymaster MySUPER--secret-0123passw0rd # sentinel down-after-milliseconds <master-name> <milliseconds> # # Number of milliseconds the master (or any attached slave or sentinel) should # be unreachable (as in, not acceptable reply to PING, continuously, for the # specified period) in order to consider it in S_DOWN state (Subjectively # Down). # 这段的意思是sentinel在和主失联多少毫秒后，可以做出主节点S_DOWN的判断。 # Default is 30 seconds. sentinel down-after-milliseconds mymaster 30000 # sentinel parallel-syncs <master-name> <numslaves> # # How many slaves we can reconfigure to point to the new slave simultaneously # during the failover. Use a low number if you use the slaves to serve query # to avoid that all the slaves will be unreachable at about the same # time while performing the synchronization with the master. sentinel parallel-syncs mymaster 1 # sentinel failover-timeout <master-name> <milliseconds> # # Specifies the failover timeout in milliseconds. It is used in many ways: # # - The time needed to re-start a failover after a previous failover was # already tried against the same master by a given Sentinel, is two # times the failover timeout. # # - The time needed for a slave replicating to a wrong master according # to a Sentinel current configuration, to be forced to replicate # with the right master, is exactly the failover timeout (counting since # the moment a Sentinel detected the misconfiguration). # # - The time needed to cancel a failover that is already in progress but # did not produced any configuration change (SLAVEOF NO ONE yet not # acknowledged by the promoted slave). # # - The maximum time a failover in progress waits for all the slaves to be # reconfigured as slaves of the new master. However even after this time # the slaves will be reconfigured by the Sentinels anyway, but not with # the exact parallel-syncs progression as specified. # # Default is 3 minutes. sentinel failover-timeout mymaster 180000 # SCRIPTS EXECUTION # # sentinel notification-script and sentinel reconfig-script are used in order # to configure scripts that are called to notify the system administrator # or to reconfigure clients after a failover. The scripts are executed # with the following rules for error handling: # # If script exits with "1" the execution is retried later (up to a maximum # number of times currently set to 10). # # If script exits with "2" (or an higher value) the script execution is # not retried. # # If script terminates because it receives a signal the behavior is the same # as exit code 1. # # A script has a maximum running time of 60 seconds. After this limit is # reached the script is terminated with a SIGKILL and the execution retried. # NOTIFICATION SCRIPT # # sentinel notification-script <master-name> <script-path> # 这里可以配置发生failover时的可执行脚本，可以配置邮件发送脚本。 # Call the specified notification script for any sentinel event that is # generated in the WARNING level (for instance -sdown, -odown, and so forth). # This script should notify the system administrator via email, SMS, or any # other messaging system, that there is something wrong with the monitored # Redis systems. # # The script is called with just two arguments: the first is the event type # and the second the event description. # # The script must exist and be executable in order for sentinel to start if # this option is provided. # # Example: # # sentinel notification-script mymaster /var/redis/notify.sh # CLIENTS RECONFIGURATION SCRIPT # # sentinel client-reconfig-script <master-name> <script-path> # # When the master changed because of a failover a script can be called in # order to perform application-specific tasks to notify the clients that the # configuration has changed and the master is at a different address. # # The following arguments are passed to the script: # # <master-name> <role> <state> <from-ip> <from-port> <to-ip> <to-port> # # <state> is currently always "failover" # <role> is either "leader" or "observer" # # The arguments from-ip, from-port, to-ip, to-port are used to communicate # the old address of the master and the new address of the elected slave # (now a master). # # This script should be resistant to multiple invocations. # # Example: # # sentinel client-reconfig-script mymaster /var/redis/reconfig.sh
二、Sentinel的启动
redis-sentinel /redis/sentinel_1/sentinel.conf
或
redis-server /redis/sentinel_1/sentinel.conf
以上两种方式都可以，一般我们需要启动至少3个以上的奇数个这样的sentinel进程。
三、Sentinel API（即Sentinel shell内可以使用的命令）
参考：https://redis.io/topics/sentinel 的Sentinel API部分。
例如我们可以使用sentinel failover来实现手动failover，还可以通过sentinel remove/monitor来实现主节点的重新配置。
四、其他注意事项
Redis Sentinel在和docker使用时需要特别注意，由于docker存在端口映射可能会导致sentinel的自动failover失效。
同样的NAT和端口映射等机制也会导致Sentinel失效，需要进行特别的配置。
总结下来就是：凡是涉及到IP转换或者端口映射时，部署Sentinel都需要注意。