如果在指定的时间内，没有收到主节点的有效回复，则判断其为主观下线。需要注意的是，该参数不仅用来判断主节点状态，同样也用来判断该主节点下面的从节点及其它Sentinel的状态。其默认值为30s。
sentinel parallel-syncs mymaster 1：在failover期间，允许多少个slave同时指向新的主节点。如果numslaves设置较大的话，虽然复制操作并不会阻塞主节点，但多个节点同时指向新的主节点，会增加主节点的网络和磁盘IO负载。
sentinel failover-timeout mymaster 180000：定义故障切换超时时间。默认180000，单位秒，即3min。需要注意的是，该时间不是总的故障切换的时间，而是适用于故障切换的多个场景。
# 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.
第一种适用场景：如果Redis Sentinel对一个主节点故障切换失败，那么下次再对该主节点做故障切换的起始时间是failover-timeout的2倍。这点从Sentinel的日志就可体现出来（28234:X 08 Oct 16:04:04.385 # Next failover delay: I will not start a failover before Mon Oct 8 16:10:04 2018）
sentinel notification-script：定义通知脚本，当Sentinel出现WARNING级别的事件时，会调用该脚本，其会传入两个参数：事件类型，事件描述。
sentinel client-reconfig-script：当主节点发生切换时，会调用该参数定义的脚本，其会传入以下参数：<master-name> <role> <state> <from-ip> <from-port> <to-ip> <to-port>
关于脚本，其必须遵循一定的规则。
# 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.
sentinel deny-scripts-reconfig：不允许使用SENTINEL SET设置notification-script和client-reconfig-script。
Sentinel的常见操作
•PING This command simply returns PONG.
•SENTINEL masters Show a list of monitored masters and their state.
•SENTINEL master <master name> Show the state and info of the specified master.
•SENTINEL slaves <master name> Show a list of slaves for this master, and their state.
•SENTINEL sentinels <master name> Show a list of sentinel instances for this master, and their state.
•SENTINEL get-master-addr-by-name <master name> Return the ip and port number of the master with that name. If a failover is in progress or terminated successfully for this master it returns the address and port of the promoted slave.
•SENTINEL reset <pattern> This command will reset all the masters with matching name. The pattern argument is a glob-style pattern. The reset process clears any previous state in a master (including a failover in progress), and removes every slave and sentinel already discovered and associated with the master.
•SENTINEL failover <master name> Force a failover as if the master was not reachable, and without asking for agreement to other Sentinels (however a new version of the configuration will be published so that the other Sentinels will update their configurations).
•SENTINEL ckquorum <master name> Check if the current Sentinel configuration is able to reach the quorum needed to failover a master, and the majority needed to authorize the failover. This command should be used in monitoring systems to check if a Sentinel deployment is ok.
•SENTINEL flushconfig Force Sentinel to rewrite its configuration on disk, including the current Sentinel state. Normally Sentinel rewrites the configuration every time something changes in its state (in the context of the subset of the state which is persisted on disk across restart). However sometimes it is possible that the configuration file is lost because of operation errors, disk failures, package upgrade scripts or configuration managers. In those cases a way to to force Sentinel to rewrite the configuration file is handy. This command works even if the previous configuration file is completely missing.
•SENTINEL MONITOR <name> <ip> <port> <quorum> This command tells the Sentinel to start monitoring a new master with the specified name, ip, port, and quorum. It is identical to the sentinel monitor configuration directive in sentinel.conf configuration file
•SENTINEL REMOVE <name> is used in order to remove the specified master: the master will no longer be monitored, and will totally be removed from the internal state of the Sentinel, so it will no longer listed by SENTINEL masters and so forth.
•SENTINEL SET <name> <option> <value> The SET command is very similar to the CONFIG SET command of Redis, and is used in order to change configuration parameters of a specific master. Multiple option / value pairs can be specified (or none at all). All the configuration parameters that can be configured via sentinel.conf are also configurable using the SET command.
sentinel masters
输出被监控的主节点的状态信息