简介

Sentinel的作用就是主从切换：Redis-Sentinel是Redis官方推荐的高可用性(HA)解决方案，当用Redis做Master-slave的高可用方案时，假如master宕机了，Redis本身是没有实现自动进行主备切换的，而Redis-sentinel本身也是一个独立运行的进程，它能监控多个master-slave集群，发现master宕机后能进行自动切换。

实现去中心化；redis的主从结构是典型的中心结构，既一个主机master多个从属slave，那么一旦中心主机宕机，从属要么就是报错要么就是提供过期数据；哨兵sentinel就是为了去中心化，开启sentinel模式后，中心主机宕机，所有的从机会重新投票选取一个从机变为主机，当前主机恢复则前主机自动变为当前主机的从机；

 主要功能：1、监控；监控redis是否良好运行；2、通知；当某个redis节点出错通知其他机器；3、切换；如果master节点出错，通知所有slave并从中选取一个切换为master,把其他slave设置到新master的从机；4、支持sentinel集群；sentinel本身是独立的进程；建议做sentinel集群防止，单个sentinel宕机

快速开始

运行方法：

方法一：redis-sentinel /path/sentinel.conf

方法二：redis-server /path/sentinel.conf

sentinel使用的端口是26379

配置文件

# Example sentinel.conf

# port <sentinel-port>

# The port that this sentinel instance will run on

# sentinel实例运行的端口

port

# 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 interferer with administrative tasks such as

# unmounting filesystems.

dir /tmp

# sentinel monitor <master-name> <ip> <redis-port> <quorum>

# master-name : master Redis Server名称

# ip : master Redis Server的IP地址

# redis-port : master Redis Server的端口号

# quorum : 主实例判断为失效至少需要 quorum 个 Sentinel 进程的同意，只要同意 Sentinel 的数量不达标，自动failover就不会执行

#

# Tells Sentinel to monitor this master, and to consider it in O_DOWN

# (Objectively Down) state only if at least <quorum> sentinels agree.

#

# 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.

#

# 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 - and the three characters ".-_".

#

sentinel monitor mymaster 127.0.0.1

# 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).

# 选项指定了 Sentinel 认为Redis实例已经失效所需的毫秒数。当实例超过该时间没有返回PING，或者直接返回错误， 
# 那么 Sentinel 将这个实例标记为主观下线（subjectively down，简称 SDOWN ）

#

# Default is  seconds.

sentinel down-after-milliseconds mymaster

# 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.

# 选项指定了在执行故障转移时， 最多可以有多少个从Redis实例在同步新的主实例， 
# 在从Redis实例较多的情况下这个数字越小，同步的时间越长，完成故障转移所需的时间就越长。

sentinel parallel-syncs mymaster

# 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.

# 如果在该时间（ms）内未能完成failover操作，则认为该failover失败

#

# Default is  minutes.

sentinel failover-timeout mymaster

# 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 "" the execution is retried later (up to a maximum

# number of times currently set to ).

#

# If script exits with "" (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 .

#

# A script has a maximum running time of  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>

#

# 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.

# 指定sentinel检测到该监控的redis实例指向的实例异常时，调用的报警脚本。该配置项可选，但是很常用。

#

# 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

文档：

https://redis.io/topics/sentinel

https://redis.io/topics/sentinel-clients

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