Redis 高可用--Sentinel

官网：

https://redis.io/topics/sentinel

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

所有的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一下几种参数，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。