主从复制概述
主从复制，是指将一台Redis服务器的数据，复制到其他的Redis服务器。前者称为主节点(master)，后者称为从节点(slave)；数据的复制是单向的，只能由主节点到从节点。Master以写为主，Slave已读为主 默认情况下，每台Redis服务器都是主节点；且一个主节点可以有多个从节点(或没有从节点)，但一个从节点只能有一个主节点。
主从复制的作用 主从复制的作用主要包括： 数据冗余：主从复制实现了数据的热备份，是持久化之外的一种数据冗余方式。 故障恢复：当主节点出现问题时，可以由从节点提供服务，实现快速的故障恢复；实际上是一种服务的冗余。 负载均衡：在主从复制的基础上，配合读写分离，可以由主节点提供写服务，由从节点提供读服务（即写Redis数据时应用连接主节点，读Redis数据时应用连接从节点），分担服务器负载；尤其是在写少读多的场景下，通过多个从节点分担读负载，可以大大提高Redis服务器的并发量。 高可用基石：除了上述作用以外，主从复制还是哨兵和集群能够实施的基础，因此说主从复制是Redis高可用的基础。
环境配置
只配置从库，不用配置主库
代码语言：javascript复制127.0.0.1:6379> info replication
# Replication
role:master
connected_slaves:0
master_replid:f47467f195c4ed51c50938a60c17de4b01bb0f72
master_replid2:0000000000000000000000000000000000000000
master_repl_offset:0
second_repl_offset:-1
repl_backlog_active:0
repl_backlog_size:1048576
repl_backlog_first_byte_offset:0
repl_backlog_histlen:0复制3个配置文件，然后修改对应的信息 1、端口 2、pid名字 3、log文件名字 4、 dump.rdp名字 一主二从 从机
代码语言：javascript复制127.0.0.1:6380> SLAVEOF 127.0.0.1 6379 #slaveof host 6379
OK
127.0.0.1:6380> info replication
# Replication
role:slave
master_host:127.0.0.1
master_port:6379
master_link_status:up
master_last_io_seconds_ago:2
master_sync_in_progress:0
slave_repl_offset:42
slave_priority:100
slave_read_only:1
connected_slaves:0
master_replid:da314952dadd611849cb227b8706af7bcd8af486
master_replid2:0000000000000000000000000000000000000000
master_repl_offset:42
second_repl_offset:-1
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:1
repl_backlog_histlen:42
127.0.0.1:6381> SLAVEOF 127.0.0.1 6379 #slaveof host 6379
OK
127.0.0.1:6381> info replication
# Replication
role:slave
master_host:127.0.0.1
master_port:6379
master_link_status:up
master_last_io_seconds_ago:8
master_sync_in_progress:0
slave_repl_offset:224
slave_priority:100
slave_read_only:1
connected_slaves:0
master_replid:da314952dadd611849cb227b8706af7bcd8af486
master_replid2:0000000000000000000000000000000000000000
master_repl_offset:224
second_repl_offset:-1
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:113
repl_backlog_histlen:112建议在配置文件中设置，用命令的话是暂时的 主机可写，从机可读
如果主机断开连接，从机依旧可以连接到主机，但是没有写的权限，一旦主机恢复，从机就可以再次读取主机信息。 如果是用命令行配置的从机，一旦断开连接就会变成主机，需要重新配置，但是在配置文件配置的就不会出现这种情况
主从复制（复制原理）
Slave启动成功连接到master后会发送一个sync同步命令 Master接到命令,启动后台的存盘进程,同时收集所有接收到的用于修改数据集命令,在后台进程执行完毕之后, master将传送整个数据文件到slave , 并完成一-次完全同步。 全量复制:而slave服务在接收到数据库文件数据后,将其存盘并加载到内存中。 增量复制: Master继续将新的所有收集到的修改命令依次传给slave ,完成同步 但是只要是重新连接master , 一次完全同步(全量复制)将被自动执行
层层链路
上一个M链接下一个S
这个时候如果主机（79）宕机，我们可以用SLAVEOF no one80从机就会变成主机,如果79主机恢复，需要从新配置
哨兵模式
主从切换技术的方法是:当主服务器宕机后,需要手动把一台从服务器切换为主服务器,这就需要人工干预 ,费事费力,还会造成一段时间内服务不可用。这不是一种推荐的方式，更多时候，我们优先考虑哨兵模式。Redis从2.8开始正式提供 了Sentinel (哨兵)架构来解决这个问题。 谋朝篡位的自动版,能够后台监控主机是否故障,如果故障了根据投票数自动将从库转换为主库. 哨兵模式是一种特殊的模式,首先Redis提供了哨兵的命令,哨兵是一一个独立的进程 ,作为进程,它会独立运行。其原理是哨兵通过发送命令,等待Redis服务器响应,从而监控运行的多个Redis实例。
这里的哨兵有两个作用
- 通过发送命令,让Redis服务器返回监控其运行状态,包括主服务器和从服务器。
- 当哨兵监测到master宕机,会自动将slave切换成master ,然后通过发布订阅模式通知其他的从服务器,修改配置文件,让它们切换主机。 然而一个哨兵进程对Redis服务器进行监控,可能会出现问题,为此,我们可以使用多个哨兵进行监控。各个哨兵之间还会进行监控,这样就形成了多哨兵模式。
假设主服务器宕机,哨兵1先检测到这个结果,系统并不会马上进行failover过程,仅仅是哨兵1主观的认为主服务器不可用,这个现象成为主观下线。当后面的哨兵也检测到主服务器不可用,并且数量达到一定值时,那么哨兵之间就会进行一次投票 ,投票的结果由一个哨兵发起,进行failover[故障转移]操作。切换成功后,就会通过发布订阅模式,让各个哨兵把自己监控的从服务器实现切换主机,这个过程称为客观下线。
测试
配置哨兵配置文件 vim sentinel.conf
代码语言：javascript复制# 监听端口
port 26379
# 后台运行
daemonize yes
#pid文件
pidfile /var/run/redis-sentinel.pid
# 哨兵得日志文件
logfile "sentinel.log"
# 日志文件存放路径
dir /www/server/redis/
# 设置初始master以及法定认为下线人数：
#sentinel monitor
sentinel monitor mymaster 192.168.0.10 6379 1
# master主观下线时间,默认30秒，30秒内没有回复pong，则认为下线了
#sentinel down-after-milliseconds
sentinel down-after-milliseconds mymaster 30000
# 指定故障转移超时时间，默认为3分钟
sentinel failover-timeout mymaster 180000
# 设置通知脚本，发生故障转移可以向管理员发送通知（可选）
sentinel notification-script mymaster /www/server/redis/notify.sh
# 禁止修改脚本，避免脚本重置
sentinel deny-scripts-reconfig yes启动哨兵模式
代码语言：javascript复制[root@cs bin]# redis-sentinel xhconfig/sentinel.conf 如果master节点断开了，这个时候就会从从机随机选一个从机当主机
如果主机回来了，只能归并到新的主机下，当作从机
优点: 1、哨兵集群,基于主从复制模式,所有的主从配置优点,它全有 2、主从可以切换,故障可以转移,系统的可用性就会更好 3、哨兵模式就是主从模式的升级,手动到自动,更加健壮! 缺点: 1、Redis 不好啊在线扩容的,集群容量一旦到达上限,在线扩容就十分麻烦! 2、实现哨兵模式的配置其实是很麻烦的,里面有很多选择!
代码语言：javascript复制哨兵全部配置
[root@cs bin]# cat /opt/redis-5.0.13/sentinel.conf
# 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.
#
# 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
# port
# The port that this sentinel instance will run on
port 26379
# By default Redis Sentinel does not run as a daemon. Use 'yes' if you need it.
# Note that Redis will write a pid file in /var/run/redis-sentinel.pid when
# daemonized.
daemonize no
# When running daemonized, Redis Sentinel writes a pid file in
# /var/run/redis-sentinel.pid by default. You can specify a custom pid file
# location here.
pidfile /var/run/redis-sentinel.pid
# Specify the log file name. Also the empty string can be used to force
# Sentinel to log on the standard output. Note that if you use standard
# output for logging but daemonize, logs will be sent to /dev/null
logfile ""
# sentinel announce-ip
# sentinel announce-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
# 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
#
# Tells Sentinel to monitor this master, and to consider it in O_DOWN
# (Objectively Down) state only if at least 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.
#
# Replicas are auto-discovered, so you don't need to specify replicas in
# any way. Sentinel itself will rewrite this configuration file adding
# the replicas using additional configuration options.
# Also note that the configuration file is rewritten when a
# replica 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
#
# Set the password to use to authenticate with the master and replicas.
# Useful if there is a password set in the Redis instances to monitor.
#
# Note that the master password is also used for replicas, so it is not
# possible to set a different password in masters and replicas 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
#
# Number of milliseconds the master (or any attached replica 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).
#
# Default is 30 seconds.
sentinel down-after-milliseconds mymaster 30000
# sentinel parallel-syncs
#
# How many replicas we can reconfigure to point to the new replica simultaneously
# during the failover. Use a low number if you use the replicas to serve query
# to avoid that all the replicas will be unreachable at about the same
# time while performing the synchronization with the master.
sentinel parallel-syncs mymaster 1
# sentinel failover-timeout
#
# 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 replica 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 replica).
#
# - The maximum time a failover in progress waits for all the replicas to be
# reconfigured as replicas of the new master. However even after this time
# the replicas 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
#
# 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
#
# 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:
#
#
#
# is currently always "failover"
# 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 replica
# (now a master).
#
# This script should be resistant to multiple invocations.
#
# Example:
#
# sentinel client-reconfig-script mymaster /var/redis/reconfig.sh
# SECURITY
#
# By default SENTINEL SET will not be able to change the notification-script
# and client-reconfig-script at runtime. This avoids a trivial security issue
# where clients can set the script to anything and trigger a failover in order
# to get the program executed.
sentinel deny-scripts-reconfig yes
# REDIS COMMANDS RENAMING
#
# Sometimes the Redis server has certain commands, that are needed for Sentinel
# to work correctly, renamed to unguessable strings. This is often the case
# of CONFIG and SLAVEOF in the context of providers that provide Redis as
# a service, and don't want the customers to reconfigure the instances outside
# of the administration console.
#
# In such case it is possible to tell Sentinel to use different command names
# instead of the normal ones. For example if the master "mymaster", and the
# associated replicas, have "CONFIG" all renamed to "GUESSME", I could use:
#
# SENTINEL rename-command mymaster CONFIG GUESSME
#
# After such configuration is set, every time Sentinel would use CONFIG it will
# use GUESSME instead. Note that there is no actual need to respect the command
# case, so writing "config guessme" is the same in the example above.
#
# SENTINEL SET can also be used in order to perform this configuration at runtime.
#
# In order to set a command back to its original name (undo the renaming), it
# is possible to just rename a command to itsef:
#
# SENTINEL rename-command mymaster CONFIG CONFIG
