Redisreplication主从复制原理及配置-创新互联
本文主要介绍Redis replication 主从复制原理和配置及基本操作 等
主要参考官方文档：
/tupian/20230521/
/tupian/20230521/replication.html
一.原理
This system works using three main mechanisms:
系统工作的三个主要机制：
1. When a master and a slave instances are well-connected, the master keeps the slave updated by sending a stream of commands to the slave, in order to replicate the effects on the dataset happening in the master side due to: client writes, keys expired or evicted, any other action changing the master dataset.
当master和slave实例连接良好时，master通过从slave发送命令流来更新slave
2. When the link between the master and the slave breaks, for network issues or because a timeout is sensed in the master or the slave, the slave reconnects and attempts to proceed with a partial resynchronization: it means that it will try to just obtain the part of the stream of commands it missed during the disconnection.
当master和slave连接中断时，因为网络故障或者感知到master或者slave超时。Slave重连并试图进行重新同步：意味着获取失去连接时丢失的命令流
3. When a partial resynchronization is not possible, the slave will ask for a full resynchronization. This will involve a more complex process in which the master needs to create a snapshot of all its data, send it to the slave, and then continue sending the stream of commands as the dataset changes.
当无法重新部分同步(partial resynchronization)时，slave将要求完全重新同步(full resynchronization)。它将包含一个复杂过程，这过程中master需要为所有数据创建一个快照，发送到slave，再继续对变化的数据发送命令流
数据安全性概念
Redis的持久化存储主要提供两种方式：RDB与AOF
/tupian/20230521/persistence.html
1. RDB(Snapshot)模式
是默认的，即保存某一时刻的数据到硬盘，在下一次触发snapshot前如果服务器crash，在上次snapshot之后修改的数据会丢失。
主要配置redis.conf参数：
save
stop-writes-on-bgsave-error yes
rdbcompression yes
dbfilename dump.rdb
dir ./
参数说明：
save
stop-writes-on-bgsave-error：在bgsave遇到error的时候是否停止持久化，默认是yes代表是，no代表不是
rdbcompression：是否压缩，默认是yes代表是，no代表不是，如果想节省CPU的话就设为no，但rdb文件会比较大
dbfilename：持久化的文件名字，默认是dump.rdb
dir：持久化的目录名字，默认是redis.conf所在的目录./
2.AOF(append-only file)模式
需要手动开启，开启后以追加的方式默认每秒钟将记录的修改操作保存到硬盘
主要配置redis.conf参数：
appendonly yes
appendfilename "appendonly.aof"
appendfsync everysec
dir ./
### 以下rewrite参数，AOF模式以追加方式记录所有写操作命令到硬盘，文件会越来越大，为缓解这种问题，redis使用了BGREWRITEAOF用于删除重复多余的写命令，类似BGSAVE，rewrite的时候会删除旧的AOF文件
###
no-appendfsync-on-rewrite no
auto-aof-rewrite-percentage 100
auto-aof-rewrite-min-size 64mb
参数说明：
appendonly：是否启动aof，默认是no代表不启用
appendfilename：aof的文件名，默认是appendonly.aof
appendfsync：触发的间隔，默认是everysec代表每秒，还可使用always代表有改变都触发，性能差些但数据最安全，no代表让OS自己决定什么时候执行，性能最好但数据不安全
dir：持久化的目录名字，默认是redis.conf所在的目录./
no-appendfsync-on-rewrite：执行rewrite时appendfsync是否暂停，默认no代表不暂停
auto-aof-rewrite-percentage：aof rewrite触发时机，当前aof文件大小超过上一次rewrite后aof文件的百分比后触发rewrite。如200，即当前的aof文件超过上一次重写后aof文件的2倍时才会再次rewrite
auto-aof-rewrite-min-size aof文件重写最小文件大小，即最开始aof文件必须要达到这个文件时才触发，后面的每次重写就不会根据这个变量了(根据上一次重写完成之后的大小).此变量仅初始化启动redis有效.如果是redis恢复时，则lastSize等于初始aof文件大小.
在使用redis复制的设置中，强烈建议在主服务器和从服务器中打开持久性。如果不能开启的话，例如由于磁盘速度非常慢而引起的延迟问题，则应避免在重启OS后自动重新启动服务，以免数据丢失。官方对此有做举例说明：
To better understand why masters with persistence turned off configured to auto restart are dangerous, check the following failure mode where data is wiped from the master and all its slaves:
1. We have a setup with node A acting as master, with persistence turned down, and nodes B and C replicating from node A.
2. Node A crashes, however it has some auto-restart system, that restarts the process. However since persistence is turned off, the node restarts with an empty data set.
3. Nodes B and C will replicate from node A, which is empty, so they'll effectively destroy their copy of the data.
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