MyCat专题(五)-MyCat基于MySQL实现主从切换

1.简介

主从切换：顾名思义，就是MySQL写节点出现宕机后，会自动切换到备用节点（也就是把从机也配置成writeHost），这样的话，第一个writeHost故障后，会自动切换到第二个，第二个故障后自动切换到第三个…以此类推；

如果当是1主3从模式，可以把第1个从节点配置为writeHost2，第2个和第3个从节点则配置为writeHost1的readHost；

一般情况下，都会结合读写分离一起实现，本文也是接着上一篇读写分离的文章的示例改造，如果没有查看的话可以先看https://blog.csdn.net/BruceLiu_code/article/details/104727028【MyCat基于MySQL实现读写分离】

2.简介

要实现mysql主从切换，主要有以下几点：

【a】修改schema.xml，配置主从切换switchType=”2” 与心跳show slave status

：

<mycat:schema xmlns:mycat="http://io.mycat/">

	<schema name="TESTDB" checkSQLschema="false" sqlMaxLimit="100">
			<table name="user" primaryKey="id" rule="mod-long" dataNode="dn1,dn2" />
	schema>
	 
	<dataNode name="dn1" dataHost="localhost1" database="septwriteread" />
	 <dataNode name="dn2" dataHost="localhost1" database="septwriteread02" />
	 
	<dataHost name="localhost1" maxCon="1000" minCon="10" balance="1"
					  writeType="0" dbType="mysql" dbDriver="native" switchType="2"  slaveThreshold="100">
			<heartbeat>show slave statusheartbeat>
			
			
			<writeHost host="hostM1" url="192.168.70.128:3306" user="root"
							   password="123">
					
					<readHost host="hostS1" url="192.168.70.130:3306" user="root" password="123" />
			writeHost>
			
			<writeHost host="hostM2" url="192.168.70.130:3306" user="root"
							   password="123">
			writeHost>
	dataHost>
	
mycat:schema>

注意点：

MyCAT心跳检查询句配置为 show slave status ，dataHost 上定义两个新属性： switchType=”2” 与slaveThreshold=”100”，此时意味着开启MySQL主从复制状态绑定的读写分离与切换机制，Mycat心跳机制通过检测 show slave status 中的 “Seconds_Behind_Master”,”Slave_IO_Running”,”Slave_SQL_Running”三个字段来确定当前主从同步的状态及Seconds_Behind_Master主从复制时延，当Seconds_Behind_Master>slaveThreshold时，读写分离筛选器会过滤掉此Slave机器，防止读到很久以前的旧数据，当主节点宕机后，切换逻辑会检查Slave上的Seconds_Behind_Master是否为0，为0时则表示主仅同步，可安全切换，否则不会切换。

【b】关闭主服务器192.168.70.128的mysql服务

systemctl stop mariadb.service

查看日志可见，主MySQL已经与从服务器失去了连接。

java.net.ConnectException: Connection refused
	at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method) ~[?:1.8.0_144]
	at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:717) ~[?:1.8.0_144]
	at io.mycat.net.NIOConnector.finishConnect(NIOConnector.java:164) ~[Mycat-server-1.6.7.1-release.jar:?]
	at io.mycat.net.NIOConnector.finishConnect(NIOConnector.java:143) ~[Mycat-server-1.6.7.1-release.jar:?]
	at io.mycat.net.NIOConnector.run(NIOConnector.java:98) ~[Mycat-server-1.6.7.1-release.jar:?]
2019-12-26 13:55:58.402  INFO [$_NIOConnector] (io.mycat.net.AbstractConnection.close(AbstractConnection.java:520)) - close connection,reason:java.net.ConnectException: Connection refused ,MySQLConnection [id=0, lastTime=1577339758387, user=root, schema=septwriteread02, old shema=septwriteread02, borrowed=false, fromSlaveDB=false, threadId=0, charset=utf8, txIsolation=3, autocommit=true, attachment=null, respHandler=null, host=192.168.70.128, port=3306, statusSync=null, writeQueue=0, modifiedSQLExecuted=false]
2019-12-26 13:55:58.402  INFO [$_NIOConnector] (io.mycat.sqlengine.SQLJob.connectionError(SQLJob.java:125)) - can't get connection for sql :show slave status
2019-12-26 13:55:58.402 ERROR [$_NIOConnector] (io.mycat.backend.heartbeat.MySQLHeartbeat.nextDector(MySQLHeartbeat.java:215)) - set Error 2  DBHostConfig [hostName=hostM1, url=192.168.70.128:3306]

【c】登录mycat，执行如下查询：

select * from user;

查看mycat日志，看下查询请求被分发到哪一个节点上执行

2019-12-26 13:58:43.432 DEBUG [$_NIOREACTOR-0-RW] (io.mycat.backend.datasource.PhysicalDBPool.getRWBanlanceCon(PhysicalDBPool.java:551)) - select read source hostM2 for dataHost:localhost1
2019-12-26 13:58:43.432 DEBUG [$_NIOREACTOR-0-RW] (io.mycat.backend.mysql.nio.MySQLConnection.synAndDoExecute(MySQLConnection.java:463)) - con need syn ,total syn cmd 2 commands SET names utf8mb4;schema change:true con:MySQLConnection [id=14, lastTime=1577339923432, user=root, schema=septwriteread, old shema=septwriteread02, borrowed=true, fromSlaveDB=false, threadId=45, charset=utf8, txIsolation=3, autocommit=true, attachment=dn1{SHOW STATUS}, respHandler=SingleNodeHandler [node=dn1{SHOW STATUS}, packetId=0], host=192.168.70.130, port=3306, statusSync=null, writeQueue=0, modifiedSQLExecuted=false]
2019-12-26 13:58:43.437 DEBUG [$_NIOREACTOR-0-RW] (io.mycat.server.NonBlockingSession.releaseConnection(NonBlockingSession.java:386)) - release connection MySQLConnection [id=14, lastTime=1577339923408, user=root, schema=septwriteread, old shema=septwriteread, borrowed=true, fromSlaveDB=false, threadId=45, charset=utf8, txIsolation=3, autocommit=true, attachment=dn1{SHOW STATUS}, respHandler=SingleNodeHandler [node=dn1{SHOW STATUS}, packetId=-95], host=192.168.70.130, port=3306, statusSync=io.mycat.backend.mysql.nio.MySQLConnection$StatusSync@7911636, writeQueue=0, modifiedSQLExecuted=false]
2019-12-26 13:58:43.437 DEBUG [$_NIOREACTOR-0-RW] (io.mycat.backend.datasource.PhysicalDatasource.releaseChannel(PhysicalDatasource.java:633)) - release channel MySQLConnection [id=14, lastTime=1577339923408, user=root, schema=septwriteread, old shema=septwriteread, borrowed=true, fromSlaveDB=false, threadId=45, charset=utf8, txIsolation=3, autocommit=true, attachment=null, respHandler=null, host=192.168.70.130, port=3306, statusSync=null, writeQueue=0, modifiedSQLExecuted=false]

可见，查询请求被分发到M2节点上，即备用的writeHost。

【d】接着，我们执行插入操作

insert into user(id,name ) values(6666666,database());

插入成功，然后我们查看 dnindex.properties，可见当前writeHost为第二个writeHost（备用写节点192.168.8.131），Mycat的dnindex属性文件中writeHost顺序是从0开始，即0代表第一个

cat dnindex.properties

3.注意点

(1) MySQL主从复制配置中，如果涉及到函数或存储过程的同步复制，需要在/etc/my.cnf中的[mysqld]段中增加配置log_bin_trust_function_creators=true 或在客户端中设置 set global log_bin_trust_function_creators = 1；

(2) 如果要做读写分离下的主从切换，那么从节点也有可能会变为写节点，因此从节点就不能设置为只读 read_only=1 ；