mycat实现mysql读写分离

1. Mycat高可用参数

有关Mycat高可用的schema.xml配置参数相关说明：

balance属性
负载均衡类型，目前的取值有如下4种。
* balance="0" : 不开启读写分离机制，所有读操作都发送到当前可用的writeHost上。
* balance="1" : 全部的readHost与stand by writeHost都参与select语句的负载均衡，简而言之，当为双主双从模式（M1->S1，M2->S2，并且M1与M2互为主备）时，在正常情况下，M2、S1和S2都参与select语句的负载均衡。
* balance="2" : 所有的读操作都随机地在writeHost、readHost上分发。
* balance="3" : 所有的读请求都随机分发到writeHost对应的readHost上执行，writeHost不负担读压力。

writeType属性
负载均衡类型目前的取值有两种。
* writeType="0" : 所有的写操作都发送到配置的第1个writeHost上，writeHost挂了则切到writeHost2上，重新恢复writeHost1节点后，不会再切回来，还是以writeHost2为准，切换记录在配置文件dnindex.properties中。
* writeType="0" : 所有的写操作都随机地发送到配置的writeHost上，Mycat1.5版本以后不再推荐使用该值。

switchType属性
* switchType="-1" : 表示不自动切换。
* switchType="1" : 为默认值（或为0），表示自动切换。
* switchType="2" : 表示基于MySQL主从同步的状态决定是否切换，心跳语句如下：

show slave status

switchType="3" : 表示基于 MySQL Galary Cluster的切换机制，心跳语句如下：

show status like 'wsrep%'

2. MySQL读写分离master-slave

对于MySQL，主流的读写分离是master-slave和galera cluster，下面分别介绍如何通过Mycat来实现这两种读写分离。

2.1 MySQL主从读写分离环境

Mycat:192.168.33.11 Port: 8086/9066 MySQL-master:192.168.33.11 Port:3306 MySQL-slaveA :192.168.33.12 Port :3306 MySQL-slaveB:192.168.33.13 Port: 33

MySQL为一主两从，192.168.33.11同时为192.168.33.12和192.168.33.13的主

[root@testA ~]# mysql -uroot -p123456
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 117
Server version: 5.7.19-log Source distribution

Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> grant replication slave on *.* to 'repl'@'192.168.33.12' identified by '123456';              
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> grant replication slave on *.* to 'repl'@'192.168.33.13' identified by '123456';       
Query OK, 0 rows affected, 1 warning (0.02 sec)

mysql>

[root@testB ~]# mysql -uroot -p123456
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 61
Server version: 5.7.19-log Source distribution

Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> change master to master_host='192.168.33.11',master_port=3306,master_user='repl',master_password='123456',MASTER_AUTO_POSITION=1;
Query OK, 0 rows affected, 2 warnings (2.61 sec)

mysql> start slave;
Query OK, 0 rows affected (0.23 sec)

[root@testC ~]# mysql -uroot -p123456
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 48
Server version: 5.7.19-log Source distribution

Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> change master to master_host='192.168.33.11',master_port=3306,master_user='repl',master_password='123456',MASTER_AUTO_POSITION=1;
Query OK, 0 rows affected, 2 warnings (2.66 sec)

mysql> start slave;
Query OK, 0 rows affected (0.23 sec)

查看主从，都正常。

2.2 验证测试

vim conf/schema.xml

<!DOCTYPE mycat:schema SYSTEM "schema.dtd">
<mycat:schema xmlns:mycat="http://io.mycat/">
    <schema name="test01" checkSQLschema="false" sqlMaxLimit="100" dataNode="dn1">
    </schema>
    <dataNode name="dn1" dataHost="dh1" database="test01"/>
    <dataHost balance="0" maxCon="1000" minCon="10" name="dh1" writeType="0" switchType="2" slaveThreshold="100" dbType="mysql" dbDriver="native">
        <heartbeat>show slave status</heartbeat>
        <writeHost host="hostM1" url="192.168.33.11:3306" password="123456" user="root"/>
        <writeHost host="hostS2" url="192.168.33.12:3306" password="123456" user="root"/>
        <writeHost host="hostS3" url="192.168.33.13:3306" password="123456" user="root"/>
    </dataHost>
</mycat:schema>

测试SQL：

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CREATE TABLE t1  (`time`  datetime NOT NULL ,`db_nm`  varchar(20) NULL);  
INSERT INTO t1 (time,db_nm) VALUES (now(), database());  
INSERT INTO t1 (time,db_nm) VALUES (now(), database());  
INSERT INTO t1 (time,db_nm) VALUES (now(), database());

[root@testA mycat]# mysql -uroot -p123456 -h127.0.0.1 -P8066 test01
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 3
Server version: 5.6.29-mycat-1.6-RELEASE-20161028204710 MyCat Server (OpenCloundDB)

Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> CREATE TABLE t1  (`time`  datetime NOT NULL ,`db_nm`  varchar(20) NULL);  
INSERT INTO t1 (time,db_nm) VALUES (now(), database());  
INSERT INTO t1 (time,db_nm) VALUES (now(), database());  
INSERT INTO t1 (time,db_nm) VALUES (now(), database()); Query OK, 0 rows affected (0.04 sec)

mysql> INSERT INTO t1 (time,db_nm) VALUES (now(), database());  
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO t1 (time,db_nm) VALUES (now(), database());  
Query OK, 1 row affected (0.01 sec)

mysql> INSERT INTO t1 (time,db_nm) VALUES (now(), database()); 
Query OK, 1 row affected (0.00 sec)

mysql> select * from t1;
+---------------------+--------+
| time                | db_nm  |
+---------------------+--------+
| 2017-11-01 17:38:03 | test01 |
| 2017-11-01 17:38:03 | test01 |
| 2017-11-01 17:38:03 | test01 |
+---------------------+--------+
3 rows in set (0.01 sec)

开启DEBUG日志，

vim conf/log4j2.c

...
<Configuration status="DEBUG">
...
        <asyncRoot level="DEBUG" includeLocation="true">
...

可以从DEBUG日志中看到，读操作发送到当前的writeHost上。

tail -f logs/wrapper.log

INFO   | jvm 1    | 2017/11/01 18:39:21 | 2017-11-01 18:39:21,738 [DEBUG][$_NIOREACTOR-1-RW] select read source hostM1 for dataHost:dh1  (io.mycat.backend.datasource.PhysicalDBPool:PhysicalDBPool.java:456)

2.3 读写分离2种配置的区别

配置方式一

<writeHost host="hostM1" url="192.168.33.11:3306" password="123456" user="root"/>
<writeHost host="hostS2" url="192.168.33.12:3306" password="123456" user="root"/>
<writeHost host="hostS3" url="192.168.33.13:3306" password="123456" user="root"/>

配置方式二

<writeHost host="hostM1" url="192.168.33.11:3306" password="123456" user="root">
    <readHost host="hostS2" url="192.168.33.12:3306" password="123456" user="root"/>
    <readHost host="hostS3" url="192.168.33.13:3306" password="123456" user="root"/>
</writeHost>

在hostM1、hostS1、hostS2均正常工作时，写均在hostM1上。读会有如下区别：

balance	方式一	方式二
0	读–>hostM1	读–>hostM1
1	读–>hostS2 or hostS3	读–>hostS2 or hostS3
2	读–>hostM1 or hostS2 or hostS3	读–>hostM1 or hostS2 or hostS3
3	读–>hostM1	读–>hostS2 or hostS3

在hostM1失去心跳，宕机时，两种配置方式的读写会有如下区别：

balance	方式一		方式二
balance	switchType=-1	switchType=0或1	switchType=-1	switchType=0或1
0	读–>X 写–>X	读–>hostS2 写–>hostS2	读–>X 写–>X
1	读–>hostS2 写–>X	读–>hostS2 写–>hostS2
2	读–>hostS2 写–>X	读–>hostS2 写–>hostS2
3	读–>X 写–>X	读–>hostS2 写–>hostS2

主从切换记录文件
cat conf/dnindex.properties

#update
#Mon Oct 23 18:44:40 CST 2017
dh1=0

注意

* 当发生主从切换后，要查看集群内所有mycat的此文件是否一致。
* 当主挂了以后，备接替主以后，再加入集群的DB是从的角色。
* 主从切换后可能会存在数据的丢失。
* mycat的主从切换，并不是mysql的主从切换，mysql的主从设置并不会改变。