# 《Mycat核心技术》第13章：取模分片
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- 本章难度：★★☆☆☆
- 本章重点：介绍Mycat取模分片，掌握Mycat取模分片的核心原理与算法，能够结合自身实际项目将Mycat取模分片灵活应用到自身实际项目中。
大家好，我是CurleyG~~
今天给大家介绍《Mycat核心技术》的第13章：给大家简单介绍下Mycat取模分片，好了，开始今天的内容。
# 一、简单描述
取模分片，就是根据数据表的某一个字段，通常是某一个整数型的字段，对其进行十进制的求模运算，将运算结果作为Mycat的路由结果，具体规则如下：
此分片算法根据id进行十进制求模运算，相比固定的分片hash，这种分片算法在批量插入时会增加事务一致性的难度。
# 二、实现取模分片
# 2.1 配置rule.xml
在rule.xml中添加如下配置
<tableRule name="mod-long">
<rule>
<columns>id</columns>
<algorithm>mod-long</algorithm>
</rule>
</tableRule>
<function name="mod-long" class="org.opencloudb.route.function.PartitionByMod">
<!-- how many data nodes -->
<property name="count">3</property>
</function>
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# 2.2 配置schema.xml
<?xml version="1.0"?>
<!DOCTYPE mycat:schema SYSTEM "schema.dtd">
<mycat:schema xmlns:mycat="http://org.opencloudb/" >
<schema name="lyzdb" checkSQLschema="false" sqlMaxLimit="100">
<!-- global table is auto cloned to all defined data nodes ,so can join
with any table whose sharding node is in the same data node -->
<table name="t_user" primaryKey="id" dataNode="dn1,dn2,dn3" rule="mod-long"/>
</schema>
<!-- <dataNode name="dn1$0-743" dataHost="localhost1" database="db$0-743"
/> -->
<dataNode name="dn1" dataHost="localhost1" database="db1" />
<dataNode name="dn2" dataHost="localhost1" database="db2" />
<dataNode name="dn3" dataHost="localhost1" database="db3" />
<!--<dataNode name="dn4" dataHost="sequoiadb1" database="SAMPLE" />
<dataNode name="jdbc_dn1" dataHost="jdbchost" database="db1" />
<dataNode name="jdbc_dn2" dataHost="jdbchost" database="db2" />
<dataNode name="jdbc_dn3" dataHost="jdbchost" database="db3" /> -->
<dataHost name="localhost1" maxCon="1000" minCon="10" balance="0"
writeType="0" dbType="mysql" dbDriver="native" switchType="1" slaveThreshold="100">
<heartbeat>select user()</heartbeat>
<!-- can have multi write hosts -->
<writeHost host="hostM1" url="192.168.209.137:3306" user="root" password="root"></writeHost>
<!--<writeHost host="hostS1" url="localhost:3316" user="root"-->
<!--password="123456" />-->
<!-- <writeHost host="hostM2" url="localhost:3316" user="root" password="123456"/> -->
</dataHost>
</mycat:schema>
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至此，Mycat的配置工作就算完成了，下面，我们一起来测试下Mycat的路由结果。
# 三、测试路由结果
# 3.1 创建数据表
mysql> explain CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME);
+-----------+------------------------------------------------------------------------------------------------------------------------+
| DATA_NODE | SQL |
+-----------+------------------------------------------------------------------------------------------------------------------------+
| dn1 | CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME) |
| dn2 | CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME) |
| dn3 | CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME) |
+-----------+------------------------------------------------------------------------------------------------------------------------+
3 rows in set (0.00 sec)
mysql> CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME);
Query OK, 0 rows affected (0.21 sec)
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Mycat日志如下：
08/13 21:44:36.871 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME), route={
1 -> dn1{CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME)}
2 -> dn2{CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME)}
3 -> dn3{CREATE TABLE t_user (ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,name VARCHAR(64),sex VARCHAR(2), CREATE_TIME DATETIME)}
} rrs
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由此可见，当创建数据表时，Mycat将SQL路由到所有的数据节点。
# 3.2 录入数据
mysql> explain insert into t_user(id,name, sex, CREATE_TIME) values(1,'lyz01', 'n', NOW());
+-----------+-----------------------------------------------------------------------------+
| DATA_NODE | SQL |
+-----------+-----------------------------------------------------------------------------+
| dn2 | insert into t_user(id,name, sex, CREATE_TIME) values(1,'lyz01', 'n', NOW()) |
+-----------+-----------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql>
mysql>
mysql> insert into t_user(id,name, sex, CREATE_TIME) values(1,'lyz01', 'n', NOW());
Query OK, 1 row affected (0.10 sec)
mysql> explain insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW());
+-----------+-----------------------------------------------------------------------------+
| DATA_NODE | SQL |
+-----------+-----------------------------------------------------------------------------+
| dn3 | insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW()) |
+-----------+-----------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW());
Query OK, 1 row affected (0.04 sec)
mysql> explain insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW());
+-----------+-----------------------------------------------------------------------------+
| DATA_NODE | SQL |
+-----------+-----------------------------------------------------------------------------+
| dn1 | insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW()) |
+-----------+-----------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW());
Query OK, 1 row affected (0.01 sec)
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Mycat日志如下：
08/13 21:51:45.221 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]insert into t_user(id,name, sex, CREATE_TIME) values(1,'lyz01', 'n', NOW()), route={
1 -> dn2{insert into t_user(id,name, sex, CREATE_TIME) values(1,'lyz01', 'n', NOW())}
} rrs
insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW());
08/13 21:52:53.005 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW()), route={
1 -> dn3{insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW())}
} rrs
insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW());
08/13 21:54:00.546 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW()), route={
1 -> dn1{insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW())}
} rrs
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由此可见，录入数据时，Mycat根据id进行十进制求模运算，将运算结果作为路由结果，将SQL路由到指定的数据分片节点，符合我们配置的分片规则。
# 3.3 指定数据分片字段查询
08/13 21:51:45.221 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]insert into t_user(id,name, sex, CREATE_TIME) values(1,'lyz01', 'n', NOW()), route={
1 -> dn2{insert into t_user(id,name, sex, CREATE_TIME) values(1,'lyz01', 'n', NOW())}
} rrs
insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW());
08/13 21:52:53.005 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW()), route={
1 -> dn3{insert into t_user(id,name, sex, CREATE_TIME) values(2,'lyz02', 'n', NOW())}
} rrs
insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW());
08/13 21:54:00.546 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW()), route={
1 -> dn1{insert into t_user(id,name, sex, CREATE_TIME) values(3,'lyz03', 'n', NOW())}
} rrs
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Mycat日志如下：
08/13 21:55:57.257 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]select * from t_user where id = 1, route={
1 -> dn2{select * from t_user where id = 1}
} rr
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由此可见，执行简单的查询，如果指定分片字段，则走分片查询单个分片节点。
# 3.4 按照分片字段范围查询
mysql> explain select * from t_user where id >=3;
+-----------+----------------------------------------------+
| DATA_NODE | SQL |
+-----------+----------------------------------------------+
| dn1 | SELECT * FROM t_user WHERE id >= 3 LIMIT 100 |
| dn2 | SELECT * FROM t_user WHERE id >= 3 LIMIT 100 |
| dn3 | SELECT * FROM t_user WHERE id >= 3 LIMIT 100 |
+-----------+----------------------------------------------+
3 rows in set (0.02 sec)
mysql> select * from t_user where id >=3;
+----+-------+------+---------------------+
| ID | name | sex | CREATE_TIME |
+----+-------+------+---------------------+
| 3 | lyz03 | n | 2017-08-13 21:54:00 |
+----+-------+------+---------------------+
1 row in set (0.04 sec)
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Mycat日志如下：
08/13 21:58:08.598 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]select * from t_user where id >=3, route={
1 -> dn1{SELECT *
FROM t_user
WHERE id >= 3
LIMIT 100}
2 -> dn2{SELECT *
FROM t_user
WHERE id >= 3
LIMIT 100}
3 -> dn3{SELECT *
FROM t_user
WHERE id >= 3
LIMIT 100}
} rrs
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由此可见，如果分片字段范围的查询，则走所有节点去检索，哪怕只有一条数据在一个分片上，route路由也是走所有的分片进行检索查询。
# 3.5 按照非分片字段查询
mysql> explain select * from t_user where name = 'lyz01';
+-----------+-----------------------------------------------------+
| DATA_NODE | SQL |
+-----------+-----------------------------------------------------+
| dn1 | SELECT * FROM t_user WHERE name = 'lyz01' LIMIT 100 |
| dn2 | SELECT * FROM t_user WHERE name = 'lyz01' LIMIT 100 |
| dn3 | SELECT * FROM t_user WHERE name = 'lyz01' LIMIT 100 |
+-----------+-----------------------------------------------------+
3 rows in set (0.00 sec)
mysql>
mysql> select * from t_user where name = 'lyz01';
+----+-------+------+---------------------+
| ID | name | sex | CREATE_TIME |
+----+-------+------+---------------------+
| 1 | lyz01 | n | 2017-08-13 21:51:45 |
+----+-------+------+---------------------+
1 row in set (0.01 sec)
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Mycat日志如下：
08/13 21:59:01.166 DEBUG [$_NIOREACTOR-0-RW] (NonBlockingSession.java:113) -ServerConnection [id=1, schema=lyzdb, host=192.168.209.1, user=lyz,txIsolation=3, autocommit=true, schema=lyzdb]select * from t_user where name = 'lyz01', route={
1 -> dn1{SELECT *
FROM t_user
WHERE name = 'lyz01'
LIMIT 100}
2 -> dn2{SELECT *
FROM t_user
WHERE name = 'lyz01'
LIMIT 100}
3 -> dn3{SELECT *
FROM t_user
WHERE name = 'lyz01'
LIMIT 100}
} rrs
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由此可见，如果不走分片字段的查询，即使是单个数据，也要route路由所有的分片，走所有的分片进行查询。
