面试题：SMM架构如何做读写分离？

Posted 2021-05-02 Java面试那些事儿

来源：http://1t.click/atjG

读写分离要做的事情就是对于一条SQL该选择哪个数据库去执行，至于谁来做选择数据库这件事儿，无非两个，要么中间件帮我们做，要么程序自己做。因此，一般来讲，读写分离有两种实现方式。

第一种是依靠中间件（比如：MyCat），也就是说应用程序连接到中间件，中间件帮我们做SQL分离；第二种是应用程序自己去做分离。这里我们选择程序自己来做，主要是利用Spring提供的路由数据源，以及AOP

然而，应用程序层面去做读写分离最大的弱点（不足之处）在于无法动态增加数据库节点，因为数据源配置都是写在配置中的，新增数据库意味着新加一个数据源，必然改配置，并重启应用。当然，好处就是相对简单。

# AbstractRoutingDataSource   

基于特定的查找key路由到特定的数据源。它内部维护了一组目标数据源，并且做了路由key与目标数据源之间的映射，提供基于key查找数据源的方法。

# 实践   

 maven依赖

<?xml version="1.0" encoding="UTF-8"?><project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion>
<groupId>com.cjs.example</groupId><artifactId>cjs-datasource-demo</artifactId><version>0.0.1-SNAPSHOT</version><packaging>jar</packaging>
<name>cjs-datasource-demo</name><description></description>
<parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>2.0.5.RELEASE</version><relativePath/> <!-- lookup parent from repository --></parent>
<properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding><java.version>1.8</java.version></properties>
<dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-aop</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-jdbc</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>1.3.2</version></dependency><dependency><groupId>org.apache.commons</groupId><artifactId>commons-lang3</artifactId><version>3.8</version></dependency>
<dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><scope>runtime</scope></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope></dependency></dependencies>
<build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin>

<!--<plugin> <groupId>org.mybatis.generator</groupId> <artifactId>mybatis-generator-maven-plugin</artifactId> <version>1.3.5</version> <dependencies> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.46</version> </dependency> </dependencies> <configuration> <configurationFile>${basedir}/src/main/resources/myBatisGeneratorConfig.xml</configurationFile> <overwrite>true</overwrite> </configuration> <executions> <execution> <id>Generate MyBatis Artifacts</id> <goals> <goal>generate</goal> </goals> </execution> </executions> </plugin>-->
</plugins></build></project>

数据源配置

application.yml

spring: datasource: master: jdbc-url: jdbc:mysql://192.168.102.31:3306/test username: root password: 123456 driver-class-name: com.mysql.jdbc.Driver slave1: jdbc-url: jdbc:mysql://192.168.102.56:3306/test username: pig # 只读账户 password: 123456 driver-class-name: com.mysql.jdbc.Driver slave2: jdbc-url: jdbc:mysql://192.168.102.36:3306/test username: pig # 只读账户 password: 123456 driver-class-name: com.mysql.jdbc.Driver

多数据源配置

package com.cjs.example.config;
import com.cjs.example.bean.MyRoutingDataSource;import com.cjs.example.enums.DBTypeEnum;import org.springframework.beans.factory.annotation.Qualifier;import org.springframework.boot.context.properties.ConfigurationProperties;import org.springframework.boot.jdbc.DataSourceBuilder;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Configuration;
import javax.sql.DataSource;import java.util.HashMap;import java.util.Map;
/** * 关于数据源配置，参考SpringBoot官方文档第79章《Data Access》 * 79. Data Access * 79.1 Configure a Custom DataSource * 79.2 Configure Two DataSources */
@Configurationpublic class DataSourceConfig {
@Bean@ConfigurationProperties("spring.datasource.master")public DataSource masterDataSource() {return DataSourceBuilder.create().build(); }
@Bean@ConfigurationProperties("spring.datasource.slave1")public DataSource slave1DataSource() {return DataSourceBuilder.create().build(); }
@Bean@ConfigurationProperties("spring.datasource.slave2")public DataSource slave2DataSource() {return DataSourceBuilder.create().build(); }
@Beanpublic DataSource myRoutingDataSource(@Qualifier("masterDataSource") DataSource masterDataSource,@Qualifier("slave1DataSource") DataSource slave1DataSource,@Qualifier("slave2DataSource") DataSource slave2DataSource) { Map<Object, Object> targetDataSources = new HashMap<>(); targetDataSources.put(DBTypeEnum.MASTER, masterDataSource); targetDataSources.put(DBTypeEnum.SLAVE1, slave1DataSource); targetDataSources.put(DBTypeEnum.SLAVE2, slave2DataSource); MyRoutingDataSource myRoutingDataSource = new MyRoutingDataSource(); myRoutingDataSource.setDefaultTargetDataSource(masterDataSource); myRoutingDataSource.setTargetDataSources(targetDataSources);return myRoutingDataSource; }
}

这里，我们配置了4个数据源，1个master，2两个slave，1个路由数据源。前3个数据源都是为了生成第4个数据源，而且后续我们只用这最后一个路由数据源。

MyBatis配置

package com.cjs.example.config;
import org.apache.ibatis.session.SqlSessionFactory;import org.mybatis.spring.SqlSessionFactoryBean;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Configuration;import org.springframework.core.io.support.PathMatchingResourcePatternResolver;import org.springframework.jdbc.datasource.DataSourceTransactionManager;import org.springframework.transaction.PlatformTransactionManager;import org.springframework.transaction.annotation.EnableTransactionManagement;
import javax.annotation.Resource;import javax.sql.DataSource;
@EnableTransactionManagement@Configurationpublic class MyBatisConfig {
@Resource(name = "myRoutingDataSource")private DataSource myRoutingDataSource;
@Beanpublic SqlSessionFactory sqlSessionFactory() throws Exception { SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean(); sqlSessionFactoryBean.setDataSource(myRoutingDataSource); sqlSessionFactoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources("classpath:mapper/*.xml"));return sqlSessionFactoryBean.getObject(); }
@Beanpublic PlatformTransactionManager platformTransactionManager() {return new DataSourceTransactionManager(myRoutingDataSource); }}

由于Spring容器中现在有4个数据源，所以我们需要为事务管理器和MyBatis手动指定一个明确的数据源。

设置路由key / 查找数据源

目标数据源就是那前3个这个我们是知道的，但是使用的时候是如果查找数据源的呢？

首先，我们定义一个枚举来代表这三个数据源

package com.cjs.example.enums;public enum DBTypeEnum {    MASTER, SLAVE1, SLAVE2;}

接下来，通过ThreadLocal将数据源设置到每个线程上下文中

package com.cjs.example.bean;
import com.cjs.example.enums.DBTypeEnum;
import java.util.concurrent.atomic.AtomicInteger;
public class DBContextHolder {
private static final ThreadLocal<DBTypeEnum> contextHolder = new ThreadLocal<>();
private static final AtomicInteger counter = new AtomicInteger(-1);
public static void set(DBTypeEnum dbType) { contextHolder.set(dbType); }
public static DBTypeEnum get() {return contextHolder.get(); }
public static void master() {set(DBTypeEnum.MASTER); System.out.println("切换到master"); }
public static void slave() {// 轮询int index = counter.getAndIncrement() % 2;if (counter.get() > 9999) { counter.set(-1); }if (index == 0) {set(DBTypeEnum.SLAVE1); System.out.println("切换到slave1"); }else {set(DBTypeEnum.SLAVE2); System.out.println("切换到slave2"); } }
}

获取路由key

package com.cjs.example.bean;
import org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource;import org.springframework.lang.Nullable;
public class MyRoutingDataSource extends AbstractRoutingDataSource {@Nullable@Overrideprotected Object determineCurrentLookupKey() {return DBContextHolder.get(); }
}

设置路由key

默认情况下，所有的查询都走从库，插入/修改/删除走主库。我们通过方法名来区分操作类型（CRUD）

package com.cjs.example.aop;
import com.cjs.example.bean.DBContextHolder;import org.apache.commons.lang3.StringUtils;import org.aspectj.lang.JoinPoint;import org.aspectj.lang.annotation.Aspect;import org.aspectj.lang.annotation.Before;import org.aspectj.lang.annotation.Pointcut;import org.springframework.stereotype.Component;
@Aspect@Componentpublic class DataSourceAop {
@Pointcut("!@annotation(com.cjs.example.annotation.Master) " +"&& (execution(* com.cjs.example.service..*.select*(..)) " +"|| execution(* com.cjs.example.service..*.get*(..)))")public void readPointcut() {
 }
@Pointcut("@annotation(com.cjs.example.annotation.Master) " +"|| execution(* com.cjs.example.service..*.insert*(..)) " +"|| execution(* com.cjs.example.service..*.add*(..)) " +"|| execution(* com.cjs.example.service..*.update*(..)) " +"|| execution(* com.cjs.example.service..*.edit*(..)) " +"|| execution(* com.cjs.example.service..*.delete*(..)) " +"|| execution(* com.cjs.example.service..*.remove*(..))")public void writePointcut() {
 }
@Before("readPointcut()")public void read() { DBContextHolder.slave(); }
@Before("writePointcut()")public void write() { DBContextHolder.master(); }

/** * 另一种写法：if...else... 判断哪些需要读从数据库，其余的走主数据库 */// @Before("execution(* com.cjs.example.service.impl.*.*(..))")// public void before(JoinPoint jp) {// String methodName = jp.getSignature().getName();//// if (StringUtils.startsWithAny(methodName, "get", "select", "find")) {// DBContextHolder.slave();// }else {// DBContextHolder.master();// }// }}

有一般情况就有特殊情况，特殊情况是某些情况下我们需要强制读主库，针对这种情况，我们定义一个主键，用该注解标注的就读主库

package com.cjs.example.annotation;public @interface Master {}

例如，假设我们有一张表member

package com.cjs.example.service.impl;
import com.cjs.example.annotation.Master;import com.cjs.example.entity.Member;import com.cjs.example.entity.MemberExample;import com.cjs.example.mapper.MemberMapper;import com.cjs.example.service.MemberService;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.stereotype.Service;import org.springframework.transaction.annotation.Transactional;
import java.util.List;
@Servicepublic class MemberServiceImpl implements MemberService {
@Autowiredprivate MemberMapper memberMapper;
@Transactional@Overridepublic int insert(Member member) {return memberMapper.insert(member); }
@Master@Overridepublic int save(Member member) {return memberMapper.insert(member); }
@Overridepublic List<Member> selectAll() {return memberMapper.selectByExample(new MemberExample()); }
@Master@Overridepublic String getToken(String appId) {// 有些读操作必须读主数据库// 比如，获取微信access_token，因为高峰时期主从同步可能延迟// 这种情况下就必须强制从主数据读return null; }}

# 测试   

package com.cjs.example;
import com.cjs.example.entity.Member;import com.cjs.example.service.MemberService;import org.junit.Test;import org.junit.runner.RunWith;import org.springframework.beans.factory.annotation.Autowired;import org.springframework.boot.test.context.SpringBootTest;import org.springframework.test.context.junit4.SpringRunner;
@RunWith(SpringRunner.class)@SpringBootTestpublic class CjsDatasourceDemoApplicationTests {
@Autowiredprivate MemberService memberService;
@Testpublic void testWrite() { Member member = new Member(); member.setName("zhangsan"); memberService.insert(member); }
@Testpublic void testRead() {for (int i = 0; i < 4; i++) { memberService.selectAll(); } }
@Testpublic void testSave() { Member member = new Member(); member.setName("wangwu"); memberService.save(member); }
@Testpublic void testReadFromMaster() { memberService.getToken("1234"); }
}

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