现在大型的电子商务系统,在数据库层面大都采用读写分离技术,就是一个Master数据库,多个Slave数据库。Master库负责数据更新和实时数据查询,Slave库当然负责非实时数据查询。因为在实际的应用中,数据库都是读多写少(读取数据的频率高,更新数据的频率相对较少),而读取数据通常耗时比较长,占用数据库服务器的CPU较多,从而影响用户体验。我们通常的做法就是把查询从主库中抽取出来,采用多个从库,使用负载均衡,减轻每个从库的查询压力。
采用读写分离技术的目标:有效减轻Master库的压力,又可以把用户查询数据的请求分发到不同的Slave库,从而保证系统的健壮性。我们看下采用读写分离的背景。
随着网站的业务不断扩展,数据不断增加,用户越来越多,数据库的压力也就越来越大,采用传统的方式,比如:数据库或者SQL的优化基本已达不到要求,这个时候可以采用读写分离的策 略来改变现状。
具体到开发中,如何方便的实现读写分离呢?目前常用的有两种方式:
1 第一种方式是我们最常用的方式,就是定义2个数据库连接,一个是MasterDataSource,另一个是SlaveDataSource。更新数据时我们读取MasterDataSource,查询数据时我们读取SlaveDataSource。这种方式很简单,我就不赘述了。
2 第二种方式动态数据源切换,就是在程序运行时,把数据源动态织入到程序中,从而选择读取主库还是从库。主要使用的技术是:annotation,Spring AOP ,反射。下面会详细的介绍实现方式。
在介绍实现方式之前,我们先准备一些必要的知识,spring 的AbstractRoutingDataSource 类
AbstractRoutingDataSource这个类 是spring2.0以后增加的,我们先来看下AbstractRoutingDataSource的定义:
public abstract class AbstractRoutingDataSource extends AbstractDataSource implements InitializingBean {}
AbstractRoutingDataSource继承了AbstractDataSource ,而AbstractDataSource 又是DataSource 的子类。DataSource 是javax.sql 的数据源接口,定义如下:
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public interface DataSource extends CommonDataSource,Wrapper { /** * <p>Attempts to establish a connection with the data source that * this <code>DataSource</code> object represents. * * @return a connection to the data source * @exception SQLException if a database access error occurs */ Connection getConnection() throws SQLException; /** * <p>Attempts to establish a connection with the data source that * this <code>DataSource</code> object represents. * * @param username the database user on whose behalf the connection is * being made * @param password the user's password * @return a connection to the data source * @exception SQLException if a database access error occurs * @since 1.4 */ Connection getConnection(String username, String password) throws SQLException; } |
DataSource 接口定义了2个方法,都是获取数据库连接。我们在看下AbstractRoutingDataSource 如何实现了DataSource接口:
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public Connection getConnection() throws SQLException { return determineTargetDataSource().getConnection(); } public Connection getConnection(String username, String password) throws SQLException { return determineTargetDataSource().getConnection(username, password); } |
很显然就是调用自己的determineTargetDataSource() 方法获取到connection。determineTargetDataSource方法定义如下:
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protected DataSource determineTargetDataSource() { Assert.notNull( this .resolvedDataSources, "DataSource router not initialized" ); Object lookupKey = determineCurrentLookupKey(); DataSource dataSource = this .resolvedDataSources.get(lookupKey); if (dataSource == null && ( this .lenientFallback || lookupKey == null )) { dataSource = this .resolvedDefaultDataSource; } if (dataSource == null ) { throw new IllegalStateException( "Cannot determine target DataSource for lookup key [" + lookupKey + "]" ); } return dataSource; } |
我们最关心的还是下面2句话:
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Object lookupKey = determineCurrentLookupKey(); DataSource dataSource = this .resolvedDataSources.get(lookupKey); |
determineCurrentLookupKey方法返回lookupKey,resolvedDataSources方法就是根据lookupKey从Map中获得数据源。resolvedDataSources 和determineCurrentLookupKey定义如下:
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private Map<Object, DataSource> resolvedDataSources; protected abstract Object determineCurrentLookupKey() |
看到以上定义,我们是不是有点思路了,resolvedDataSources是Map类型,我们可以把MasterDataSource和SlaveDataSource存到Map中,如下:
key | value |
master | MasterDataSource |
slave | SlaveDataSource |
我们在写一个类DynamicDataSource 继承AbstractRoutingDataSource,实现其determineCurrentLookupKey() 方法,该方法返回Map的key,master或slave。
好了,说了这么多,有点烦了,下面我们看下怎么实现。
上面已经提到了我们要使用的技术,我们先看下annotation的定义:
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@Retention (RetentionPolicy.RUNTIME) @Target (ElementType.METHOD) public @interface DataSource { String value(); } |
我们还需要实现spring的抽象类AbstractRoutingDataSource,就是实现determineCurrentLookupKey方法:
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public class DynamicDataSource extends AbstractRoutingDataSource { @Override protected Object determineCurrentLookupKey() { // TODO Auto-generated method stub return DynamicDataSourceHolder.getDataSouce(); } } public class DynamicDataSourceHolder { public static final ThreadLocal<String> holder = new ThreadLocal<String>(); public static void putDataSource(String name) { holder.set(name); } public static String getDataSouce() { return holder.get(); } } |
从DynamicDataSource 的定义看出,他返回的是DynamicDataSourceHolder.getDataSouce()值,我们需要在程序运行时调用DynamicDataSourceHolder.putDataSource()方法,对其赋值。下面是我们实现的核心部分,也就是AOP部分,DataSourceAspect定义如下:
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public class DataSourceAspect { public void before(JoinPoint point) { Object target = point.getTarget(); String method = point.getSignature().getName(); Class<?>[] classz = target.getClass().getInterfaces(); Class<?>[] parameterTypes = ((MethodSignature) point.getSignature()) .getMethod().getParameterTypes(); try { Method m = classz[ 0 ].getMethod(method, parameterTypes); if (m != null && m.isAnnotationPresent(DataSource. class )) { DataSource data = m .getAnnotation(DataSource. class ); DynamicDataSourceHolder.putDataSource(data.value()); System.out.println(data.value()); } } catch (Exception e) { // TODO: handle exception } } } |
为了方便测试,我定义了2个数据库,shop模拟Master库,test模拟Slave库,shop和test的表结构一致,但数据不同,数据库配置如下:
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< bean id = "masterdataSource" class = "org.springframework.jdbc.datasource.DriverManagerDataSource" > < property name = "driverClassName" value = "com.mysql.jdbc.Driver" /> < property name = "url" value = "jdbc:mysql://127.0.0.1:3306/shop" /> < property name = "username" value = "root" /> < property name = "password" value = "yangyanping0615" /> </ bean > < bean id = "slavedataSource" class = "org.springframework.jdbc.datasource.DriverManagerDataSource" > < property name = "driverClassName" value = "com.mysql.jdbc.Driver" /> < property name = "url" value = "jdbc:mysql://127.0.0.1:3306/test" /> < property name = "username" value = "root" /> < property name = "password" value = "yangyanping0615" /> </ bean > < beans:bean id = "dataSource" class = "com.air.shop.common.db.DynamicDataSource" > < property name = "targetDataSources" > < map key-type = "java.lang.String" > <!-- write --> < entry key = "master" value-ref = "masterdataSource" /> <!-- read --> < entry key = "slave" value-ref = "slavedataSource" /> </ map > </ property > < property name = "defaultTargetDataSource" ref = "masterdataSource" /> </ beans:bean > < bean id = "transactionManager" class = "org.springframework.jdbc.datasource.DataSourceTransactionManager" > < property name = "dataSource" ref = "dataSource" /> </ bean > <!-- 配置SqlSessionFactoryBean --> < bean id = "sqlSessionFactory" class = "org.mybatis.spring.SqlSessionFactoryBean" > < property name = "dataSource" ref = "dataSource" /> < property name = "configLocation" value = "classpath:config/mybatis-config.xml" /> </ bean > |
在spring的配置中增加aop配置
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<!-- 配置数据库注解aop --> < aop:aspectj-autoproxy ></ aop:aspectj-autoproxy > < beans:bean id = "manyDataSourceAspect" class = "com.air.shop.proxy.DataSourceAspect" /> < aop:config > < aop:aspect id = "c" ref = "manyDataSourceAspect" > < aop:pointcut id = "tx" expression = "execution(* com.air.shop.mapper.*.*(..))" /> < aop:before pointcut-ref = "tx" method = "before" /> </ aop:aspect > </ aop:config > <!-- 配置数据库注解aop --> |
下面是MyBatis的UserMapper的定义,为了方便测试,登录读取的是Master库,用户列表读取Slave库:
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public interface UserMapper { @DataSource ( "master" ) public void add(User user); @DataSource ( "master" ) public void update(User user); @DataSource ( "master" ) public void delete( int id); @DataSource ( "slave" ) public User loadbyid( int id); @DataSource ( "master" ) public User loadbyname(String name); @DataSource ( "slave" ) public List<User> list(); } |
好了,运行我们的Eclipse看看效果,输入用户名admin 登录看看效果
以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持服务器之家。
原文链接:http://www.cnblogs.com/surge/p/3582248.html