seata分布式事务原理_springboot多数据源整合分布式事务中间件seata

seata是阿里推出的分布式事务解决方案，今天我来分享一下springboot多数据源怎么整合seata解决分布式事务的问题。

首先说一下本文使用的实验环境：

springboot：2.1.6.RELEASE

orm框架：mybatis

数据库：mysql

数据库连接池：HikariCP

seata server：1.3.0

配置seata

首先下载seata server安装包，本文使用版本是1.3.0，下载地址如下：

https://github.com/seata/seata/releases

下载完成后，解压后在seata目录下建一个目录logs,在里面建一个文件seata_gc.log,如果不创建这个log文件，启动会报找不到文件的错误。

启动server，我本地使用windows环境的启动命令如下：

seata-server.bat -p 8091 -h 127.0.0.1 -m file

关于启动命令的说明，我摘自官网(http://seata.io/en-us/docs/user/quickstart.html)

Usage: sh seata-server.sh(for linux and mac) or cmd seata-server.bat(for windows) [options]  Options:    --host, -h      The host to bind.      Default: 0.0.0.0    --port, -p      The port to listen.      Default: 8091    --storeMode, -m      log store mode : file、db      Default: file    --helpe.g.sh seata-server.sh -p 8091 -h 127.0.0.1 -m file

下面是启动成功后的日志：

00:21:10,892 |-INFO in ch.qos.logback.core.rolling.RollingFileAppender[ERROR] - Active log file name: /root/logs/seata/txc.8091.error.log00:21:10,892 |-INFO in ch.qos.logback.core.rolling.RollingFileAppender[ERROR] - File property is set to [/root/logs/seata/txc.8091.error.log]00:21:10,892 |-INFO in ch.qos.logback.classic.joran.action.RootLoggerAction - Setting level of ROOT logger to INFO00:21:10,892 |-INFO in ch.qos.logback.core.joran.action.AppenderRefAction - Attaching appender named [ALL] to Logger[ROOT]00:21:10,893 |-INFO in ch.qos.logback.core.joran.action.AppenderRefAction - Attaching appender named [WARN] to Logger[ROOT]00:21:10,893 |-INFO in ch.qos.logback.core.joran.action.AppenderRefAction - Attaching appender named [ERROR] to Logger[ROOT]00:21:10,893 |-INFO in ch.qos.logback.core.joran.action.AppenderRefAction - Attaching appender named [CONSOLE] to Logger[ROOT]00:21:10,893 |-INFO in ch.qos.logback.classic.joran.action.ConfigurationAction - End of configuration.00:21:10,894 |-INFO in ch.qos.logback.classic.joran.JoranConfigurator@2038ae61 - Registering current configuration as safe fallback point2020-08-06 00:21:11.194  INFO --- [           main] io.seata.config.FileConfiguration        : The configuration file used is registry.conf2020-08-06 00:21:11.264  INFO --- [           main] io.seata.config.FileConfiguration        : The configuration file used is file.conf2020-08-06 00:21:11.959  INFO --- [           main] i.s.core.rpc.netty.NettyServerBootstrap  : Server started, listen port: 8091

搭建工程

连接seata server的方式有多种，这儿我们采用文件的方式：

registry {
        # file 、nacos 、eureka、redis、zk、consul、etcd3、sofa  type = "file"  file {
          name = "file.conf"  }}

关于springboot多数据源的配置，如果你还不太熟悉，看一下下面这篇文章，本文不再介绍。

https://blog.csdn.net/zjj2006/article/details/52213754

本文的示例来自seata官方示例，我做了修改，需要3个数据库:seata_pay、seata_order和seata_storage，每个数据库都有一张undo_log表记录回滚日志。sql语句如下：

# OrderDROP DATABASE IF EXISTS seata_order;CREATE DATABASE seata_order;CREATE TABLE seata_order.orders(    id               INT(11) NOT NULL AUTO_INCREMENT,    user_id          INT(11)        DEFAULT NULL,    product_id       INT(11)        DEFAULT NULL,    pay_amount       DECIMAL(10, 0) DEFAULT NULL,    status           VARCHAR(100)   DEFAULT NULL,    add_time         DATETIME       DEFAULT CURRENT_TIMESTAMP,    last_update_time DATETIME       DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,    PRIMARY KEY (id)) ENGINE = InnoDB  AUTO_INCREMENT = 1  DEFAULT CHARSET = utf8;CREATE TABLE seata_order.undo_log(    id            BIGINT(20)   NOT NULL AUTO_INCREMENT,    branch_id     BIGINT(20)   NOT NULL,    xid           VARCHAR(100) NOT NULL,    context       VARCHAR(128) NOT NULL,    rollback_info LONGBLOB     NOT NULL,    log_status    INT(11)      NOT NULL,    log_created   DATETIME     NOT NULL,    log_modified  DATETIME     NOT NULL,    PRIMARY KEY (id),    UNIQUE KEY ux_undo_log (xid, branch_id)) ENGINE = InnoDB  AUTO_INCREMENT = 1  DEFAULT CHARSET = utf8;# StorageDROP DATABASE IF EXISTS seata_storage;CREATE DATABASE seata_storage;CREATE TABLE seata_storage.product(    id               INT(11) NOT NULL AUTO_INCREMENT,    price            DOUBLE   DEFAULT NULL,    stock            INT(11)  DEFAULT NULL,    last_update_time DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,    PRIMARY KEY (id)) ENGINE = InnoDB  AUTO_INCREMENT = 1  DEFAULT CHARSET = utf8;INSERT INTO seata_storage.product (id, price, stock) VALUES (1, 5, 10);CREATE TABLE seata_storage.undo_log(    id            BIGINT(20)   NOT NULL AUTO_INCREMENT,    branch_id     BIGINT(20)   NOT NULL,    xid           VARCHAR(100) NOT NULL,    context       VARCHAR(128) NOT NULL,    rollback_info LONGBLOB     NOT NULL,    log_status    INT(11)      NOT NULL,    log_created   DATETIME     NOT NULL,    log_modified  DATETIME     NOT NULL,    PRIMARY KEY (id),    UNIQUE KEY ux_undo_log (xid, branch_id)) ENGINE = InnoDB  AUTO_INCREMENT = 1  DEFAULT CHARSET = utf8;# PayDROP DATABASE IF EXISTS seata_pay;CREATE DATABASE seata_pay;CREATE TABLE seata_pay.account(    id               INT(11) NOT NULL AUTO_INCREMENT,    balance          DOUBLE   DEFAULT NULL,    last_update_time DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,    PRIMARY KEY (id)) ENGINE = InnoDB  AUTO_INCREMENT = 1  DEFAULT CHARSET = utf8;CREATE TABLE seata_pay.undo_log(    id            BIGINT(20)   NOT NULL AUTO_INCREMENT,    branch_id     BIGINT(20)   NOT NULL,    xid           VARCHAR(100) NOT NULL,    context       VARCHAR(128) NOT NULL,    rollback_info LONGBLOB     NOT NULL,    log_status    INT(11)      NOT NULL,    log_created   DATETIME     NOT NULL,    log_modified  DATETIME     NOT NULL,    PRIMARY KEY (id),    UNIQUE KEY ux_undo_log (xid, branch_id)) ENGINE = InnoDB  AUTO_INCREMENT = 1  DEFAULT CHARSET = utf8;INSERT INTO seata_pay.account (id, balance) VALUES (1, 1);SELECT auto_incrementFROM information_schema.TABLESWHERE TABLE_SCHEMA = 'seata_order'  AND TABLE_NAME = 'undo_log'

整个springboot工程的目录如下：

这里用的是动态数据源，跟我之前文章讲的mybatis多数据源配置不同，这里我主要讲一下这一块

首先看一下mybatis config文件mybatis.xml，所有数据库的xml映射文件都写入一个xml里面，代码如下：

/span>        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"        "http://mybatis.org/dtd/mybatis-3-config.dtd">                settings>            typeAliases>                            mappers>configuration>

再看一下application.properties中数据源的配置，这儿我们创建了3个数据库，所以需要配置3个数据源：

spring.application.name=springboot-seata######seata_pay#############datasource.pay.jdbc-url=jdbc:mysql://192.168.59.1:3306/seata_pay?useAffectedRows=true&serverTimezone=UTC&characterEncoding=utf-8datasource.pay.username=rootdatasource.pay.password=123456datasource.pay.driver-class-name=com.mysql.cj.jdbc.Driver######seata_storage#############datasource.storage.jdbc-url=jdbc:mysql://192.168.59.1:3306/seata_storage?useAffectedRows=true&serverTimezone=UTC&characterEncoding=utf-8datasource.storage.username=rootdatasource.storage.password=123456datasource.storage.driver-class-name=com.mysql.cj.jdbc.Driver######seata_order#############datasource.order.jdbc-url=jdbc:mysql://192.168.59.1:3306/seata_order?useAffectedRows=true&serverTimezone=UTC&characterEncoding=utf-8datasource.order.username=rootdatasource.order.password=123456datasource.order.driver-class-name=com.mysql.cj.jdbc.Driverspring.cloud.alibaba.seata.tx-service-group=springboot-seata

mybatis动态数据源配置类如下：

@Configuration@MapperScan(basePackages={
      "boot.mapper"}, sqlSessionFactoryRef = "sqlSessionFactoryBean")public class DataSourceProxyConfig {
          @Bean("originOrder")    @ConfigurationProperties(prefix = "datasource.order")    public DataSource dataSourceMaster() {
              return DataSourceBuilder.create().build();    }    @Bean("originStorage")    @ConfigurationProperties(prefix = "datasource.storage")    public DataSource dataSourceStorage() {
              return DataSourceBuilder.create().build();    }    @Bean("originPay")    @ConfigurationProperties(prefix = "datasource.pay")    public DataSource dataSourcePay() {
              return DataSourceBuilder.create().build();    }    @Bean(name = "order")    public DataSourceProxy masterDataSourceProxy(@Qualifier("originOrder") DataSource dataSource) {
              return new DataSourceProxy(dataSource);    }    @Bean(name = "storage")    public DataSourceProxy storageDataSourceProxy(@Qualifier("originStorage") DataSource dataSource) {
              return new DataSourceProxy(dataSource);    }    @Bean(name = "pay")    public DataSourceProxy payDataSourceProxy(@Qualifier("originPay") DataSource dataSource) {
              return new DataSourceProxy(dataSource);    }    @Bean("dynamicDataSource")    public DataSource dynamicDataSource(@Qualifier("order") DataSource dataSourceOrder,                                        @Qualifier("storage") DataSource dataSourceStorage,                                        @Qualifier("pay") DataSource dataSourcePay) {
      //这儿是动态数据源配置的关键，3个数据源放在了一个map里面        DynamicRoutingDataSource dynamicRoutingDataSource = new DynamicRoutingDataSource();        Map dataSourceMap = new HashMap<>(3);        dataSourceMap.put(DataSourceKey.ORDER.name(), dataSourceOrder);        dataSourceMap.put(DataSourceKey.STORAGE.name(), dataSourceStorage);        dataSourceMap.put(DataSourceKey.PAY.name(), dataSourcePay);        dynamicRoutingDataSource.setDefaultTargetDataSource(dataSourceOrder);        dynamicRoutingDataSource.setTargetDataSources(dataSourceMap);        DynamicDataSourceContextHolder.getDataSourceKeys().addAll(dataSourceMap.keySet());        return dynamicRoutingDataSource;    }    @Bean    @ConfigurationProperties(prefix = "mybatis")    public SqlSessionFactoryBean sqlSessionFactoryBean(@Qualifier("dynamicDataSource") DataSource dataSource) {
              SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();        sqlSessionFactoryBean.setDataSource(dataSource);        sqlSessionFactoryBean.setConfigLocation(new PathMatchingResourcePatternResolver().getResource("classpath:mybatis/mybatis.xml"));//这儿只能放一个config文件，所以3个数据源的xml文件放到了一个文件里面        return sqlSessionFactoryBean;    }}

在service调用dao时，需要切换数据源，代码如下：

DynamicDataSourceContextHolder.setDataSourceKey(DataSourceKey.ORDER);//切换到seata_order数据库

这里面的切换本质上是将数据库名称放到一个ThreadLocal上

public class DynamicDataSourceContextHolder {
          private static final ThreadLocal CONTEXT_HOLDER = ThreadLocal.withInitial(DataSourceKey.ORDER::name);    private static List dataSourceKeys = new ArrayList<>();    public static void setDataSourceKey(DataSourceKey key) {
              CONTEXT_HOLDER.set(key.name());    }    public static String getDataSourceKey() {
              return CONTEXT_HOLDER.get();    }    public static void clearDataSourceKey() {
              CONTEXT_HOLDER.remove();    }    public static List getDataSourceKeys() {
              return dataSourceKeys;    }}

而ThreadLocal上面的数据库为动态数据源使用

public class DynamicRoutingDataSource extends AbstractRoutingDataSource {
          @Override    protected Object determineCurrentLookupKey() {
              log.info("当前数据源 [{}]", DynamicDataSourceContextHolder.getDataSourceKey());        return DynamicDataSourceContextHolder.getDataSourceKey();    }}

这样seata在获取连接的时候，就可以取到当前的数据库连接，因为每个库里面都有一个undo_log表需要写回滚日志，所以必须能够保证seata能够动态获取当前的数据库。

运行结果

执行Application的main函数启动工程。

在上面的建表语句中，我们建了订单seata_order、支付seata_pay、库存seata_storage3个数据库，支付数据库中的account表插入了一条记录，余额是1，库存数据库的product表插入了1条记录，商品数量(stock)是10。

我们用postman模拟发送一个请求：

url：http://localhost:8083/order/placeOrder，content：{
       "userId":1, "productId":1, "price":1}

执行成功，这时orders表插入了1条记录，account表余额减为0，product表商品数量减为9。如下3个图，

这是我们再发一次上面的http请求，会失败，因为账户余额不足。这次我们采用debug方式，看一下undo_log的数据。程序执行到下图中的断点时，会产生undo_log,如下图：

我们看下undo_log中字段rollback_info数据：

{
        "@class":"io.seata.rm.datasource.undo.BranchUndoLog",  "xid":"192.168.59.132:8091:34937248742391808",  "branchId":34937257391046656,  "sqlUndoLogs":[    "java.util.ArrayList",    [      {
              "@class":"io.seata.rm.datasource.undo.SQLUndoLog",        "sqlType":"INSERT",        "tableName":"orders",        "beforeImage":{
                "@class":"io.seata.rm.datasource.sql.struct.TableRecords$EmptyTableRecords",          "tableName":"orders",          "rows":[            "java.util.ArrayList",            [                          ]          ]        },        "afterImage":{
                "@class":"io.seata.rm.datasource.sql.struct.TableRecords",          "tableName":"orders",          "rows":[            "java.util.ArrayList",            [              {
                      "@class":"io.seata.rm.datasource.sql.struct.Row",                "fields":[                  "java.util.ArrayList",                  [                    {
                            "@class":"io.seata.rm.datasource.sql.struct.Field",                      "name":"id",                      "keyType":"PRIMARY_KEY",                      "type":4,                      "value":2                    },                    {
                            "@class":"io.seata.rm.datasource.sql.struct.Field",                      "name":"user_id",                      "keyType":"NULL",                      "type":4,                      "value":1                    },                    {
                            "@class":"io.seata.rm.datasource.sql.struct.Field",                      "name":"product_id",                      "keyType":"NULL",                      "type":4,                      "value":1                    },                    {
                            "@class":"io.seata.rm.datasource.sql.struct.Field",                      "name":"pay_amount",                      "keyType":"NULL",                      "type":3,                      "value":[                        "java.math.BigDecimal",                        1                      ]                    },                    {
                            "@class":"io.seata.rm.datasource.sql.struct.Field",                      "name":"status",                      "keyType":"NULL",                      "type":12,                      "value":"INIT"                    },                    {
                            "@class":"io.seata.rm.datasource.sql.struct.Field",                      "name":"add_time",                      "keyType":"NULL",                      "type":93,                      "value":[                        "java.sql.Timestamp",                        [                          1596793692000,                          0                        ]                      ]                    },                    {
                            "@class":"io.seata.rm.datasource.sql.struct.Field",                      "name":"last_update_time",                      "keyType":"NULL",                      "type":93,                      "value":[                        "java.sql.Timestamp",                        [                          1596793692000,                          0                        ]                      ]                    }                  ]                ]              }            ]          ]        }      }    ]  ]}

可以看到undo_log记录了要回滚的表中的每个字段的值，比如id=2，回滚时使用。

这时我们看seata server的日志，如下：

2020-08-06 21:49:08.740  INFO --- [Thread_1_12_500] io.seata.server.coordinator.DefaultCore  : Rollback global transaction successfully, xid = 192.168.59.132:8091:34937248742391808.

原理简介

其实分布式事务的原理还是使用了单数据库的事务原理，我们可以把分布式事务中每个数据库看做是单数据库的表。首先每个事务有一个全局的事务id，叫做xid，上面的第二个例子中值是192.168.59.132:8091:34937248742391808。

有了这个xid后，我们就可以记录undo_log了，undo_log中记录了这个xid的，每次提交事务前都要先写undo_log，后提交事务，这时你一定恍然大悟，这不就是mysql中的wal机制吗？

而rollback_info字段记录了要回滚的表的记录中的每个字段和对应值，这样就方便的回滚了。这时你肯定又恍然大悟，与其说是回滚，难道这不就是交易补偿啊？

理解了这个，在理解seata官方的解释就容易多了，如下图：

seata中有3个角色，TC其实就是seata server，RM是单个数据库的事务管理器，TM是定义开启和提交回滚全局事务的组件。官方定义如下：

Transaction Coordinator(TC): Maintain status of global and branch transactions, drive the global commit or rollback.Transaction Manager(TM): Define the scope of global transaction: begin a global transaction, commit or rollback a global transaction.Resource Manager(RM): Manage resources that branch transactions working on, talk to TC for registering branch transactions and reporting status of branch transactions, and drive the branch transaction commit or rollback.

总结

本文主要介绍了springboot多数据源整合seata的使用，也简单介绍了一些原理。seata对分布式事务的管理思想其实还是单个数据库事务的思想。后面有时间再详细介绍seata的原理。

源码地址：

https://github.com/jinjunzhu/springboot-seata.git

参考：

http://seata.io/en-us/docs/user/quickstart.htmlhttps://github.com/seata/seatahttps://github.com/seata/seata-samples