(二十)SpringCloud Alibaba Seata处理分布式事务
1、分布式问题
分布式前:
- 单机单库没这个问题
- 从1:1 -> 1:N -> N: N
分布式之后:
一句话:一次业务操作需要跨多个数据源或需要跨多个系统进行远程调用,就会产生分布式事务问题
2、Seata简介
2.1 是什么
Seata是一款开源的分布式事务解决方案,致力于在微服务架构下提供高性能和简单易用的分布式事务服务
官网地址:seata.io/zh-cn/
2.2 能干嘛
一个典型的分布式事务过程
分布式事务处理过程的——ID+三组件模型
Transaction ID XID:全局唯一的事务ID
3组件概念:
- Transaction Coordinator(TC) :事务协调器,维护全局事务的运行状态,负责协调并驱动全局事务的提交或回滚;
- Transaction Manager(TM) :控制全局事务的边界,负责开启一个全局事务,并最终发起全局提交或全局回滚的决议;
- Resource Manager(RM) :控制分支事务,负责分支注册,状态汇报,并接收事务协调器的指令,驱动分支(本地)事务的提交和回滚;
2.3 去那下
2.4 怎么玩
Spring 本地@Transactional
全局@GlobalTransactional:SEATA的分布式交易解决方案
3、Seata-Server安装
3.1 官网地址
3.2 下载版本
seata-server-0.9.0
3.3 解压
seata-server-0.9.0.zip解压到指定目录并修改conf目录下的file.conf配置文件
-
先备份原始file.conf文件
-
主要修改:自定义事务组名称+事务日志存储模式为db+数据库连接信息
-
file.conf修改:
-
service模块:
vgroup_mapping.my_test_tx_group = "fsp_tx_group"
-
store模块:
mode = "db"
url = "jdbc:mysql://127.0.0.1:3306/seata" user = "root" password = "你自己的密码"
-
3.4 mysql5.7数据库新建库seata
3.5 在seata库里建表
建表db_store.sql在\seata-server-0.9.0\seata\conf目录里面
赋值db_store.sql里的内容,粘贴到SQLYog中执行
3.6 修改 registry.conf配置文件
修改seata-server-0.9.0\seata\conf目录下的registry.conf配置文件
registry {
#file 、nacos 、eureka、redis、zk、consul、etcd3、sofa
type = "nacos"
nacos {
serverAddr = "localhost:8848"
namespace = ""
cluster = "default"
}
3.7 先启动Nacos端口号8848
3.8 再启动seata-server
找到目录softs\seata-server-0.9.0\seata\bin下,执行cmd,输入seata-server.bat
4、订单/库存/账户业务数据库准备
4.1 说明
以下演示都需要先启动Nacos后启动Seata,保证两个都OK
Seata没启动报错no available server to connect
4.2 分布式事务业务说明
下订单-->扣库存-->减账户(余额)
4.3 创建业务数据库
- seata_order: 存储订单的数据库
- seata_storage:存储库存的数据库
- seata_account: 存储账户信息的数据库
4.3 按照上述3库分别建对应业务表
-
seata_order库下建t_order表
CREATE TABLE t_order( `id` BIGINT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY, `user_id` BIGINT(11) DEFAULT NULL COMMENT '用户id', `product_id` BIGINT(11) DEFAULT NULL COMMENT '产品id', `count` INT(11) DEFAULT NULL COMMENT '数量', `money` DECIMAL(11,0) DEFAULT NULL COMMENT '金额', `status` INT(1) DEFAULT NULL COMMENT '订单状态:0:创建中; 1:已完结' ) ENGINE=INNODB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8; SELECT * FROM t_order;
-
seata_storage库下建t_storage表
CREATE TABLE t_storage( `id` BIGINT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY, `product_id` BIGINT(11) DEFAULT NULL COMMENT '产品id', `'total` INT(11) DEFAULT NULL COMMENT '总库存', `used` INT(11) DEFAULT NULL COMMENT '已用库存', `residue` INT(11) DEFAULT NULL COMMENT '剩余库存' ) ENGINE=INNODB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8; INSERT INTO seata_storage.t_storage(`id`,`product_id`,`total`,`used`,`residue`) VALUES('1','1','100','0','100'); SELECT * FROM t_storage;
-
seata_account库下建t_account表
CREATE TABLE t_account( `id` BIGINT(11) NOT NULL AUTO_INCREMENT PRIMARY KEY COMMENT 'id', `user_id` BIGINT(11) DEFAULT NULL COMMENT '用户id', `total` DECIMAL(10,0) DEFAULT NULL COMMENT '总额度', `used` DECIMAL(10,0) DEFAULT NULL COMMENT '已用余额', `residue` DECIMAL(10,0) DEFAULT '0' COMMENT '剩余可用额度' ) ENGINE=INNODB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8; INSERT INTO seata_account.t_account(`id`,`user_id`,`total`,`used`,`residue`) VALUES('1','1','1000','0','1000') SELECT * FROM t_account;
4.4 按照上述3库分别建对应的回滚日志表
订单-库存-账户3个库下都需要建各自的回滚日志表
粘贴该目录:\seata-server-0.9.0\seata\conf目录下的db_undo_log.sql
然后在每个数据库中就新建该表
4.5 最终效果
5、订单/库存/账户业务微服务准备
5.1 业务需求
下订单->减库存->扣余额->改(订单)状态
5.2 新建订单Order-Module
5.2.1 新建model
seata-order-service2001
5.2.2 POM
<dependencies>
<!--nacos-->
<dependency>
<groupId>com.alibaba.cloud</groupId>
<artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
</dependency>
<!--seata 因为兼容版本问题,所以需要剔除它自带的seata的包 -->
<dependency>
<groupId>com.alibaba.cloud</groupId>
<artifactId>spring-cloud-starter-alibaba-seata</artifactId>
<exclusions>
<exclusion>
<artifactId>seata-all</artifactId>
<groupId>io.seata</groupId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>io.seata</groupId>
<artifactId>seata-all</artifactId>
<version>0.9.0</version>
</dependency>
<!--feign-->
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency>
<!--web-actuator-->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
<!--spring cloud alibaba nacos-->
<dependency>
<groupId>com.alibaba.cloud</groupId>
<artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
</dependency>
<!--SpringCloud alibaba sentinel-datasource-nacos:后续做持久化用到-->
<dependency>
<groupId>com.alibaba.csp</groupId>
<artifactId>sentinel-datasource-nacos</artifactId>
</dependency>
<!--SpringCloud alibaba Sentinel-->
<dependency>
<groupId>com.alibaba.cloud</groupId>
<artifactId>spring-cloud-starter-alibaba-sentinel</artifactId>
</dependency>
<!--mysql-druid-->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.47</version>
</dependency>
<dependency>
<groupId>com.alibaba</groupId>
<artifactId>druid-spring-boot-starter</artifactId>
<version>1.1.10</version>
</dependency>
<!--Mybatis和SpringBoot整合-->
<dependency>
<groupId>org.mybatis.spring.boot</groupId>
<artifactId>mybatis-spring-boot-starter</artifactId>
<version>2.0.0</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<optional>true</optional>
</dependency>
</dependencies>
5.2.3 YML
server:
port: 2001
spring:
application:
name: seata-order-service
cloud:
alibaba:
seata:
#自定义事务组名称需要与seata-server中的对应
tx-service-group: fsp_tx_group
nacos:
discovery:
server-addr: localhost:8848
datasource:
driver-class-name: com.mysql.jdbc.Driver
url: jdbc:mysql://localhost:3306/seata_order?useUnicode=true&characterEncoding=UTF-8&useSSL=false&serverTimezone=GMT
username: root
password: root
feign:
hystrix:
enabled: false
logging:
level:
io:
seata: info
mybatis:
mapperLocations: classpath:mapper/*.xml
5.2.4 file.conf
transport {
# tcp udt unix-domain-socket
type = "TCP"
#NIO NATIVE
server = "NIO"
#enable heartbeat
heartbeat = true
#thread factory for netty
thread-factory {
boss-thread-prefix = "NettyBoss"
worker-thread-prefix = "NettyServerNIOWorker"
server-executor-thread-prefix = "NettyServerBizHandler"
share-boss-worker = false
client-selector-thread-prefix = "NettyClientSelector"
client-selector-thread-size = 1
client-worker-thread-prefix = "NettyClientWorkerThread"
# netty boss thread size,will not be used for UDT
boss-thread-size = 1
#auto default pin or 8
worker-thread-size = 8
}
shutdown {
# when destroy server, wait seconds
wait = 3
}
serialization = "seata"
compressor = "none"
}
service {
vgroup_mapping.fsp_tx_group = "default"
default.grouplist = "127.0.0.1:8091"
enableDegrade = false
disable = false
max.commit.retry.timeout = "-1"
max.rollback.retry.timeout = "-1"
disableGlobalTransaction = false
}
client {
async.commit.buffer.limit = 10000
lock {
retry.internal = 10
retry.times = 30
}
report.retry.count = 5
tm.commit.retry.count = 1
tm.rollback.retry.count = 1
}
## transaction log store
store {
## store mode: file、db
mode = "db"
## file store
file {
dir = "sessionStore"
# branch session size , if exceeded first try compress lockkey, still exceeded throws exceptions
max-branch-session-size = 16384
# globe session size , if exceeded throws exceptions
max-global-session-size = 512
# file buffer size , if exceeded allocate new buffer
file-write-buffer-cache-size = 16384
# when recover batch read size
session.reload.read_size = 100
# async, sync
flush-disk-mode = async
}
## database store
db {
## the implement of javax.sql.DataSource, such as DruidDataSource(druid)/BasicDataSource(dbcp) etc.
datasource = "dbcp"
## mysql/oracle/h2/oceanbase etc.
db-type = "mysql"
driver-class-name = "com.mysql.jdbc.Driver"
url = "jdbc:mysql://127.0.0.1:3306/seata"
user = "root"
password = "root"
min-conn = 1
max-conn = 3
global.table = "global_table"
branch.table = "branch_table"
lock-table = "lock_table"
query-limit = 100
}
}
lock {
## the lock store mode: local、remote
mode = "remote"
local {
## store locks in user's database
}
remote {
## store locks in the seata's server
}
}
recovery {
#schedule committing retry period in milliseconds
committing-retry-period = 1000
#schedule asyn committing retry period in milliseconds
asyn-committing-retry-period = 1000
#schedule rollbacking retry period in milliseconds
rollbacking-retry-period = 1000
#schedule timeout retry period in milliseconds
timeout-retry-period = 1000
}
transaction {
undo.data.validation = true
undo.log.serialization = "jackson"
undo.log.save.days = 7
#schedule delete expired undo_log in milliseconds
undo.log.delete.period = 86400000
undo.log.table = "undo_log"
}
## metrics settings
metrics {
enabled = false
registry-type = "compact"
# multi exporters use comma divided
exporter-list = "prometheus"
exporter-prometheus-port = 9898
}
support {
## spring
spring {
# auto proxy the DataSource bean
datasource.autoproxy = false
}
}
5.2.5 registry.conf
registry {
# file 、nacos 、eureka、redis、zk、consul、etcd3、sofa
type = "nacos"
nacos {
serverAddr = "localhost:8848"
namespace = ""
cluster = "default"
}
eureka {
serviceUrl = "http://localhost:8761/eureka"
application = "default"
weight = "1"
}
redis {
serverAddr = "localhost:6379"
db = "0"
}
zk {
cluster = "default"
serverAddr = "127.0.0.1:2181"
session.timeout = 6000
connect.timeout = 2000
}
consul {
cluster = "default"
serverAddr = "127.0.0.1:8500"
}
etcd3 {
cluster = "default"
serverAddr = "http://localhost:2379"
}
sofa {
serverAddr = "127.0.0.1:9603"
application = "default"
region = "DEFAULT_ZONE"
datacenter = "DefaultDataCenter"
cluster = "default"
group = "SEATA_GROUP"
addressWaitTime = "3000"
}
file {
name = "file.conf"
}
}
config {
# file、nacos 、apollo、zk、consul、etcd3
type = "file"
nacos {
serverAddr = "localhost"
namespace = ""
}
consul {
serverAddr = "127.0.0.1:8500"
}
apollo {
app.id = "seata-server"
apollo.meta = "http://192.168.1.204:8801"
}
zk {
serverAddr = "127.0.0.1:2181"
session.timeout = 6000
connect.timeout = 2000
}
etcd3 {
serverAddr = "http://localhost:2379"
}
file {
name = "file.conf"
}
}
5.2.6 domain
Order类:
package com.atguigu.springcloud.alibaba.domain;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import java.math.BigDecimal;
@Data
@AllArgsConstructor
@NoArgsConstructor
public class Order
{
private Long id;
private Long userId;
private Long productId;
private Integer count;
private BigDecimal money;
private Integer status; //订单状态:0:创建中;1:已完结
}
CommonResult类:
package com.atguigu.springcloud.alibaba.domain;
import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
@Data
@AllArgsConstructor
@NoArgsConstructor
public class CommonResult<T>
{
private Integer code;
private String message;
private T data;
public CommonResult(Integer code, String message)
{
this(code,message,null);
}
}
5.2.7 Dao接口与实现
package com.atguigu.springcloud.alibaba.dao;
import com.atguigu.springcloud.alibaba.domain.Order;
import org.apache.ibatis.annotations.Mapper;
import org.apache.ibatis.annotations.Param;
@Mapper
public interface OrderDao {
//新建订单
void create(Order order);
//修改订单的状态 从零改为1
void update(@Param("userId") Long userId, @Param("status") Integer status);
}
resources文件夹下新建mapper文件夹后添加:OrderMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.atguigu.springcloud.alibaba.dao.OrderDao">
<resultMap id="BaseResultMap" type="com.atguigu.springcloud.alibaba.domain.Order">
<id column="id" property="id" jdbcType="BIGINT"/>
<result column="user_id" property="userId" jdbcType="BIGINT"/>
<result column="product_id" property="productId" jdbcType="BIGINT"/>
<result column="count" property="count" jdbcType="INTEGER"/>
<result column="money" property="money" jdbcType="DECIMAL"/>
<result column="status" property="status" jdbcType="INTEGER"/>
</resultMap>
<insert id="create">
insert into t_order(id,user_id,product_id,count,money,status)
values (null,#{userId},#{productId},#{count},#{money},0);
</insert>
<update id="update">
update t_order set status = 1 where user_id = #{userId} and status = #{status}
</update>
</mapper>
5.2.8 Service接口与实现
OrderService:
package com.atguigu.springcloud.alibaba.service;
import com.atguigu.springcloud.alibaba.domain.Order;
public interface OrderService {
//新建订单
void create(Order order);
}
AccountService:
package com.atguigu.springcloud.alibaba.service;
import com.atguigu.springcloud.alibaba.domain.CommonResult;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;
import java.math.BigDecimal;
@FeignClient(value = "seata-account-service")
public interface AccountService {
@PostMapping(value = "/account/decrease")
CommonResult decrease(@RequestParam("userId")Long userId,
@RequestParam("money")BigDecimal money);
}
StorageService:
package com.atguigu.springcloud.alibaba.service;
import com.atguigu.springcloud.alibaba.domain.CommonResult;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestParam;
@FeignClient(value = "seata-storage-service")//调用库存微服务
public interface StorageService {
@PostMapping(value = "/storage/decrease")
CommonResult decrease(@RequestParam("productId") Long productId,@RequestParam("count") Integer count);
}
OrderServiceImpl:
package com.atguigu.springcloud.alibaba.service.impl;
import com.atguigu.springcloud.alibaba.dao.OrderDao;
import com.atguigu.springcloud.alibaba.domain.Order;
import com.atguigu.springcloud.alibaba.service.AccountService;
import com.atguigu.springcloud.alibaba.service.OrderService;
import com.atguigu.springcloud.alibaba.service.StorageService;
import io.seata.spring.annotation.GlobalTransactional;
import lombok.extern.slf4j.Slf4j;
import org.springframework.stereotype.Service;
import javax.annotation.Resource;
@Service
@Slf4j
public class OrderServiceImpl implements OrderService {
@Resource
private OrderDao orderDao;
@Resource
private StorageService storageService;
@Resource
private AccountService accountService;
/**
* 创建订单->调用库存服务扣减库存->调用账户服务扣减账户余额->修改订单状态
*/
@GlobalTransactional
@Override
public void create(Order order) {
System.out.println("-------->开始新建订单");
//新建订单
orderDao.create(order);
//扣减库存
System.out.println("---->订单微服务开始调用库存,做扣减Count");
storageService.decrease(order.getProductId(),order.getCount());
System.out.println("---->订单微服务开始调用库存,做扣减end");
//做扣减账户
System.out.println("--->订单微服务开始调用账户,做扣减Money");
accountService.decrease(order.getUserId(),order.getMoney());
System.out.println("--->订单微服务调用账户,做扣减end");
//修改订单状态,从0到1代表已经完成
System.out.println("--->修改订单状态开始");
orderDao.update(order.getUserId(),0);
System.out.println("--->修改订单状态结束(#^.^#)");
}
}
5.2.9 Controller
package com.atguigu.springcloud.alibaba.controller;
import com.atguigu.springcloud.alibaba.domain.CommonResult;
import com.atguigu.springcloud.alibaba.domain.Order;
import com.atguigu.springcloud.alibaba.service.OrderService;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import javax.annotation.Resource;
import javax.smartcardio.CommandAPDU;
@RestController
public class OrderController {
@Resource
private OrderService orderService;
@GetMapping("/order/create")
public CommonResult create(Order order){
orderService.create(order);
return new CommonResult(200,"订单创建成功");
}
@RequestMapping("/order/test")
public String test(){
System.out.println("test 2001");
return "test 2001";
}
}
5.2.10 Config配置
MyBatisConfig:
package com.atguigu.springcloud.alibaba.config;
import org.mybatis.spring.annotation.MapperScan;
import org.springframework.context.annotation.Configuration;
@Configuration
@MapperScan({"com.atguigu.springcloud.alibaba.dao"})
public class MyBatisConfig {
}
DataSourceProxyConfig:
package com.atguigu.springcloud.alibaba.config;
import com.alibaba.druid.pool.DruidDataSource;
import io.seata.rm.datasource.DataSourceProxy;
import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.SqlSessionFactoryBean;
import org.mybatis.spring.transaction.SpringManagedTransactionFactory;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
import javax.sql.DataSource;
/**
* 使用Seata对数据源进行代理
*/
@Configuration
public class DataSourceProxyConfig {
@Value("${mybatis.mapperLocations}")
private String mapperLocations;
@Bean
@ConfigurationProperties(prefix = "spring.datasource")
public DataSource druidDataSource(){
return new DruidDataSource();
}
@Bean
public DataSourceProxy dataSourceProxy(DataSource dataSource) {
return new DataSourceProxy(dataSource);
}
@Bean
public SqlSessionFactory sqlSessionFactoryBean(DataSourceProxy dataSourceProxy) throws Exception {
SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
sqlSessionFactoryBean.setDataSource(dataSourceProxy);
sqlSessionFactoryBean.setMapperLocations(new PathMatchingResourcePatternResolver().getResources(mapperLocations));
sqlSessionFactoryBean.setTransactionFactory(new SpringManagedTransactionFactory());
return sqlSessionFactoryBean.getObject();
}
}
5.2.11 主启动类
package com.atguigu.springcloud.alibaba;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.openfeign.EnableFeignClients;
@EnableDiscoveryClient
@EnableFeignClients
@SpringBootApplication(exclude = DataSourceAutoConfiguration.class)//取消数据源的自动配置,使用Seata对数据源进行代理
public class SeataOrderMainApp2001 {
public static void main(String[] args) {
SpringApplication.run(SeataOrderMainApp2001.class,args);
}
}
no available server to connect.
5.3 新建库存Storage-Module
5.4 新建账户Account-Module
6、Test
6.1 测试之前数据库情况:
6.2 访问:
http://localhost:2001/order/create?userId=1&productId=1&count=10&money=100
6.3 访问之后数据库情况:
6.4 后台情况:
2001
2002:
2003:
6.4 超时异常,没加@GlobalTransactional:
-
AccountServiceImpl添加超时
public void decrease(Long userId, BigDecimal money) { System.out.println(("------->account-service中扣减账户余额开始")); //模拟超时异常,全局事务回滚 try { TimeUnit.SECONDS.sleep(20); }catch (InterruptedException e) { e.printStackTrace(); } accountDao.decrease(userId,money); System.out.println("------->account-service中扣减账户余额结束"); }
-
重启2003,再次访问
-
数据库情况
-
故障情况
- 当库存和账户余额扣减后,订单状态并没有设置为已经完成,没有从零改为1
- 而且由于feign的重试机制,账户余额还有可能被多次扣减
6.5 超时异常,添加@GlobalTransactional
-
AccountServiceImpl添加超时
-
OrderServiceImpl
@GlobalTransactional
-
再次测试:
-
查看数据库:
- 下单后数据库数据并没有任何改变,记录都添加不进来
7、Seata之原理简介
7.1 Seata
Simple Extensible Autonomous Transaction Architecture:简单可扩展自治事务框架
2020起初,参加工作后用1.0以后的版本
7.2 再看TC/TM/RM三大组件
TC:seata的服务器
TM:事物的发起者,业务的入口。 @GlobalTransactional(name = "txl-create-order", rollbackFor = Exception.class)
RM:事务的参与者,一个数据库就是一个RM。
分布式事务的执行流程:
- TM开启分布式事务(TM向TC注册全局事务记录)
- 换业务场景,编排数据库,服务等事务内资源(RM向TC汇报资源准备状态)
- TM结束分布式事务,事务一阶段结束(TM通知TC提交/回滚分布式事务)
- TC汇总事务信息,决定分布式事务是提交还是回滚
- TC通知所有RM提交/回滚资源,事务二阶段结束。
7.3 AT模式如何做到对业务的无侵入
1)是什么
2)一阶段加载
3)二阶段提交
4)二阶段回滚