16.Spring Cloud Alibaba学习笔记–Seata案例–原理

TC/TM/RM三个组件的近似理解

TC（Transaction Coordinator） —— Seata 服务器
TM（Transaction Manager）—— 带有 @GlobalTransactional 的方法，事务的发起方
RM（Resource Manager）—— 订单 / 库存 / 账户这三个数据库，事务的参与方

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）；
按业务场景，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 汇报事务信息，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。

AT模式如何做到对业务的无侵入

AT模式简介

前提

基于支持本地 ACID 事务的关系型数据库。

Java 应用，通过 JDBC 访问数据库。

整体机制

两阶段提交协议的演变：

一阶段：业务数据和回滚日志记录在同一个本地事务中提交，释放本地锁和连接资源。

二阶段：

提交异步化，非常快速地完成。

回滚通过一阶段的回滚日志进行反向补偿。

参考：

http://seata.io/zh-cn/docs/overview/what-is-seata.html

AT 模式对应于阿里云的全局事务服务（Global Transaction Service，简称 GTS）。

一阶段加载

在一阶段，Seata 拦截“业务 SQL”：

解析 SQL 语义，找到“业务 SQL”要更新的业务数据，在业务数据被更新前，将其保存成“before image”（前置镜像）；
执行“业务 SQL”更新业务数据；
在业务数据更新之后，其保存成”after image” （后置镜像），最后生成行锁。

以上操作全部在一个数据库事务内完成，这样保证了一阶段操作的原子性。

官方的过程可参考：

http://seata.io/zh-cn/docs/dev/mode/at-mode.html

二阶段提交

二阶段如果顺利提交的话，因为“业务 SQL”在一阶段已经提交至数据库，所以 Seata 框架只需将一阶段保存的快照数据和行锁删掉，完成数据清理即可。

二阶段回滚

二阶段如果是回滚的话，Seata 就需要回滚一阶段已经执行的“业务 SQL”，还原业务数据。回滚方式便是用“before image”还原业务数据；但在还原前要首先要校验脏写 ，对比”数据库当前业务数据”和”after image”，如果两份数据完全一致就说明没有脏写，可以还原业务数据，如果不一致就说明有脏写，出现脏写就需要转人工处理 。

通过 DEBUG 了解 Seata 原理

打断点

1、以 DEBUG 模式启动

seata-order-service2001，seata-storage-service2002，seata-account-service2003

2、打断点

在 seata-account-service2003 的 AccountServiceImpl 中打断点

3、刷新网址

http://localhost:2001/order/create?userId=1&productId=1&count=10&money=100

4、查看数据库

seata -> branch_table

branch_id	xid	transaction_id	resource_id	lock_key	branch_type	status	client_id	gmt_create	gmt_modified
2051276927	192.168.25.141:8091:2051276925	2051276925	jdbc:mysql://192.168.25.158:3306/seata_order	t_order:17	AT	2	seata-order-service:192.168.25.141:61317	2020-8-16 12:14:23	2020-8-16 12:14:23
2051276931	192.168.25.141:8091:2051276925	2051276925	jdbc:mysql://192.168.25.158:3306/seata_storage	t_storage:1	AT	2	seata-storage-service:192.168.25.141:61184	2020-8-16 12:14:23	2020-8-16 12:14:23
2051276934	192.168.25.141:8091:2051276925	2051276925	jdbc:mysql://192.168.25.158:3306/seata_account	t_account:1	AT	2	seata-account-service:192.168.25.141:61232	2020-8-16 12:14:23	2020-8-16 12:14:23

seata -> global_table

xid	transaction_id	status	application_id	transaction_service_group	transaction_name	timeout	begin_time	application_data	gmt_create	gmt_modified
192.168.25.141:8091:2051276925	2051276925	5	seata-order-service	fsp_tx_group	fsp-create-order	60000	1597551263304		2020-8-16 12:14:23	2020-8-16 12:14:54

seata -> lock_table

row_key	xid	transaction_id	branch_id	resource_id	table_name	pk	gmt_create	gmt_modified
jdbc:mysql://192.168.25.158:3306/seata_account^^^t_account^^^1	192.168.25.141:8091:2051276925	2051276925	2051276934	jdbc:mysql://192.168.25.158:3306/seata_account	t_account	1	2020-8-16 12:14:23	2020-8-16 12:14:23
jdbc:mysql://192.168.25.158:3306/seata_order^^^t_order^^^17	192.168.25.141:8091:2051276925	2051276925	2051276927	jdbc:mysql://192.168.25.158:3306/seata_order	t_order	17	2020-8-16 12:14:23	2020-8-16 12:14:23
jdbc:mysql://192.168.25.158:3306/seata_storage^^^t_storage^^^1	192.168.25.141:8091:2051276925	2051276925	2051276931	jdbc:mysql://192.168.25.158:3306/seata_storage	t_storage	1	2020-8-16 12:14:23	2020-8-16 12:14:23

seata_order -> undo_log

id	branch_id	xid	context	rollback_info	log_status	log_created	log_modified	ext
11	2051276927	192.168.25.141:8091:2051276925	serializer=jackson	见下方	0	2020-8-16 12:14:23	2020-8-16 12:14:23

seata_order -> undo_log -> rollback_info

{
  "@class": "io.seata.rm.datasource.undo.BranchUndoLog",
  "xid": "192.168.25.141:8091:2051276925",
  "branchId": 2051276927,
  "sqlUndoLogs": [
    "java.util.ArrayList",
    [
      {
        "@class": "io.seata.rm.datasource.undo.SQLUndoLog",
        "sqlType": "INSERT",
        "tableName": "t_order",
        "beforeImage": {
          "@class": "io.seata.rm.datasource.sql.struct.TableRecords$EmptyTableRecords",
          "tableName": "t_order",
          "rows": [
            "java.util.ArrayList",
            []
          ]
        },
        "afterImage": {
          "@class": "io.seata.rm.datasource.sql.struct.TableRecords",
          "tableName": "t_order",
          "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": "PrimaryKey",
                      "type": -5,
                      "value": [
                        "java.lang.Long",
                        17
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "user_id",
                      "keyType": "NULL",
                      "type": -5,
                      "value": [
                        "java.lang.Long",
                        1
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "product_id",
                      "keyType": "NULL",
                      "type": -5,
                      "value": [
                        "java.lang.Long",
                        1
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "count",
                      "keyType": "NULL",
                      "type": 4,
                      "value": 10
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "money",
                      "keyType": "NULL",
                      "type": 3,
                      "value": [
                        "java.math.BigDecimal",
                        100
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "status",
                      "keyType": "NULL",
                      "type": 4,
                      "value": 0
                    }
                  ]
                ]
              }
            ]
          ]
        }
      }
    ]
  ]
}

seata_storage -> undo_log

id	branch_id	xid	context	rollback_info	log_status	log_created	log_modified	ext
11	2051276931	192.168.25.141:8091:2051276925	serializer=jackson	见下方	0	2020-8-16 12:14:23	2020-8-16 12:14:23

seata_storage -> undo_log -> rollback_info

{
  "@class": "io.seata.rm.datasource.undo.BranchUndoLog",
  "xid": "192.168.25.141:8091:2051276925",
  "branchId": 2051276931,
  "sqlUndoLogs": [
    "java.util.ArrayList",
    [
      {
        "@class": "io.seata.rm.datasource.undo.SQLUndoLog",
        "sqlType": "UPDATE",
        "tableName": "t_storage",
        "beforeImage": {
          "@class": "io.seata.rm.datasource.sql.struct.TableRecords",
          "tableName": "t_storage",
          "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": "PrimaryKey",
                      "type": -5,
                      "value": [
                        "java.lang.Long",
                        1
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "used",
                      "keyType": "NULL",
                      "type": 4,
                      "value": 20
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "residue",
                      "keyType": "NULL",
                      "type": 4,
                      "value": 80
                    }
                  ]
                ]
              }
            ]
          ]
        },
        "afterImage": {
          "@class": "io.seata.rm.datasource.sql.struct.TableRecords",
          "tableName": "t_storage",
          "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": "PrimaryKey",
                      "type": -5,
                      "value": [
                        "java.lang.Long",
                        1
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "used",
                      "keyType": "NULL",
                      "type": 4,
                      "value": 30
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "residue",
                      "keyType": "NULL",
                      "type": 4,
                      "value": 70
                    }
                  ]
                ]
              }
            ]
          ]
        }
      }
    ]
  ]
}

seata_account -> undo_log

id	branch_id	xid	context	rollback_info	log_status	log_created	log_modified	ext
2	2051276842	192.168.25.141:8091:2051276834	serializer=jackson	{}	1	2020-8-16 12:06:26	2020-8-16 12:06:26
7	2051276934	192.168.25.141:8091:2051276925	serializer=jackson	见下方	0	2020-8-16 12:14:23	2020-8-16 12:14:23

seata_account -> undo_log -> rollback_info

{
  "@class": "io.seata.rm.datasource.undo.BranchUndoLog",
  "xid": "192.168.25.141:8091:2051276925",
  "branchId": 2051276934,
  "sqlUndoLogs": [
    "java.util.ArrayList",
    [
      {
        "@class": "io.seata.rm.datasource.undo.SQLUndoLog",
        "sqlType": "UPDATE",
        "tableName": "t_account",
        "beforeImage": {
          "@class": "io.seata.rm.datasource.sql.struct.TableRecords",
          "tableName": "t_account",
          "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": "PrimaryKey",
                      "type": -5,
                      "value": [
                        "java.lang.Long",
                        1
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "residue",
                      "keyType": "NULL",
                      "type": 3,
                      "value": [
                        "java.math.BigDecimal",
                        800
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "used",
                      "keyType": "NULL",
                      "type": 3,
                      "value": [
                        "java.math.BigDecimal",
                        200
                      ]
                    }
                  ]
                ]
              }
            ]
          ]
        },
        "afterImage": {
          "@class": "io.seata.rm.datasource.sql.struct.TableRecords",
          "tableName": "t_account",
          "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": "PrimaryKey",
                      "type": -5,
                      "value": [
                        "java.lang.Long",
                        1
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "residue",
                      "keyType": "NULL",
                      "type": 3,
                      "value": [
                        "java.math.BigDecimal",
                        700
                      ]
                    },
                    {
                      "@class": "io.seata.rm.datasource.sql.struct.Field",
                      "name": "used",
                      "keyType": "NULL",
                      "type": 3,
                      "value": [
                        "java.math.BigDecimal",
                        300
                      ]
                    }
                  ]
                ]
              }
            ]
          ]
        }
      }
    ]
  ]
}

放行后继续执行

异步任务阶段的分支提交请求将异步和批量地删除相应 UNDO LOG 记录。

seata -> branch_table
seata -> global_table
seata -> lock_table
seata_order -> undo_log
seata_storage -> undo_log
seata_account -> undo_log

内容被全部删除。

16.Spring Cloud Alibaba学习笔记

Seata案例--原理