面试官问我：MySQL中的XA事务崩溃了如何恢复？？

MySQL XA事务问题

服务器强制断电后重启，此时MySQL报错，查看MySQL启动日志时，发现如下所示的错误信息。

InnoDB: The log sequence number in ibdata files does not match

InnoDB: the log sequence number in the ib_logfiles!

100224 23:24:20 InnoDB: Database was not shut down normally!

InnoDB: Starting crash recovery.

InnoDB: Reading tablespace information from the .ibd files...

InnoDB: Restoring possible half-written data pages from the doublewrite

InnoDB: buffer...

InnoDB: Transaction 0 4497755 was in the XA prepared state.

InnoDB: Transaction 0 4468551 was in the XA prepared state.

InnoDB: Transaction 0 4468140 was in the XA prepared state.

InnoDB: 3 transaction(s) which must be rolled back or cleaned up

InnoDB: in total 0 row operations to undo

InnoDB: Trx id counter is 0 5312768

InnoDB: Starting in background the rollback of uncommitted transactions

100224 23:24:20 InnoDB: Rollback of non-prepared transactions completed

100224 23:24:20 InnoDB: Started; log sequence number 0 3805002509

100224 23:24:20 InnoDB: Starting recovery for XA transactions...

100224 23:24:20 InnoDB: Transaction 0 4497755 in prepared state after recovery

100224 23:24:20 InnoDB: Transaction contains changes to 8 rows

100224 23:24:20 InnoDB: Transaction 0 4468551 in prepared state after recovery

100224 23:24:20 InnoDB: Transaction contains changes to 1 rows

100224 23:24:20 InnoDB: Transaction 0 4468140 in prepared state after recovery

100224 23:24:20 InnoDB: Transaction contains changes to 1 rows

100224 23:24:20 InnoDB: 3 transactions in prepared state after recovery

100224 23:24:20 [Note] Found 3 prepared transaction(s) in InnoDB

100224 23:24:20 [Warning] Found 3 prepared XA transactions

100224 23:24:20 [Note] Event Scheduler: Loaded 0 events

100224 23:24:20 [Note] /opt/mysql/bin/mysqld: ready for connections.

Version: '8.0.18' socket: '/tmp/mysql.sock' port: 3306 MySQL Community Server (GPL)

从上面的日志信息中，可以看出有三个XA的事务没有提交或回滚。那该如何恢复MySQL的XA事务呢？

恢复MySQL XA事务

首先，登录到MySQL，执行如下命令。

mysql> xa recover;

+----------+--------------+--------------+------------------------------------------------------------+

| formatID | gtrid_length | bqual_length | data |

+----------+--------------+--------------+------------------------------------------------------------+

| 131075 | 30 | 28 | 1-7f000001:bae5:4b6928eb:f06397f000001:bae5:4b6928eb:f0650 |

| 131075 | 30 | 28 | 1-7f000001:bae5:4b6928eb:fb5c37f000001:bae5:4b6928eb:fb5cd |

| 131075 | 30 | 28 | 1-7f000001:bae5:4b6928eb:f03ea7f000001:bae5:4b6928eb:f0400 |

+----------+--------------+--------------+------------------------------------------------------------+

数据表示信息如下：

formatIDis the formatIDpart of the transaction xid

gtrid_lengthis the length in bytes of the gtridpart of the xid

bqual_lengthis the length in bytes of the bqualpart of the xid

datais the concatenation of the gtridand bqualparts of the xid

这是三个XA事务的信息，准备直接回滚。

mysql> xa rollback '1-7f000001:bae5:4b6928eb:fb5c3','7f000001:bae5:4b6928eb:fb5cd',131075;

Query OK, 0 rows affected (0.41 sec)

MySQL XA事务补充

XA事务支持限于InnoDB存储引擎。

MySQL XA实施是针对外部XA的，其中，MySQL服务器作为资源管理器，而客户端程序作为事务管理器。未实施“内部XA”。这样，就允许MySQL服务器内的单独存储引擎作为RM(资源管理器)，而服务器本身作为TM(事务管理器)。处理包含1个以上存储引擎的XA事务时，需要内部XA。内部XA的实施是不完整的，这是因为，它要求存储引擎在表处理程序层面上支持两阶段提交，目前仅对InnoDB实现了该特性。

对于XA START，不支持JOIN和RESUME子句。

对于XA END，不支持SUSPEND [FOR MIGRATE]子句。

在全局事务内，对于每个XA事务，xid值的bqual部分应是不同的，该要求是对当前MySQL XA实施的限制。它不是XA规范的组成部分。

如果XA事务达到PREPARED状态而且MySQL服务器宕机，当服务器重启后，能够继续处理事务。就像原本应当的那样。但是，如果客户端连接中止而服务器继续运行，服务器将回滚任何未完成的XA事务，即使该事务已达到PREPARED状态也同样。它应能提交或回滚PREPARED XA事务，但在不更改二进制日志机制的情况下不能这样。