MySQL的XA事务恢复过程分析

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mysql数据库开机报错：
InnoDB: The log sequence number in ibdata files does not match
InnoDB: the log sequence number in the ib_logfiles!
100224 12:24:20 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 12:24:20 InnoDB: Rollback of non-prepared transactions completed
100224 12:24:20 InnoDB: Started; log sequence number 0 3805002509
100224 12:24:20 InnoDB: Starting recovery for XA transactions...
100224 12:24:20 InnoDB: Transaction 0 4497755 in prepared state after recovery
100224 12:24:20 InnoDB: Transaction contains changes to 8 rows
100224 12:24:20 InnoDB: Transaction 0 4468551 in prepared state after recovery
100224 12:24:20 InnoDB: Transaction contains changes to 1 rows
100224 12:24:20 InnoDB: Transaction 0 4468140 in prepared state after recovery
100224 12:24:20 InnoDB: Transaction contains changes to 1 rows
100224 12:24:20 InnoDB: 3 transactions in prepared state after recovery
100224 12:24:20 [Note] Found 3 prepared transaction(s) in InnoDB
100224 12:24:20 [Warning] Found 3 prepared XA transactions
100224 12:24:20 [Note] Event Scheduler: Loaded 0 events
100224 12:24:20 [Note] /opt/mysql/bin/mysqld: ready for connections.
Version: '5.1.39' socket: '/tmp/mysql.sock' port: 3306 MySQL Community Server (GPL)
意味着有三个XA的事务没有提交或回滚。
登录到mysql
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)
再启动就正常了。

MySQL XA
I.5. 对XA事务的限制
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。

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