1. XA-2PC (two phase commit, 两阶段提交 )
XA是由X/Open组织提出的分布式事务的规范(X代表transaction; A代表accordant？)。XA规范主要定义了(全局)事务管理器(TM: Transaction Manager)和(局部)资源管理器(RM: Resource Manager)之间的接口。XA为了实现分布式事务，将事务的提交分成了两个阶段：也就是2PC (tow phase commit)，XA协议就是通过将事务的提交分为两个阶段来实现分布式事务。
1.1 prepare 阶段：
第一阶段，事务管理器向所有涉及到的数据库服务器发出prepare"准备提交"请求，数据库收到请求后执行数据修改和日志记录等处理，处理完成后只是把事务的状态改成"可以提交",然后把结果返回给事务管理器。
1.2 commit 阶段：
事务管理器收到回应后进入第二阶段，如果在第一阶段内有任何一个数据库的操作发生了错误，或者事务管理器收不到某个数据库的回应，则认为事务失败，回撤所有数据库的事务。数据库服务器收不到第二阶段的确认提交请求，也会把"可以提交"的事务回撤。如果第一阶段中所有数据库都提交成功，那么事务管理器向数据库服务器发出"确认提交"请求，数据库服务器把事务的"可以提交"状态改为"提交完成"状态，然后返回应答。
2. MySQL 中的XA实现
Support for transactions is available for the InnoDB storage engine. The MySQL XA implementation is based on the X/Open CAE document Distributed Transaction Processing: The XA Specification.
Currently, among the MySQL Connectors, MySQL Connector/J 5.0.0 and higher supports XA directly, by means of a class interface that handles the XA SQL statement interface for you.
XA supports distributed transactions, that is, the ability to permit multiple separate transactional resources to participate in a global transaction. Transactional resources often are RDBMSs but may be other kinds of resources.
A global transaction involves several actions that are transactional in themselves, but that all must either complete successfully as a group, or all be rolled back as a group. In essence, this extends ACID properties “up a level” so that multiple ACID transactions can be executed in concert as components of a global operation that also has ACID properties. (However, for a distributed transaction, you must use the isolation level to achieve ACID properties. It is enough to use for a nondistributed transaction, but not for a distributed transaction.)
最重要的一点：使用MySQL中的XA实现分布式事务时必须使用serializable隔离级别。
The MySQL implementation of XA MySQL enables a MySQL server to act as a Resource Manager that handles XA transactions within a global transaction. A client program that connects to the MySQL server acts as the Transaction Manager.
The process for executing a global transaction uses two-phase commit (2PC). This takes place after the actions performed by the branches of the global transaction have been executed.
In the first phase, all branches are prepared. That is, they are told by the TM to get ready to commit. Typically, this means each RM that manages a branch records the actions for the branch in stable storage. The branches indicate whether they are able to do this, and these results are used for the second phase.
In the second phase, the TM tells the RMs whether to commit or roll back. If all branches indicated when they were prepared that they will be able to commit, all branches are told to commit. If any branch indicated when it was prepared that it will not be able to commit, all branches are told to roll back.
第一阶段：为prepare阶段，TM向RM发出prepare指令，RM进行操作，然后返回成功与否的信息给TM；
第二阶段：为事务提交或者回滚阶段，如果TM收到所有RM的成功消息，则TM向RM发出提交指令；不然则发出回滚指令；
XA transaction support is limited to the InnoDB storage engine.(只有innodb支持XA分布式事务)
For "external XA" a MySQL server acts as a Resource Manager and client programs act as Transaction Managers. For "Internal XA", storage engines within a MySQL server act as RMs, and the server itself acts as a TM. Internal XA support is limited by the capabilities of individual storage engines. Internal XA is required for handling XA transactions that involve more than one storage engine. The implementation of internal XA requires that a storage engine support two-phase commit at the table handler level, and currently this is true only for InnoDB.