概念

资源管理器（resource manager）：用来管理系统资源，是通向事务资源的途径。数据库就是一种资源管理器。资源管理还应该具有管理事务提交或回滚的能力。
事务管理器（transaction manager）：事务管理器是分布式事务的核心管理者。事务管理器与每个资源管理器（resource manager）进行通信，协调并完成事务的处理。事务的各个分支由唯一命名进行标识
Xid 接口 Xid, Xid 接口是 X/Open 事务标识符 XID 结构的 Java 映射。此接口指定三个访问器方法，以检索全局事务格式 ID、全局事务 ID 和分支限定符。Xid 接口供事务管理器和资源管理器使用。此接口对应用程序不可见。

XA 不能自动提交。

分段提交

XA需要两阶段提交: prepare 和 commit. 
第一阶段为 准备（prepare）阶段。即所有的参与者准备执行事务并锁住需要的资源。参与者ready时，向transaction manager报告已准备就绪。

第二阶段为提交阶段（commit）。当transaction manager确认所有参与者都ready后，向所有参与者发送commit命令。

假设有两个Connection, con1, con2, 大体的过程如下 .

con1 = XAResouce1.getConnection...
con2 = XAResouce2.getConnection...   con1 do some thing.
con2 do some thing.
after they finish.     pre1 = XAResouce1.prepare();
pre2 = XAResouce2.prepare();     if( both pre1 and pre2 are OK）{
XAResouce1 and 2 commit
}else {
XAResouce1 and 2 rollback
}    

事务协调/管理者

因为XA 事务是基于两阶段提交协议的，所以需要有一个事务协调者（transaction manager）来保证所有的事务参与者都完成了准备工作(第一阶段)。如果事务协调者（transaction manager）收到所有参与者都准备好的消息，就会通知所有的事务都可以提交了（第二阶段）。MySQL 在这个XA事务中扮演的是参与者的角色，而不是事务协调者（transaction manager）。

测试用例

import com.alibaba.druid.pool.xa.DruidXADataSource;
import com.mysql.jdbc.jdbc2.optional.MysqlXid;import javax.sql.XAConnection;
import javax.transaction.xa.XAException;
import javax.transaction.xa.XAResource;
import javax.transaction.xa.Xid;
import java.io.IOException;
import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.Properties;class DistributeTransaction {private Properties props;private String propertyfile = "jdbc.properties";private String sql_1 = "delete from test3 where pk_t=3;";private String sql_2 = "INSERT INTO test(name) VALUES('tyz');";DistributeTransaction() {Connection connection_1 = null;Connection connection_2 = null;DruidXADataSource xaDataSource_1 = null;DruidXADataSource xaDataSource_2 = null;Xid xid_1 = null;Xid xid_2 = null;XAConnection xaConnection_1 = null;XAConnection xaConnection_2 = null;XAResource xaResource_1 = null;XAResource xaResource_2 = null;try {props = new Properties();props.load(getClass().getResourceAsStream(propertyfile));} catch (IOException io) {System.err.println("Error while accessing the properties file (" + propertyfile + "). Abort.");System.exit(1);}DruidXADataSource[] xaDataSources = initXADataSource();xaDataSource_1 = xaDataSources[0];xaDataSource_2 = xaDataSources[1];XAConnection[] xaConnections = initXAConnection(xaDataSource_1, xaDataSource_2);xaConnection_1 = xaConnections[0];xaConnection_2 = xaConnections[1];xaResource_1 = initXAResource(xaConnection_1);xaResource_2 = initXAResource(xaConnection_2);connection_1 = getDatabaseConnection(xaConnection_1);connection_2 = getDatabaseConnection(xaConnection_2);// create a separate branch for a common transactionXid[] xids = createXID();xid_1 = xids[0];xid_2 = xids[1];try {execBranch(connection_1, xaResource_1, xid_1, sql_1);execBranch(connection_2, xaResource_2, xid_2, sql_2);if (prepareCommit(xaResource_1, xid_1) == XAResource.XA_OK &&prepareCommit(xaResource_2, xid_2) == XAResource.XA_OK) {commitBranch(xaResource_1, xid_1);commitBranch(xaResource_2, xid_2);} else {throw new RuntimeException();}} catch (Exception e) {rollbackBranch(xaResource_1, xid_1);rollbackBranch(xaResource_2, xid_2);}}DruidXADataSource[] initXADataSource() {System.out.print("Create a XADataSource_1 data source: ");DruidXADataSource xaDataSource_1 = new DruidXADataSource();xaDataSource_1.setDbType(props.getProperty("db1.dbtype"));xaDataSource_1.setUrl(props.getProperty("db1.url"));xaDataSource_1.setUsername(props.getProperty("db1.username"));xaDataSource_1.setPassword(props.getProperty("db1.password"));System.out.println("Okay.");System.out.print("Create a XADataSource_2 data source: ");DruidXADataSource xaDataSource_2 = new DruidXADataSource();xaDataSource_2.setDbType(props.getProperty("db2.dbtype"));xaDataSource_2.setUrl(props.getProperty("db2.url"));xaDataSource_2.setUsername(props.getProperty("db2.username"));xaDataSource_2.setPassword(props.getProperty("db2.password"));System.out.println("Okay.");return new DruidXADataSource[]{xaDataSource_1, xaDataSource_2};}XAConnection[] initXAConnection(DruidXADataSource xaDataSource_1, DruidXADataSource xaDataSource_2) {XAConnection xaconn_1 = null;XAConnection xaconn_2 = null;try {System.out.print("Set up DB_1 XA connection: ");xaconn_1 = xaDataSource_1.getXAConnection();System.out.println("Okay.");System.out.print("Set up DB_2 XA connection: ");xaconn_2 = xaDataSource_2.getXAConnection();System.out.println("Okay.");} catch (SQLException e) {sqlerr(e);}return new XAConnection[]{xaconn_1, xaconn_2};}XAResource initXAResource(XAConnection xacon) {XAResource xares = null;try {System.out.print("Setting up a XA resource: ");xares = xacon.getXAResource();System.out.println("Okay.");} catch (SQLException e) {sqlerr(e);}return xares;}Connection getDatabaseConnection(XAConnection xacon) {Connection con = null;try {System.out.print("Establish database connection: ");con = xacon.getConnection();con.setAutoCommit(false);System.out.println("Okay.");} catch (SQLException e) {sqlerr(e);}return con;}Xid[] createXID() {Xid xid_1 = null;byte[] gid_1 = new byte[1];byte[] bid_1 = new byte[1];gid_1[0] = (Byte.decode(props.getProperty("xid.global"))).byteValue();bid_1[0] = (Byte.decode(props.getProperty("xid.branch.db_1"))).byteValue();System.out.print("Creating an XID (" + Byte.toString(gid_1[0]) + ", " + Byte.toString(bid_1[0]) + ") for DB_1: ");xid_1 = new MysqlXid(gid_1, bid_1, 0);System.out.println("Okay.");Xid xid_2 = null;byte[] gid_2 = new byte[1];byte[] bid_2 = new byte[1];gid_2[0] = (Byte.decode(props.getProperty("xid.global"))).byteValue();bid_2[0] = (Byte.decode(props.getProperty("xid.branch.db_2"))).byteValue();System.out.print("Creating an XID (" + Byte.toString(gid_2[0]) + ", " + Byte.toString(bid_2[0]) + ") for DB_2: ");xid_2 = new MysqlXid(gid_2, bid_2, 0);System.out.println("Okay.");return new Xid[]{xid_1, xid_2};}void execBranch(Connection con, XAResource xares, Xid xid, String sql) {try {xares.start(xid, XAResource.TMNOFLAGS);Statement stmt = con.createStatement();stmt.executeUpdate(sql);xares.end(xid, XAResource.TMSUCCESS);} catch (XAException e) {System.err.println("XA exception caught:");System.err.println("Cause  : " + e.getCause());System.err.println("Message: " + e.getMessage());e.printStackTrace();throw new RuntimeException(e);} catch (SQLException e) {sqlerr(e);throw new RuntimeException(e);}}int prepareCommit(XAResource xares, Xid xid) {int rc = 0;System.out.print("Prepare XA branch (" +Byte.toString((xid.getGlobalTransactionId())[0]) + ", " +Byte.toString((xid.getBranchQualifier())[0]) + "): ");try {xares.prepare(xid);} catch (XAException e) {xaerr(e);throw new RuntimeException(e);}System.out.println("Okay.");return rc;}void commitBranch(XAResource xares, Xid xid) {System.out.print("Commit XA branch (" +Byte.toString((xid.getGlobalTransactionId())[0]) + ", " +Byte.toString((xid.getBranchQualifier())[0]) + "): ");try {// second parameter is 'false' since we have a two phase commitxares.commit(xid, false);} catch (XAException e) {xaerr(e);throw new RuntimeException(e);}System.out.println("Okay.");}void rollbackBranch(XAResource xares, Xid xid) {System.out.print("Rollback XA branch (" +Byte.toString((xid.getGlobalTransactionId())[0]) + ", " +Byte.toString((xid.getBranchQualifier())[0]) + "): ");try {xares.rollback(xid);} catch (XAException e) {xaerr(e);throw new RuntimeException(e);}System.out.println("Okay.");}void sqlerr(SQLException exception) {System.err.println("FAILED.");while (exception != null) {System.err.println("==> SQL Exception caught");System.err.println("--> SQLCODE : " + exception.getErrorCode());System.err.println("--> SQLSTATE: " + exception.getSQLState());System.err.println("--> Message : " + exception.getMessage());exception = exception.getNextException();}}void xaerr(XAException exception) {System.err.println("FAILED.");System.err.println("==> XA Exception caught");System.err.println("--> Cause  : " + exception.getCause());System.err.println("--> Message: " + exception.getMessage());exception.printStackTrace();}public static void main (String args[]) {new DistributeTransaction();}}

XA性能局限性

效率低下，准备阶段的成本持久，全局事务状态的成本持久，性能与本地事务相差10倍左右；
提交前，出现故障难以恢复和隔离问题。