(参考并转载自http://blog.donews.com/Ralph/archive/2004/11/18/174646.aspx)

关于分布式事务，我觉得有必要了解清楚以下几点：

(1)目前流行的分布式事务协议DTP，

(2)Java如何定义实现规范

(3)常用的第三方JTA以及使用配置方法

1.分布式事务的协议：DTP 和 Two - Phase Commit Protocol

在分布式事务管理领域，X/Open的DTP模型是业界最被广泛接受分布式事务管理模型。几乎所有的事务处理产品，关系数据库以及消息产品都支持该DTP模型中定义的接口。下面是对DTP模型的一段描述：

X/Open’s

DTP model defines three components: application programs, resource

managers(such as RDBMS), and a transaction manager. This model also

specifies functional interfaces between application programs and the

transaction manager (known as the TX interface), and between the

transaction manager and the resource managers (the XA interface). With

products complying to these interfaces, one can implement transactions

with the two phase commit and recovery protocol to preserve atomicity

of transactions.

DTP 模型中有一个非常重要的模型，叫Two - Phase Commit

Protocol。该协议作用于transaction manager和resource managers之间，保证所有的resource

managers要么都commit, 要么都abort。

具体的TPC protocol执行过程如下：

The first

phase of this protocol is the preparation phase, during which the

transaction manager conducts a voting among all the resource managers

registered for the target transaction. The transaction manager calls

the prepare method on each resource manager, which will return a

X_READONLY or XA_OK constant. The first value implies that the

operations conducted on the target resource are read only, and there

are no operations to be committed. The second value indicates that the

resource manager is ready to commit the operations. In case the

resource manager wants the transaction to be rolled back, it throws an

appropriate XAException.

The second phase is a commit or recover

phase. Depending on whether all resource managers are ready for a

commit or not, one of these phases will be invoked by the transaction

manager. In case all the resource managers return a X_OK in the first

phase, the transaction manager calls the commit method on each resource

manager. Please note that the two phase protocol is not fool-proof, and

the commit calls may throw appropriate XAException in return. In such a

case, the transaction manager should initiate the recovery phase. In

case the voting determines a rollback, the transaction calls the

recover on each prepared resource manager.

2. Java Transaction Service

sun并未提供Java Persistence API的实现，只是定义了一套接口和规范。Java的事务实现构架遵循了DTP模型，主要有两部分组成：JTS(Java Transaction Service)和JTA(Java Transaction API)。JTS主要规定了DTP模型中transaction manager的实现方式，而JTA则定义了application programs, transaction manager及resource manager之间的接口。

JTA可以分为三类， 分别为针对application接口，transaction manager接口和resource manager接口。

(1)application接口包括：

javax.transaction.Synchronization：An

object intended to participate in a synchronization protocol with the

transaction manager should implement this interface. This mechanism is

based on the Observer pattern. This interface has two methods -

beforeCompletion and afterCompletion to be called before starting and

after completing, respectively, the two phase commit operation.

(2)transaction manager接口：

javax.transaction.Status:

Defines the flags for the status of a transaction. The

javax.transaction.Transaction, javax.transaction.TransactionManager,

and javax.transaction.UserTransaction interfaces provide a getStatus

method that returns one of the above status flags.

javax.transaction.Transaction:

An object of this type is created for each global transaction. This

interface provides methods for transaction completion(commit and

rollback), resource enlistment (enlistResource) and delistment

(delistResource), registration of synchronization objects

(registerSynchronization), and query of status of the transaction

(getStatus).

javax.transaction.TransactionManager: This

interface is implemented by the JTS and allows an application server to

communicate with the transaction manager to demarcate transactions

(begin, commit, rollback, suspend and resume), set the transaction for

rollback (setRollbackOnly), get the associated Transaction object

(getTransaction), set the transaction timeout interval

(setTransactionTimeout) and query the status of the transaction

(getStatus).

javax.transaction.UserTransaction: This interface

allows application components to manage transaction boundaries

explicitly. This interface provides methods to begin and end

transactions (begin, commit, and rollback), set the transaction for

rollback (setRollbackOnly), set the transaction timeout interval

(setTransactionTimeout), and get the status of the transaction

(getStatus). The application server should provide an interface to

create an object of this type.

javax.transaction.xa.Xid: This

interface is a Java mapping of the X/Open transaction identifier xid

structure. The transaction manager uses an object of this type to

associate a resource manager with a transaction.

(3)resource manager接口：

javax.transaction.xa.XAResource:

This is a Java mapping of the X/Open XA interface, and is implemented

by resource managers operating with the JTS. This interface provides

methods to start (start) and end (end) work on behalf of a specified

transaction, to prepare a transaction with the current resource

(prepare), to end transactions with the current resource (commit,

forget, recover, and rollback), to compare the current resource manager

with another resource manager (isSameRM), and to get and set the

transaction timeout (getTransactionTimeout, setTransactionTimeout).

使用JTA进行分布式事务编程有两种方法，一种是通过javax.transaction.UserTransaction,另一种是通过javax.transaction.TransactionManager。但这两种方法之间的区别不是很清楚？

运用javax.transaction.UserTransaction(以Weblogic6.1为例)：

//get UserTransaction

//in websphere, jndi name is “java:comp/UserTransaction”

UserTransaction  userTrans =   (UserTransaction)getInitialContext().lookup("javax.transaction.UserTransaction");

//begin transaction

userTrans.begin();

//transactional operations

…

//end transaction

userTrans.commit();

运用javax.transaction.TransactionManager(以Weblogic6.1为例)：

//get UserTransaction

UserTransaction  transManager =   (UserTransaction)getInitialContext().lookup("javax.transaction.TransactionManager");

//begin transaction

Transaction trans = transManager.begin();

//transactional operations

…

//end transaction

transManager.commit();

Transaction

对象封装了事务上下文，通过该对象的enlistResource()，delistResource()方法可以添加删除参与事务的resource

manager, 通过registerSynchronization()方法可以注册监听对象。

3.AtomikosTransactionsEssentials简介

关于这一点可以参考：http://momoko8443.javaeye.com/blog/190994