WCF 4 Step By Step Chapter 9 Note (Transaction)

The commonly accepted standard mechanism for handling distributed transactions is the two-phase commit protocol.

什么是两阶段提交协议 two-phase commit

实现分布式事务的关键就是两阶段提交协议。在此协议中，一个或多个资源管理器的活动均由一个称为事务协调器的单独软件组件来控制。

1)应用程序调用事务协调器中的提交方法

2) 事务协调器将联络事务中涉及的每个资源管理器，并通知它们准备提交事务（这是第一阶段的开始）

3)为 了以肯定的方式响应准备阶段，资源管理器必须将自己置于以下状态：确保能在被要求提交事务时提交事务，或在被要求回滚事务时回滚事务。大多数资源管理器会
将包含其计划更改的日记文件（或等效文件）写入持久存储区中。如果资源管理器无法准备事务，它会以一个否定响应来回应事务协调器。

4)事务协调器收集来自资源管理器的所有响应

5)在 第二阶段，事务协调器将事务的结果通知给每个资源管理器。如果任一资源管理器做出否定响应，则事务协调器会将一个回滚命令发送给事务中涉及的所有资源管理
器。如果资源管理器都做出肯定响应，则事务协调器会指示所有的资源管理器提交事务。一旦通知资源管理器提交，此后的事务就不能失败了。通过以肯定的方式响 应第一阶段，每个资源管理器均已确保，如果以后通知它提交事务，则事务不会失败

The OASIS organization has proposed the Web Services Atomic Transaction (WS-AtomicTransaction) specification, which describes a standard
mechanism for handling transactions in a Web services infrastructure.The WS-AtomicTransaction specification is primarily useful when building Web services. However,

WCF is not just concerned with Web services; you can use WCF to build applications based on many other technologies, such as COM, MSMQ, and .NET Framework Remoting. Microsoft
has provided its own transaction management features, which are built in to the current family of Microsoft Windows operating systems under the name Distributed Transaction Coordinator,
or DTC. DTC uses its own optimized transaction protocol. Transactions based on the DTC transaction protocol are referred to as OLE transactions (OLE was the name of a technology
that was the forerunner of COM). OLE transactions are ideal if you are building solutions based on Microsoft technologies.

Service:

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerSession,
TransactionIsolationLevel=IsolationLevel.RepeatableRead)]
public class ShoppingCartServiceImpl : IShoppingCartService
{
...
}

[OperationBehavior(TransactionScopeRequired=true, TransactionAutoComplete=false)]
public bool AddItemToCart(string productNumber)
{
...
}

**The TransactionAutoComplete property specifies what happens to the transaction

when the operation finishes. If you set this property to true, the transaction automatically

commits and makes all its changes permanent. Setting this property to false keeps

the transaction active; the changes are not committed yet. The default value for this

property is true

Specifying a parameter of TransactionFlowOption.Mandatory indicates that the client application must
create a transaction before calling this operation and must send the details of this transaction as part of the SOAP message header when invoking the operation.

TransactionFlowOption.Allowed, which will use a transaction created by the client if one exists (the WCF runtime will
create a new transaction

if not)

TransactionFlowOption.NotAllowed, which causes the WCF runtime to disregard any client transaction and always create a new one

public interface IShoppingCartService
{
[OperationContract(Name = "AddItemToCart", IsInitiating = true)]
[TransactionFlow(TransactionFlowOption.Mandatory)]
bool AddItemToCart(string productNumber);

2)Configuration Service to Flow Transactions from Client Applications



3)Create a Transaction in the Client Application

TransactionOptions tOpts = new TransactionOptions();
tOpts.IsolationLevel = IsolationLevel.RepeatableRead;
tOpts.Timeout = new TimeSpan(0, 1, 0);
using (TransactionScope tx =
new TransactionScope(TransactionScopeOption.RequiresNew, tOpts))
{
...
tx.Complete();
}

The TransactionScopeOption parameter of the TransactionScope constructor determines

how the WCF runtime utilizes an existing transaction. If this parameter is set to Transaction

ScopeOption.RequiresNew, the WCF runtime will always create a new transaction.

4)Configure the Client Application to Flow Transactions to the ShoppingCartService

Service

5)

[OperationBehavior(TransactionScopeRequired = true,
TransactionAutoComplete = false)]
public bool Checkout()
{
// Not currently implemented
// - just indicate that the transaction completed successfully
// and return true
OperationContext.Current.SetTransactionComplete();
return true;
}

The static Current property of the OperationContext class provides access to the execution

context of the operation. The SetTransactionComplete method indicates that the current transaction has completed successfully and can be committed when the
client application calls the Complete method of the TransactionScope object containing this transaction.If
you need to abort the transaction, just exit the method without calling

the SetTransactionComplete method,

Additionally, you
can call the SetTransactionComplete method only once in a transaction

Implementing the WS-AtomicTransaction Protocol

The NetTcpBinding binding uses OLE transactions and Microsoft’s own protocol for communicating through DTC to other Microsoft-specific services such as SQL Server. In a heterogeneous environment
involving services based on non-Microsoft technologies, you may not be

able to employ OLE transactions. Instead, you should apply a more standardized mechanism,such as the WS-AtomicTransaction protocol.

**Important The BasicHttpBinding binding does not support transactions (OLE or WS-AtomicTransaction).such as the
WS-AtomicTransaction protocol.

The choice of transaction protocol should be transparent to your services and client applications. The code that you write to initiate, control, and support transactions based on the WSAtomicTransaction protocol
is the same as that for manipulating OLE transactions, so the same service can execute using OLE transactions or WS-AtomicTransaction transactions, depending on how you configure the service

If you wish to exploit the implementation of the WS-AtomicTransaction protocol provided by the .NET Framework 4.0 with the HTTP bindings, you must configure support for the WS-AtomicTransaction
protocol in DTC.

Designing a WCF Service to Support Transactions

Transactions, Sessions, and Service Instance Context Modes

An important point to bear in mind from the points just raised is that transactions cannot span multiple
sessions.