Implementing an Implicit Transaction using Transaction Scope

Artigo
11/03/2006

The TransactionScope class provides a simple way to mark a block of code as participating in a transaction, without requiring you to interact with the transaction itself. A transaction scope can select and manage the ambient transaction automatically. Due to its ease of use and efficiency, it is recommended that you use the TransactionScope class when developing a transaction application.

In addition, you do not need to enlist resources explicitly with the transaction. Any System.Transactions resource manager (such as SQL Server 2005) can detect the existence of an ambient transaction created by the scope and automatically enlist.

Creating a transaction scope

The following sample shows a simple usage of the TransactionScope class.

'  This function takes arguments for 2 connection strings and commands to create a transaction 
'  involving two SQL Servers. It returns a value > 0 if the transaction is committed, 0 if the 
'  transaction is rolled back. To test this code, you can connect to two different databases 
'  on the same server by altering the connection string, or to another RDBMS such as Oracle 
'  by altering the code in the connection2 code block.
Public Function CreateTransactionScope( _
  ByVal connectString1 As String, ByVal connectString2 As String, _
  ByVal commandText1 As String, ByVal commandText2 As String) As Integer

    ' Initialize the return value to zero and create a StringWriter to display results.
    Dim returnValue As Integer = 0
    Dim writer As System.IO.StringWriter = New System.IO.StringWriter

    ' Create the TransactionScope to execute the commands, guaranteeing
    '  that both commands can commit or roll back as a single unit of work.
    Using scope As New TransactionScope()
        Using connection1 As New SqlConnection(connectString1)
            Try
                ' Opening the connection automatically enlists it in the 
                ' TransactionScope as a lightweight transaction.
                connection1.Open()

                ' Create the SqlCommand object and execute the first command.
                Dim command1 As SqlCommand = New SqlCommand(commandText1, connection1)
                returnValue = command1.ExecuteNonQuery()
                writer.WriteLine("Rows to be affected by command1: {0}", returnValue)

                ' If you get here, this means that command1 succeeded. By nesting
                ' the using block for connection2 inside that of connection1, you
                ' conserve server and network resources as connection2 is opened
                ' only when there is a chance that the transaction can commit.   
                 Using connection2 As New SqlConnection(connectString2)
                    Try
                        ' The transaction is escalated to a full distributed
                        ' transaction when connection2 is opened.
                        connection2.Open()

                        ' Execute the second command in the second database.
                        returnValue = 0
                        Dim command2 As SqlCommand = New SqlCommand(commandText2, connection2)
                        returnValue = command2.ExecuteNonQuery()
                        writer.WriteLine("Rows to be affected by command2: {0}", returnValue)

                    Catch ex As Exception
                        ' Display information that command2 failed.
                        writer.WriteLine("returnValue for command2: {0}", returnValue)
                        writer.WriteLine("Exception Message2: {0}", ex.Message)
                    End Try
                End Using

            Catch ex As Exception
                ' Display information that command1 failed.
                writer.WriteLine("returnValue for command1: {0}", returnValue)
                writer.WriteLine("Exception Message1: {0}", ex.Message)
            End Try
        End Using

        ' The Complete method commits the transaction. If an exception has been thrown,
        ' Complete is called and the transaction is rolled back.
        scope.Complete()
    End Using

    ' The returnValue is greater than 0 if the transaction committed.
    If returnValue > 0 Then
        writer.WriteLine("Transaction was committed.")
    Else
       ' You could write additional business logic here, for example, you can notify the caller 
       ' by throwing a TransactionAbortedException, or logging the failure.
       writer.WriteLine("Transaction rolled back.")
     End If

    ' Display messages.
    Console.WriteLine(writer.ToString())

    Return returnValue
End Function

// This function takes arguments for 2 connection strings and commands to create a transaction 
// involving two SQL Servers. It returns a value > 0 if the transaction is committed, 0 if the 
// transaction is rolled back. To test this code, you can connect to two different databases 
// on the same server by altering the connection string, or to another RDBMS such as Oracle 
// by altering the code in the connection2 code block.
static public int CreateTransactionScope(
    string connectString1, string connectString2,
    string commandText1, string commandText2)
{
    // Initialize the return value to zero and create a StringWriter to display results.
    int returnValue = 0;
    System.IO.StringWriter writer = new System.IO.StringWriter();

    // Create the TransactionScope to execute the commands, guaranteeing
    // that both commands can commit or roll back as a single unit of work.
    using (TransactionScope scope = new TransactionScope())
    {
        using (SqlConnection connection1 = new SqlConnection(connectString1))
        {
            try
            {
                // Opening the connection automatically enlists it in the 
                // TransactionScope as a lightweight transaction.
                connection1.Open();

                // Create the SqlCommand object and execute the first command.
                SqlCommand command1 = new SqlCommand(commandText1, connection1);
                returnValue = command1.ExecuteNonQuery();
                writer.WriteLine("Rows to be affected by command1: {0}", returnValue);

                // If you get here, this means that command1 succeeded. By nesting
                // the using block for connection2 inside that of connection1, you
                // conserve server and network resources as connection2 is opened
                // only when there is a chance that the transaction can commit.   
                using (SqlConnection connection2 = new SqlConnection(connectString2))
                    try
                    {
                        // The transaction is escalated to a full distributed
                        // transaction when connection2 is opened.
                        connection2.Open();

                        // Execute the second command in the second database.
                        returnValue = 0;
                        SqlCommand command2 = new SqlCommand(commandText2, connection2);
                        returnValue = command2.ExecuteNonQuery();
                        writer.WriteLine("Rows to be affected by command2: {0}", returnValue);
                    }
                    catch (Exception ex)
                    {
                        // Display information that command2 failed.
                        writer.WriteLine("returnValue for command2: {0}", returnValue);
                        writer.WriteLine("Exception Message2: {0}", ex.Message);
                    }
            }
            catch (Exception ex)
            {
                // Display information that command1 failed.
                writer.WriteLine("returnValue for command1: {0}", returnValue);
                writer.WriteLine("Exception Message1: {0}", ex.Message);
            }
        }

        // The Complete method commits the transaction. If an exception has been thrown,
        // Complete is not  called and the transaction is rolled back.
        scope.Complete();
    }

    // The returnValue is greater than 0 if the transaction committed.
    if (returnValue > 0)
    {
        writer.WriteLine("Transaction was committed.");
    }
    else
    {
        // You could write additional business logic here, for example, you can notify the caller 
        // by throwing a TransactionAbortedException, or logging the failure.
        writer.WriteLine("Transaction rolled back.");
    }

    // Display messages.
    Console.WriteLine(writer.ToString());

    return returnValue;
}

The transaction scope is started once you create a new TransactionScope object. As illustrated in the code sample, it is recommended that you create scopes with a using statement. The using statement is available both in C# and in Visual Basic, and works like a try...finally block to ensure that the scope is disposed of properly.

When you instantiate TransactionScope, the transaction manager determines which transaction to participate in. Once determined, the scope always participates in that transaction. The decision is based on two factors: whether an ambient transaction is present and the value of the TransactionScopeOption parameter in the constructor. The ambient transaction is the transaction within which your code executes. You can obtain a reference to the ambient transaction by calling the static Current property of the Transaction class. For more information on how this parameter is used, see the Managing transaction flow using TransactionScopeOption section of this topic.

Completing a transaction scope

When your application completes all the work it wants to perform in a transaction, you should call the Complete method only once to inform the transaction manager that it is acceptable to commit the transaction. It is very good practice to put the call to Complete as the last statement in the using block.

Failing to call this method aborts the transaction, because the transaction manager interprets this as a system failure, or equivalent to an exception thrown within the scope of transaction. However, calling this method does not guarantee that the transaction wil be committed. It is merely a way of informing the transaction manager of your status. After calling the Complete method, you can no longer access the ambient transaction by using the Current property, and attempting to do so will result in an exception being thrown.

If the TransactionScope object created the transaction initially, the actual work of committing the transaction by the transaction manager occurs after the last line of code in the using block. If it did not create the transaction, the commit occurs whenever Commit is called by the owner of the CommittableTransaction object. At that point the Transaction Manager calls the resource managers and informs them to either commit or rollback, based on whether the Complete method was called on the TransactionScope object.

The using statement ensures that the Dispose method of the TransactionScope object is called even if an exception occurs. The Dispose method marks the end of the transaction scope. Exceptions that occur after calling this method may not affect the transaction. This method also restores the ambient transaction to it previous state.

A TransactionAbortedException is thrown if the scope creates the transaction, and the transaction is aborted. A TransactionIndoubtException is thrown if the transaction manager cannot reach a Commit decision. No exception is thrown if the transaction is committed.

Rolling back a transaction

If you want to rollback a transaction, you should not call the Complete method within the transaction scope. For example, you can throw an exception within the scope. The transaction in which it participates in will be rolled back.

Managing transaction flow using TransactionScopeOption

Transaction scope can be nested by calling a method that uses a TransactionScope from within a method that uses its own scope, as is the case with the RootMethod method in the following example,

void RootMethod()
{
     using(TransactionScope scope = new TransactionScope())
     {
          /* Perform transactional work here */
          SomeMethod();
          scope.Complete();
     }
}

void SomeMethod()
{
     using(TransactionScope scope = new TransactionScope())
     {
          /* Perform transactional work here */
          scope.Complete();
     }
}

The top-most transaction scope is referred to as the root scope.

The TransactionScope class provides several overloaded constructors that accept an enumeration of the type TransactionScopeOption, which defines the transactional behavior of the scope.

A TransactionScope object has three options:

Join the ambient transaction, or create a new one if one does not exist.
Be a new root scope, that is, start a new transaction and have that transaction be the new ambient transaction inside its own scope.
Not take part in a transaction at all. There is no ambient transaction as a result.

If the scope is instantiated with Required, and an ambient transaction is present, the scope joins that transaction. If, on the other hand, there is no ambient transaction, then the scope creates a new transaction, and become the root scope. This is the default value. When Required is used, the code inside the scope does not need to behave differently whether it is the root or just joining the ambient transaction. It should operate identically in both cases.

If the scope is instantiated with RequiresNew, it is always the root scope. It starts a new transaction, and its transaction becomes the new ambient transaction inside the scope.

If the scope is instantiated with Suppress, it never takes part in a transaction, regardless of whether an ambient transaction is present. A scope instantiated with this value always have null as its ambient transaction.

The above options are summarized in the following table.

TransactionScopeOption	Ambient Transaction	The scope takes part in
Required	No	New Transaction (will be the root)
Requires New	No	New Transaction (will be the root)
Suppress	No	No Transaction
Required	Yes	Ambient Transaction
Requires New	Yes	New Transaction (will be the root)
Suppress	Yes	No Transaction

When a TransactionScope object joins an existing ambient transaction, disposing of the scope object may not end the transaction, unless the scope aborts the transaction. If the ambient transaction was created by a root scope, only when the root scope is disposed of, does Commit get called on the transaction. If the transaction was created manually, the transaction ends when it is either aborted, or committed by its creator.

The following example shows a TransactionScope object that creates three nested scope objects, each instantiated with a different TransactionScopeOption value.

using(TransactionScope scope1 = new TransactionScope()) 
//Default is Required 
{ 
     using(TransactionScope scope2 = new 
      TransactionScope(TransactionScopeOption.Required)) 
     {
     ...
     } 

     using(TransactionScope scope3 = new TransactionScope(TransactionScopeOption.RequiresNew)) 
     {
     ...
     } 

     using(TransactionScope scope4 = new 
        TransactionScope(TransactionScopeOption.Suppress)) 
    {
     ...
    } 
}

The example shows a code block without any ambient transaction creating a new scope (scope1) with Required. The scope scope1 is a root scope as it creates a new transaction (Transaction A) and makes Transaction A the ambient transaction. Scope1 then creates three more objects, each with a different TransactionScopeOption value. For example, scope2 is created with Required, and since there is an ambient transaction, it joins the first transaction created by scope1. Note that scope3 is the root scope of a new transaction, and that scope4 has no ambient transaction.

Although the default and most commonly used value of TransactionScopeOption is Required, each of the other values has its unique purpose.

Suppress is useful when you want to preserve the operations performed by the code section, and do not want to abort the ambient transaction if the operations fail. For example, when you want to perform logging or audit operations, or when you want to publish events to subscribers regardless of whether your ambient transaction commits or aborts. This value allows you to have a non-transactional code section inside a transaction scope, as shown in the following example.

using(TransactionScope scope1 = new TransactionScope())
{
     try
     {
          //Start of non-transactional section 
          using(TransactionScope scope2 = new
             TransactionScope(TransactionScopeOption.Suppress))
          {
               //Do non-transactional work here
          }
          //Restores ambient transaction here
   }
     catch
     {}
   //Rest of scope1
}

Voting inside a nested scope

Although a nested scope can join the ambient transaction of the root scope, calling Complete in the nested scope has no affect on the root scope. Only if all the scopes from the root scope down to the last nested scope vote to commit the transaction, will the transaction be committed.

Setting the TransactionScope timeout

Some of the overloaded constructors of TransactionScope accept a value of type TimeSpan, which is used to control the timeout of the transaction. A timeout set to zero means an infinite timeout. Infinite timeout is useful mostly for debugging, when you want to isolate a problem in your business logic by stepping through your code, and you do not want the transaction you debug to time out while you attempt to locate the problem. Be extremely careful using the infinite timeout value in all other cases, because it overrides the safeguards against transaction deadlocks.

You typically set the TransactionScope timeout to values other than default in two cases. The first is during development, when you want to test the way your application handles aborted transactions. By setting the timeout to a small value (such as one millisecond), you cause your transaction to fail and can thus observe your error handling code. The second case in which you set the value to be less than the default timeout is when you believe that the scope is involved in resource contention, resulting in deadlocks. In that case, you want to abort the transaction as soon as possible and not wait for the default timeout to expire.

When a scope joins an ambient transaction but specifies a smaller timeout than the one the ambient transaction is set to, the new, shorter timeout is enforced on the TransactionScope object, and the scope must end within the nested time specified, or the transaction is automatically aborted. If the nested scope's timeout is more than that of the ambient transaction, it has no effect.

Setting the TransactionScope isolation level

Some of the overloaded constructors of TransactionScope accept a structure of type TransactionOptions to specify an isolation level, in addition to a timeout value. By default, the transaction executes with isolation level set to Serializable. Selecting an isolation level other than Serializable is commonly used for read-intensive systems. This requires a solid understanding of transaction processing theory and the semantics of the transaction itself, the concurrency issues involved, and the consequences for system consistency.

In addition, not all resource managers support all levels of isolation, and they may elect to take part in the transaction at a higher level than the one configured.

Every isolation level besides Serializable is susceptible to inconsistency resulting from other transactions accessing the same information. The difference between the different isolation levels is in the way read and write locks are used. A lock can be held only when the transaction accesses the data in the resource manager, or it can be held until the transaction is committed or aborted. The former is better for throughput, the latter for consistency. The two kinds of locks and the two kinds of operations (read/write) give four basic isolation levels. See IsolationLevel for more information.

When using nested TransactionScope objects, all nested scopes must be configured to use exactly the same isolation level if they want to join the ambient transaction. If a nested TransactionScope object tries to join the ambient transaction yet it specifies a different isolation level, an ArgumentException is thrown.

Interop with COM+

When you create a new TransactionScope instance, you can use the EnterpriseServicesInteropOption enumeration in one of the constructors to specify how to interact with COM+. For more information on this, see Interoperability with Enterprise Services and COM+ Transactions.

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