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Lightweight Transaction Manager
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The LTM is a very fast, very inexpensive resource manager for transactions occurring in a single application domain. It s the starting point of all transactions in the Framework, and it monitors the resources being touched by a transaction and enlists the services of more robust resource managers on an as-needed basis. When the transactional work goes out-of-process (i.e., you start modifying database data), the LTM will automatically use a resource manager that supports the Promotable Single Phase Enlistment (PSPE) model of transaction management. This is a new transactional infrastructure that knows and understands the pay as you go mechanism of the LTM. If there is no PSPE manager available, the LTM enlists the DTC. And then, of course, the DTC is enlisted any time multiple remote data sources are modified. When the PSPE model can do the job, then your transactions will perform as well as an ADO.NET Transaction would in version 1.x. Why use the PSPE model if you get the same performance as you would with ADO.NET transactions It s used so that the transaction is automatically promoted to the DTC when more than one database is touched. In version 2.0 of the .NET Framework, you will automatically get a PSPE transaction when working with SQL Server 2005. If the transactional work touches another server or another database, it automatically uses the DTC. Volatile transactions automatically participate in the PSPE without invoking the DTC.
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The new functionality of transactions is made available in the System.Transactions namespace of the Framework class library. There a few ways to create transactions and enlist resources to do work within them, but the best and most common way is to use the TransactionScope object.
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CHAPTER 12 TRANSACTIONS
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Here s a method that uses TranasactionScope to do its work in a transaction. (See XAction.cs in the App_Code directory of the Web12 project.) public bool UpdateQuantity(int itemId, int quantity) { using (TransactionScope tx = new TransactionScope(TransactionScopeOption.Required)) { AuditItemUpdate(itemId, quantity); string sql = "UPDATE Inventory SET OnHand = OnHand - @quantity " + "WHERE InventoryID = @inventoryID and " + "OnHand - @quantity >= 0"; SqlConnection cn = new SqlConnection(connStr); SqlCommand cm = new SqlCommand(sql, cn); cm.Parameters.Add(new SqlParameter("@quantity", SqlDbType.Int)).Value = quantity; cm.Parameters.Add(new SqlParameter("@inventoryID", SqlDbType.Int)).Value = itemId; cn.Open(); int i = cm.ExecuteNonQuery(); cn.Close(); if (i==1) { tx.Complete(); } return Convert.ToBoolean(i); } } Here the TransactionScope is declared within the C# using statement. The TransactionOption enum has three values: Required, RequiresNew, and Suppress. These are a lot like the transaction options available in COM+. They can affect the transactional behavior of your method in the context of any transaction that exists in the call stack calling the method. In other words, with the setting of Required, if a transaction already exists, the work this method does will participate in that transaction; otherwise, a new one will be created. The syntax of the TransactionScope simplifies the semantics of transaction management so much, that it s easy to miss it entirely unless you look closely. Here s the code that s managing the transaction for this method.
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CHAPTER 12 TRANSACTIONS
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using (TransactionScope tx = new TransactionScope(TransactionScopeOption.Required)) { if (i==1) { tx.Complete(); } } Here are some things TransactionScope does for you: Any statement appearing within the brackets of the using statement will be done within the transaction. Any connection created in this block will be enlisted in the transaction. If an error occurs within the using block, the transaction will be automatically rolled back. The check of i==1 is basically a check of are you happy with the work If yes, then call Complete on the transaction for your portion of the work. Every step of the call stack must call Complete for the transaction to be committed. How is the transaction managed under the hood This depends on the database you re connecting to. SQL Server 2005 supports lightweight PSPE transactions. So if this code connects to SQL Server 2005, the transaction is managed by that provider and is no more expensive than an ADO.NET transaction. Let s contrast this block of code to similar code that does the same thing in COM+. The type defining this method does not need to inherit from ServicedComponent. This assembly does not need a strong name, which it would for registration in COM+. No special registration steps are necessary to configure the component within COM+. It performs just as well as Enterprise Services if you re using SQL Server 2000, and performs as well as an ADO.NET transaction if you re using SQL Server 2005. With SQL Server 2005, the DTC would automatically be enlisted to manage the transaction if any layer of the call stack connected to a different database. Let s build up a simple call stack to demonstrate the dynamic enlistment behavior of this infrastructure. You ll use the tables shown in Figure 12-1 to mock-up an order entry scenario. (The script to build this database is in XActionIE.sql in the Code12 directory.)
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