gtin c# Transactions and Locking in C#.NET

Creating EAN-13 in C#.NET Transactions and Locking

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Transactions and Locking
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own when a logical unit of work should begin or end only a developer who knows the business logic can determine that but SQL Server does provide the mechanisms needed to manage transactions.
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Transaction Commit Modes
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There are three basic transaction modes in which SQL Server operates to begin transactions and commit data: autocommit, explicit, and implicit modes. In addition to these, there is a new transaction mode in SQL Server 2005 that is only applicable for multiple active result sets (MARS), called batch-scoped transaction mode. The default transaction mode for SQL Server 2005 is autocommit. Using API functions and T-SQL statements, a transaction can be started in one of the three basic modes. The transaction mode can be set to either autocommit or implicit for a SQL Server connection, or a transaction can be started explicitly through coding. Let s take a look at how each of these modes works and how to use them.
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In autocommit mode, the SQL Server default mode, each T-SQL statement (select, insert, update, delete, schema changes, and so on) is committed when it finishes or is rolled back when it fails. No explicit T-SQL statements or application code is necessary to control transactions with this mode. Each transaction consists of just one T-SQL statement. Autocommit mode is useful when you are executing statements by interactive command line, using the sqlcmd utility or Query Editor, because you do not have to explicitly start and end each transaction. Each statement is treated as its own transaction by SQL Server and is committed as soon as it is finished. Every connection to SQL Server uses autocommit mode until you start an explicit transaction by using BEGIN TRANSACTION or until you specify implicit mode. Once the explicit transaction is ended (with a commit or rollback) or implicit mode is turned off, SQL Server automatically returns to autocommit mode. Autocommit is also the default mode for ADO, OLE DB, ODBC, and DB-Library.
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Explicit Mode
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An explicit transaction is one in which you explicitly define both the start and the end of the transaction. Explicit mode is used most often in application programming, in stored procedures, and T-SQL scripts. When you are executing a group of statements to perform a task, you might need to determine at what points the transaction should start and end so that either the entire group of statements succeeds or the entire group s modifications are rolled back, such is the case when multiple tables with related data must be modified together in order for a particular business function to complete properly and to maintain consistent data between those tables. For example, a bank deposit transaction may require an update to the customer balance and an insert into a historical table that stores a record with information about the deposit, such as the time and place it occurred.
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Part IV
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Microsoft SQL Server 2005 Architecture and Features
When you explicitly identify the beginning and the end of a transaction, you are using explicit mode, and the transaction is referred to as an explicit transaction. You specify an explicit transaction by using either T-SQL statements or API functions. This section explains only the T-SQL method, as the specific API functions are a more detailed developer topic and are beyond the scope of this book (For more information, see Inside Microsoft SQL Server 2005: T-SQL Programming, by Itzik Ben-gan, Dejan Sarka, and Roger Wolter, published by Microsoft Press.). It is very important that the application developer also understands the implications of starting and ending transactions within the application.
Using Explicit Transactions
Let s look at a situation in which you would need to use an explicit transaction to start and end a task. Suppose we have a stored procedure that handles the database task of creating a customer s order for an item. The steps in this procedure include selecting the customer s current account information, entering the new order ID number and the item ordered, calculating the price of the order plus taxes, and updating the customer s account to reflect payment due for the order. We want either all of these steps to be completed together or none of them to be completed so that the data will remain consistent in the database. To achieve this, we will group the statements that handle these tasks into an explicit transaction. If we do not group the statements into a transaction, we could end up with inconsistent data in the database. For example, if the network connection from the client to the server is interrupted after the new order number is entered but before the customer account is updated with the payment due, the database will be left with a new order for the customer but no charge on the customer s account. Without an explicit transaction, SQL Server would commit each statement as soon as it finished using autocommit mode, leaving the stored procedure half-completed at the time of the network disconnect. However, if the steps are defined within one explicit transaction, SQL Server automatically rolls back the entire transaction upon disconnection, and the client can later reconnect and execute the procedure again. Using explicit transactions when your task consists of several steps is also beneficial because, whether or not you specify your own ROLLBACK statements, SQL Server will automatically roll back your transactions when a severe error occurs, such as a break in communication across the network, a database crash, a client system crash, or a deadlock. (Deadlocks are covered in the section Blocking and Deadlocks later in this chapter.) The T-SQL statement used to start a transaction is BEGIN TRANSACTION (BEGIN or BEGIN TRAN are equivalent; see the syntax that follows). You specify the end of an implicit or an explicit transaction by using either COMMIT TRANSACTION or
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