Tài liệu Using Transaction Isolation Levels to Protect Data doc

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Recipe 6.12 Using Transaction Isolation Levels to Protect Data
Problem
You want to effectively use transaction isolation levels to ensure data consistency for a
range of data rows.
Solution
Set and use isolation levels as shown in the following example.
The sample code contains three event handlers:
Start Tran Button.Click
Opens a Connection and starts a transaction with the specified isolation level:
Chaos, ReadCommitted, ReadUncommitted, RepeatableRead, Serializable, or
Unspecified. Within the transaction, a DataTable is filled with the Orders table
from the Northwind database. The default view of the table is bound to the data
grid on the form.
Cancel Button.Click
Rolls back the transaction, closes the connection, and clears the data grid.
Form.Closing
Rolls back the transaction and closes the connection.
The C# code is shown in Example 6-30
.
Example 6-30. File: TransactionIsolationLevelsForm.cs
// Namespaces, variables, and constants
using System;
using System.Configuration;
using System.Windows.Forms;
using System.Data;
using System.Data.SqlClient;

private SqlConnection conn;

private SqlTransaction tran;

// . . .

private void startButton_Click(object sender, System.EventArgs e)
{
startButton.Enabled = false;

// Get the user-defined isolation level.
IsolationLevel il = IsolationLevel.Unspecified;
if(chaosRadioButton.Checked)
il = IsolationLevel.Chaos;
else if(readCommittedRadioButton.Checked)
il = IsolationLevel.ReadCommitted;
else if(readUncommittedRadioButton.Checked)
il = IsolationLevel.ReadUncommitted;
else if(repeatableReadRadioButton.Checked)
il = IsolationLevel.RepeatableRead;
else if(serializableRadioButton.Checked)
il = IsolationLevel.Serializable;
else if(unspecifiedRadioButton.Checked)
il = IsolationLevel.Unspecified;

// Open a connection.
conn = new SqlConnection(
ConfigurationSettings.AppSettings["Sql_ConnectString"]);
conn.Open( );
try
{
// Start a transaction.

tran = conn.BeginTransaction(il);
}
catch(Exception ex)
{
// Could not start the transaction. Close the connection.
conn.Close( );

MessageBox.Show(ex.Message,"Transaction Isolation Levels",
MessageBoxButtons.OK, MessageBoxIcon.Error);
startButton.Enabled = true;
return;
}

String sqlText = "SELECT * FROM Orders";
// Create a command using the transaction.
SqlCommand cmd = new SqlCommand(sqlText, conn, tran);
// Create a DataAdapter to retrieve all Orders.
SqlDataAdapter da = new SqlDataAdapter(cmd);
// Define a CommandBuilder for the DataAdapter.
SqlCommandBuilder cb = new SqlCommandBuilder(da);
// Fill table with Orders.
DataTable dt = new DataTable( );
da.Fill(dt);

// Bind the default view of the table to the grid.
dataGrid.DataSource = dt.DefaultView;

cancelButton.Enabled = true;
dataGrid.ReadOnly = false;
}

private void cancelButton_Click(object sender, System.EventArgs e)
{
cancelButton.Enabled = false;
dataGrid.ReadOnly = true;

// Roll back the transaction and close the connection.
tran.Rollback( );
conn.Close( );

// Unbind the grid.
dataGrid.DataSource = null;

startButton.Enabled = true;
}

private void UsingLockingHintsForPessimisticLockingForm_Closing(
object sender, System.ComponentModel.CancelEventArgs e)
{
// Roll back the transaction and close the connection.
tran.Rollback( );
conn.Close( );
}
Discussion
The isolation level specifies the transaction locking behavior for a connection. It
determines what changes made to data within a transaction are visible outside of the
transaction while the transaction is uncommitted.
Concurrency violations occur when multiple users or processes attempt to modify the
same data in a database at the same time without locking. Table 6-16
describes

concurrency problems.
Table 6-16. Concurrency problems
Condition Description
Lost Update
Two or more transactions select the same row and subsequently
update that row. Data is lost because the transactions are
unaware of each other and overwrite each other's updates.
Uncommitted
Dependency (Dirty
Read)
A second transaction selects a row that has been updated, but not
committed, by another transaction. The first transaction makes
more changes to the data or rolls back the changes already made
resulting in the second transaction having invalid data.
Inconsistent Analysis
(Nonrepeatable
Read)
A second transaction reads different data each time that the same
row is read. Another transaction has changed and committed the
data between the reads.
Phantom Read
An insert or delete is performed for a row belonging to a range
of rows being read by a transaction. The rows selected by the
transaction are missing the inserted rows and still contain the
deleted rows that no longer exist.
Locks ensure transactional integrity and maintain database consistency by controlling
how resources can be accessed by concurrent transactions. A lock is an object indicating
that a user has a dependency on a resource. It prevents other users from performing
operations that would adversely affect the locked resources. Locks are acquired and
released by user actions; they are managed internally by database software. Table 6-17

lists and describes resource lock modes used by ADO.NET.
Table 6-17. Resource lock modes
Lock
mode
Description
Shared
Concurrent transactions can read the locked resource. Concurrent
transactions cannot modify the locked resource while the lock is held.
Exclusive
Prevents both read and modify access to a resource by concurrent
transactions.
Isolation level defines the degree to which one transaction must be isolated from other
transactions. A higher isolation level increases data correctness but decreases concurrent
access to data. Table 6-18
describes the different isolations levels supported by
ADO.NET. The first four levels are listed in order of increasing isolation.
Table 6-18. IsolationLevel enumeration
Name Description
ReadUncommitted
No shared locks are issued. Exclusive locks are not honored.A dirty
read is possible.
ReadCommitted
Shared locks are held while data is being read by the
transaction.Dirty reads are not possible. Nonrepeatable reads or
phantom rows can still occur because data can be changed prior to
being committed.
RepeatableRead
Shared locks are placed on all data used by the query. Other users
are prevented from updating the data.Nonrepeatable reads are

prevented, but phantom reads are still possible.
Serializable
A range lock—covering individual records and the ranges between
them—is placed on the data preventing other users from updating
or inserting rows until the transaction is complete.Phantom reads
are prevented.
Chaos
Pending changes from more highly isolated transactions cannot be
overwritten.This isolation level is not supported by SQL Server.
Unspecified
A different isolation level than the one specified is being used, but
that level cannot be determined.
In ADO.NET, the isolation level can be set by creating the transaction using an overload
of the BeginTransaction( ) method of the Command or by setting the IsolationLevel
property of an existing Transaction object. The default isolation level is ReadCommitted.
Parallel transactions are not supported, so the isolation level applies to the entire
transaction. It can be changed programmatically at any time. If the isolation level is
changed within a transaction, the new level applies to all statements remaining in the
transaction.
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