IsolationLevel Énumération
Définition
Important
Certaines informations portent sur la préversion du produit qui est susceptible d’être en grande partie modifiée avant sa publication. Microsoft exclut toute garantie, expresse ou implicite, concernant les informations fournies ici.
Spécifie le comportement de verrouillage des transactions pour la connexion.
Cette énumération prend en charge une combinaison au niveau du bit de ses valeurs membres.
public enum class IsolationLevelpublic enum IsolationLevel[System.Flags]
public enum IsolationLeveltype IsolationLevel = [<System.Flags>]
type IsolationLevel = Public Enum IsolationLevel- Héritage
- Attributs
Champs
| Chaos | 16 | Les modifications en attente, provenant des transactions les plus isolées, ne peuvent pas être remplacées. |
| ReadCommitted | 4096 | Les verrous partagés sont conservés pendant la lecture des données afin d'éviter tout défaut de lecture, mais les données peuvent être modifiées avant la fin de la transaction, entraînant ainsi des données fantômes ou des lectures qui ne peuvent pas être répétées. |
| ReadUncommitted | 256 | Un défaut de lecture est possible, ce qui signifie qu'aucun verrou partagé n'est émis et qu'aucun verrou exclusif n'est respecté. |
| RepeatableRead | 65536 | Des verrous sont placés sur toutes les données utilisées dans une requête afin d’empêcher d’autres utilisateurs de mettre à jour les données. Empêche les lectures qui ne peuvent pas être répétées, mais des lignes fantômes peuvent toujours exister. |
| Serializable | 1048576 | Un verrou de plage est placé sur DataSet afin d'empêcher les autres utilisateurs de mettre à jour ou de modifier les lignes du groupe de données avant la fin de la transaction. |
| Snapshot | 16777216 | Réduit le blocage en stockant une version des données qu'une application pourra lire pendant qu'une autre les modifiera. Indique qu'il n'est pas possible de voir les modifications apportées dans une transaction à partir d'une autre transaction, même si vous réexécutez la requête. |
| Unspecified | -1 | Un niveau d'isolement différent de celui spécifié est utilisé actuellement, mais il est impossible de le déterminer. |
Exemples
Cette application montre comment utiliser IsolationLevel dans DbTransaction. L’exemple montre quels sont les comportements suivants autorisés dans les différents niveaux d’isolation :
Lectures incorrectes.
Lectures non reproductibles.
Fantômes.
Cette application s’exécute dans les niveaux d’isolation suivants :
ReadUncommitted
ReadCommitted
RepeatableRead
Sérialisable
Instantané
La classe PhantomReadThreads montre si la transaction spécifique autorise le comportement de lecture fantôme. Si la transaction autorise le comportement, les threads fonctionnent dans l’ordre suivant :
Dans le premier thread, sélectionnez les produits (Tous).
Dans le deuxième thread, insérez un nouveau produit.
Validez la transaction dans le deuxième thread.
Sélectionnez à nouveau les produits.
Validez la transaction dans le premier thread.
Si la transaction autorise le comportement, les deux opérations De sélection obtiennent les résultats différents.
La classe NonrepeatableReadThreads montre si la transaction spécifique autorise le comportement de lecture non répécable. Si la transaction autorise le comportement, les threads fonctionnent dans l’ordre suivant :
Dans le premier thread, sélectionnez le produit(ProductId=1).
Dans le deuxième thread, mettez à jour la valeur Quantity(ProductId=1).
Validez la transaction dans le deuxième thread.
Sélectionnez à nouveau le produit.
Validez la transaction dans le premier thread.
Si la transaction autorise le comportement, les deux opérations De sélection obtiennent les résultats différents.
La classe ExchangeValuesThreads illustre la différence entre la transaction Serializable et la transaction d’instantané. Pour la transaction sérialisable, les threads fonctionnent dans l’ordre suivant :
Dans le premier thread, obtenez le prix du produit(ProductId=2) et stockez-le dans la variable.
Dans le premier thread, mettez à jour le prix du produit(ProductId=1) avec le prix de product(ProductId=2).
Validez la transaction dans le premier thread.
Dans le deuxième thread, obtenez le prix du produit(ProductId=1) et stockez-le dans la variable .
Dans le deuxième thread, mettez à jour le prix du produit(ProductId=2) avec le prix de product(ProductId=1).
Validez la transaction dans le deuxième thread.
Désormais, les valeurs de Price(ProductId=1 et ProductId=2) sont identiques au prix d’origine de Product(ProductId=2).
Pour la transaction d’instantané, les threads fonctionnent dans l’ordre suivant :
Dans le premier thread, obtenez le prix du produit(ProductId=2) et stockez-le dans la variable ;
Dans le premier thread, mettez à jour le prix du produit(ProductId=1) avec le prix de product(ProductId=2).
Dans le deuxième thread, récupérez le prix du produit(ProductId=1) à partir de l’instantané et stockez-le dans la variable.
Dans le deuxième thread, mettez à jour le prix du produit(ProductId=2) avec le prix de product(ProductId=1).
Validez la transaction dans le deuxième thread.
Validez la transaction dans le premier thread.
Échangez maintenant le prix des produits(ProductId=1 et ProductId=2).
La classe DirtyReadThreads montre si la transaction spécifique autorise le comportement de lecture incorrecte. Si la transaction autorise le comportement, les threads fonctionnent dans l’ordre suivant :
Dans le premier thread, commencez une transaction et ajoutez la valeur Quantity (ProductId=1).
Dans le deuxième thread, lisez la valeur Quantity et ajoutez à nouveau la valeur.
Validez la transaction dans le deuxième thread.
Restaurez la transaction dans le premier thread.
Si la transaction autorise le comportement, la valeur Quantity est ajoutée deux fois.
using System;
using System.Data.SqlClient;
using System.Data;
using System.Threading;
using System.Threading.Tasks;
namespace CSDataIsolationLevel {
// Use the delegate to call the different threads.
public delegate void AsyncAccessDatabase(String connString, IsolationLevel level);
static class DirtyReadThreads {
public static void DirtyReadFirstThread(String connStrig, IsolationLevel level) {
Console.WriteLine("Begin the DirtyReadFirstThread.....");
using (SqlConnection conn = new SqlConnection(connStrig)) {
String cmdText = @"Use DbDataIsolationLevel;
Update dbo.Products set Quantity=Quantity+100 where ProductId=1;
WaitFor Delay '00:00:06';";
conn.Open();
using (SqlTransaction tran = conn.BeginTransaction(level, "DirtyReadFirst")) {
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
command.Transaction = tran;
command.ExecuteNonQuery();
}
if (tran != null)
tran.Rollback();
}
}
Console.WriteLine("Exit from the DirtyReadFirstThread.....");
}
public static void DirtyReadSecondThread(String connStrig, IsolationLevel level) {
Console.WriteLine("Begin the DirtyReadSecondThread.....");
using (SqlConnection conn = new SqlConnection(connStrig)) {
String cmdText = @"Use DbDataIsolationLevel;
WaitFor Delay '00:00:03';
Declare @qty int;
select @qty=Quantity from dbo.Products where ProductId=1;
Update dbo.Products set Quantity=@qty+100 where ProductId=1;";
conn.Open();
using (SqlTransaction tran = conn.BeginTransaction(level, "DirtyReadSecond")) {
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
command.Transaction = tran;
command.ExecuteNonQuery();
}
tran.Commit();
}
}
Console.WriteLine("Exit from the DirtyReadSecondThread.....");
}
}
static class NonrepeatableReadThreads {
public static void NonrepeatableReadFirstThread(String connStrig, IsolationLevel level) {
Console.WriteLine("Begin the NonrepeatableReadFirstThread.....");
using (SqlConnection conn = new SqlConnection(connStrig)) {
String cmdText = @"Use DbDataIsolationLevel;
Select ProductId,ProductName,Quantity,Price
from dbo.Products
where ProductId=1
WaitFor Delay '00:00:06';
Select ProductId,ProductName,Quantity,Price
from dbo.Products
where ProductId=1";
conn.Open();
using (SqlTransaction tran = conn.BeginTransaction(level, "NonrepeatableReadFirst")) {
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
command.Transaction = tran;
using (SqlDataReader reader = command.ExecuteReader()) {
Boolean isFirstReader = true;
do {
Console.WriteLine("It's the result of {0} read:", isFirstReader ? "first" : "second");
TransactionIsolationLevels.DisplayData(reader);
isFirstReader = !isFirstReader;
} while (reader.NextResult() && !isFirstReader);
}
}
tran.Commit();
}
}
Console.WriteLine("Exit from the NonrepeatableReadFirstThread.....");
}
public static void NonrepeatableReadSecondThread(String connStrig, IsolationLevel level) {
Console.WriteLine("Begin the NonrepeatableReadSecondThread.....");
using (SqlConnection conn = new SqlConnection(connStrig)) {
String cmdText = @"Use DbDataIsolationLevel;
WaitFor Delay '00:00:03';
Update dbo.Products set Quantity=Quantity+100 where ProductId=1;";
conn.Open();
using (SqlTransaction tran = conn.BeginTransaction(level, "NonrepeatableReadSecond")) {
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
command.Transaction = tran;
command.ExecuteNonQuery();
}
tran.Commit();
}
}
Console.WriteLine("Exit from the NonrepeatableReadSecondThread.....");
}
}
static class PhantomReadThreads {
public static void PhantomReadFirstThread(String connStrig, IsolationLevel level) {
Console.WriteLine("Begin the PhantomReadFirstThread.....");
using (SqlConnection conn = new SqlConnection(connStrig)) {
String cmdText = @"Use DbDataIsolationLevel;
Select ProductId,ProductName,Quantity,Price
from dbo.Products
WaitFor Delay '00:00:06';
Select ProductId,ProductName,Quantity,Price
from dbo.Products";
conn.Open();
using (SqlTransaction tran = conn.BeginTransaction(level, "PhantomReadFirst")) {
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
command.Transaction = tran;
using (SqlDataReader reader = command.ExecuteReader()) {
Boolean isFirstReader = true;
do {
Console.WriteLine("It's the result of {0} read:", isFirstReader ? "first" : "second");
TransactionIsolationLevels.DisplayData(reader);
isFirstReader = !isFirstReader;
} while (reader.NextResult() && !isFirstReader);
}
}
tran.Commit();
}
}
Console.WriteLine("Exit from the PhantomReadFirstThread.....");
}
public static void PhantomReadSecondThread(String connStrig, IsolationLevel level) {
Console.WriteLine("Begin the PhantomReadSecondThread.....");
using (SqlConnection conn = new SqlConnection(connStrig)) {
String cmdText = @"Use DbDataIsolationLevel;
WaitFor Delay '00:00:03';
INSERT [dbo].[Products] ([ProductName], [Quantity], [Price])
VALUES (N'White Bike', 843, 1349.00)";
conn.Open();
using (SqlTransaction tran = conn.BeginTransaction(level, "PhantomReadSecond")) {
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
command.Transaction = tran;
command.ExecuteNonQuery();
}
tran.Commit();
}
}
Console.WriteLine("Exit from the PhantomReadSecondThread.....");
}
}
// Demonstrates if the specific transaction allows the following behaviors:
// 1. Dirty reads;
// 2. Non-repeatable reads;
// 3. Phantoms.
static class TransactionIsolationLevels {
public static void DemonstrateIsolationLevel(String connString, IsolationLevel level) {
// Before connect the database, recreate the table.
OperateDatabase.CreateTable(connString);
DemonstrateIsolationLevel(connString, level, DirtyReadThreads.DirtyReadFirstThread, DirtyReadThreads.DirtyReadSecondThread);
DisplayData(connString);
Console.WriteLine();
OperateDatabase.CreateTable(connString);
DemonstrateIsolationLevel(connString, level, NonrepeatableReadThreads.NonrepeatableReadFirstThread, NonrepeatableReadThreads.NonrepeatableReadSecondThread);
Console.WriteLine();
OperateDatabase.CreateTable(connString);
DemonstrateIsolationLevel(connString, level, PhantomReadThreads.PhantomReadFirstThread, PhantomReadThreads.PhantomReadSecondThread);
Console.WriteLine();
}
// Demonstrates if the specific transaction allows the specific behaviors.
public static void DemonstrateIsolationLevel(String connString, IsolationLevel level,
AsyncAccessDatabase firstThread, AsyncAccessDatabase secondThread) {
Task[] tasks ={
Task.Factory.StartNew(()=>firstThread(connString, level)),
Task.Factory.StartNew(()=>secondThread(connString, level))
};
Task.WaitAll(tasks);
}
static class ExchangeValuesThreads {
public static void ExchangeValuesFirstThread(String connStrig, IsolationLevel level) {
Console.WriteLine("Begin the ExchangeValuesFirstThread.....");
using (SqlConnection conn = new SqlConnection(connStrig)) {
String cmdText = @"Use DbDataIsolationLevel;
Declare @price money;
select @price=Price from dbo.Products where ProductId=2;
Update dbo.Products set Price=@price where ProductId=1;
WaitFor Delay '00:00:06'; ";
conn.Open();
using (SqlTransaction tran = conn.BeginTransaction(level, "ExchangeValuesFirst")) {
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
command.Transaction = tran;
command.ExecuteNonQuery();
}
tran.Commit();
}
}
Console.WriteLine("Exit from the ExchangeValuesFirstThread.....");
}
public static void ExchangeValuesSecondThread(String connStrig, IsolationLevel level) {
Console.WriteLine("Begin the ExchangeValuesSecondThread.....");
using (SqlConnection conn = new SqlConnection(connStrig)) {
String cmdText = @"Use DbDataIsolationLevel;
WaitFor Delay '00:00:03';
Declare @price money;
select @price=Price from dbo.Products where ProductId=1;
Update dbo.Products set Price=@price where ProductId=2;";
conn.Open();
using (SqlTransaction tran = conn.BeginTransaction(level, "ExchangeValuesSecond")) {
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
command.Transaction = tran;
command.ExecuteNonQuery();
}
tran.Commit();
}
}
Console.WriteLine("Exit from the ExchangeValuesSecondThread.....");
}
}
// Demonstrates the difference between the Serializable and Snapshot transaction
public static void DemonstrateBetweenSnapshotAndSerializable(String connString) {
OperateDatabase.CreateTable(connString);
Console.WriteLine("Exchange Vaules in the Snapshot transaction:");
DemonstrateIsolationLevel(connString, IsolationLevel.Snapshot,
ExchangeValuesThreads.ExchangeValuesFirstThread,
ExchangeValuesThreads.ExchangeValuesSecondThread);
DisplayData(connString);
Console.WriteLine();
Console.WriteLine("Cannot Exchange Vaules in the Serializable transaction:");
OperateDatabase.CreateTable(connString);
DemonstrateIsolationLevel(connString, IsolationLevel.Serializable,
ExchangeValuesThreads.ExchangeValuesFirstThread,
ExchangeValuesThreads.ExchangeValuesSecondThread);
DisplayData(connString);
}
public static void DisplayData(String connString) {
using (SqlConnection conn = new SqlConnection(connString)) {
String cmdText = @"Use DbDataIsolationLevel;
Select ProductId,ProductName,Quantity,Price
from dbo.Products";
conn.Open();
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
using (SqlDataReader reader = command.ExecuteReader()) {
DisplayData(reader);
}
}
}
}
public static void DisplayData(SqlDataReader reader) {
Boolean isFirst = true;
while (reader.Read()) {
if (isFirst) {
isFirst = false;
for (int i = 0; i < reader.FieldCount; i++)
Console.Write("{0,-12} ", reader.GetName(i));
Console.WriteLine();
}
for (int i = 0; i < reader.FieldCount; i++)
Console.Write("{0,-12} ", reader[i]);
Console.WriteLine();
}
}
}
// This class includes database operations. If there's no database 'DbDataIsolationLevel', create the database.
static class OperateDatabase {
public static Boolean CreateDatabase(String connString) {
using (SqlConnection conn = new SqlConnection(connString)) {
String cmdText = @"Use Master;
if Db_Id('DbDataIsolationLevel') is null
create Database [DbDataIsolationLevel];";
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
conn.Open();
command.ExecuteNonQuery();
}
Console.WriteLine("Create the Database 'DbDataIsolationLevel'");
}
return true;
}
// If there's no table [dbo].[Products] in DbDataIsolationLevel, create the table; or recreate it.
public static Boolean CreateTable(String connString) {
using (SqlConnection conn = new SqlConnection(connString)) {
String cmdText = @"Use DbDataIsolationLevel
if Object_ID('[dbo].[Products]') is not null
drop table [dbo].[Products]
Create Table [dbo].[Products]
(
[ProductId] int IDENTITY(1,1) primary key,
[ProductName] NVarchar(100) not null,
[Quantity] int null,
[Price] money null
)";
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
conn.Open();
command.ExecuteNonQuery();
}
}
return InsertRows(connString);
}
// Insert some rows into [dbo].[Products] table.
public static Boolean InsertRows(String connString) {
using (SqlConnection conn = new SqlConnection(connString)) {
String cmdText = @"Use DbDataIsolationLevel
INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Blue Bike', 365,1075.00)
INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Red Bike', 159, 1299.00)
INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Black Bike', 638, 1159.00)";
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
conn.Open();
command.ExecuteNonQuery();
}
}
return true;
}
// Turn on or off 'ALLOW_SNAPSHOT_ISOLATION'
public static Boolean SetSnapshot(String connString, Boolean isOpen) {
using (SqlConnection conn = new SqlConnection(connString)) {
String cmdText = null;
if (isOpen)
cmdText = @"ALTER DATABASE DbDataIsolationLevel SET ALLOW_SNAPSHOT_ISOLATION ON";
else
cmdText = @"ALTER DATABASE DbDataIsolationLevel SET ALLOW_SNAPSHOT_ISOLATION OFF";
using (SqlCommand command = new SqlCommand(cmdText, conn)) {
conn.Open();
command.ExecuteNonQuery();
}
}
return true;
}
}
class Program {
static void Main(string[] args) {
String connString = "Data Source=(local);Initial Catalog=master;Integrated Security=True;Asynchronous Processing=true;";
OperateDatabase.CreateDatabase(connString);
Console.WriteLine();
Console.WriteLine("Demonstrate the ReadUncommitted transaction: ");
TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
System.Data.IsolationLevel.ReadUncommitted);
Console.WriteLine("-----------------------------------------------");
Console.WriteLine("Demonstrate the ReadCommitted transaction: ");
TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
System.Data.IsolationLevel.ReadCommitted);
Console.WriteLine("-----------------------------------------------");
Console.WriteLine("Demonstrate the RepeatableRead transaction: ");
TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
System.Data.IsolationLevel.RepeatableRead);
Console.WriteLine("-----------------------------------------------");
Console.WriteLine("Demonstrate the Serializable transaction: ");
TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
System.Data.IsolationLevel.Serializable);
Console.WriteLine("-----------------------------------------------");
Console.WriteLine("Demonstrate the Snapshot transaction: ");
OperateDatabase.SetSnapshot(connString, true);
TransactionIsolationLevels.DemonstrateIsolationLevel(connString,
System.Data.IsolationLevel.Snapshot);
Console.WriteLine("-----------------------------------------------");
Console.WriteLine("Demonstrate the difference between the Snapshot and Serializable transactions:");
TransactionIsolationLevels.DemonstrateBetweenSnapshotAndSerializable(connString);
OperateDatabase.SetSnapshot(connString, false);
Console.WriteLine();
}
}
}
Imports System.Data.SqlClient
Imports System.Data
Imports System.Threading
Imports System.Threading.Tasks
Namespace CSDataIsolationLevel
' Use the delegate to call the different threads.
Public Delegate Sub AsyncAccessDatabase(connString As [String], level As IsolationLevel)
NotInheritable Class DirtyReadThreads
Private Sub New()
End Sub
Public Shared Sub DirtyReadFirstThread(connStrig As [String], level As IsolationLevel)
Console.WriteLine("Begin the DirtyReadFirstThread.....")
Using conn As New SqlConnection(connStrig)
Dim cmdText As [String] = "Use DbDataIsolationLevel; " & vbCr & vbLf & vbCr & vbLf &
"Update dbo.Products set Quantity=Quantity+100 where ProductId=1;" & vbCr & vbLf &
"WaitFor Delay '00:00:06';"
conn.Open()
Using tran As SqlTransaction = conn.BeginTransaction(level, "DirtyReadFirst")
Using command As New SqlCommand(cmdText, conn)
command.Transaction = tran
command.ExecuteNonQuery()
End Using
If tran IsNot Nothing Then
tran.Rollback()
End If
End Using
End Using
Console.WriteLine("Exit from the DirtyReadFirstThread.....")
End Sub
Public Shared Sub DirtyReadSecondThread(connStrig As [String], level As IsolationLevel)
Console.WriteLine("Begin the DirtyReadSecondThread.....")
Using conn As New SqlConnection(connStrig)
Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
"WaitFor Delay '00:00:03'; " & vbCr & vbLf & vbCr & vbLf &
"Declare @qty int;" & vbCr & vbLf &
"select @qty=Quantity from dbo.Products where ProductId=1;" & vbCr & vbLf & vbCr & vbLf &
"Update dbo.Products set Quantity=@qty+100 where ProductId=1;"
conn.Open()
Using tran As SqlTransaction = conn.BeginTransaction(level, "DirtyReadSecond")
Using command As New SqlCommand(cmdText, conn)
command.Transaction = tran
command.ExecuteNonQuery()
End Using
tran.Commit()
End Using
End Using
Console.WriteLine("Exit from the DirtyReadSecondThread.....")
End Sub
End Class
NotInheritable Class NonrepeatableReadThreads
Private Sub New()
End Sub
Public Shared Sub NonrepeatableReadFirstThread(connStrig As [String], level As IsolationLevel)
Console.WriteLine("Begin the NonrepeatableReadFirstThread.....")
Using conn As New SqlConnection(connStrig)
Dim cmdText As [String] = "Use DbDataIsolationLevel; " & vbCr & vbLf & vbCr & vbLf &
"Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
"from dbo.Products" & vbCr & vbLf &
"where ProductId=1" & vbCr & vbLf & vbCr & vbLf &
"WaitFor Delay '00:00:06';" & vbCr & vbLf & vbCr & vbLf &
"Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
"from dbo.Products" & vbCr & vbLf &
"where ProductId=1"
conn.Open()
Using tran As SqlTransaction = conn.BeginTransaction(level, "NonrepeatableReadFirst")
Using command As New SqlCommand(cmdText, conn)
command.Transaction = tran
Using reader As SqlDataReader = command.ExecuteReader()
Dim isFirstReader As [Boolean] = True
Do
Console.WriteLine("It's the result of {0} read:", If(isFirstReader, "first", "second"))
TransactionIsolationLevels.DisplayData(reader)
isFirstReader = Not isFirstReader
Loop While reader.NextResult() AndAlso Not isFirstReader
End Using
End Using
tran.Commit()
End Using
End Using
Console.WriteLine("Exit from the NonrepeatableReadFirstThread.....")
End Sub
Public Shared Sub NonrepeatableReadSecondThread(connStrig As [String], level As IsolationLevel)
Console.WriteLine("Begin the NonrepeatableReadSecondThread.....")
Using conn As New SqlConnection(connStrig)
Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
"WaitFor Delay '00:00:03'; " & vbCr & vbLf & vbCr & vbLf &
"Update dbo.Products set Quantity=Quantity+100 where ProductId=1;"
conn.Open()
Using tran As SqlTransaction = conn.BeginTransaction(level, "NonrepeatableReadSecond")
Using command As New SqlCommand(cmdText, conn)
command.Transaction = tran
command.ExecuteNonQuery()
End Using
tran.Commit()
End Using
End Using
Console.WriteLine("Exit from the NonrepeatableReadSecondThread.....")
End Sub
End Class
NotInheritable Class PhantomReadThreads
Private Sub New()
End Sub
Public Shared Sub PhantomReadFirstThread(connStrig As [String], level As IsolationLevel)
Console.WriteLine("Begin the PhantomReadFirstThread.....")
Using conn As New SqlConnection(connStrig)
Dim cmdText As [String] = "Use DbDataIsolationLevel; " & vbCr & vbLf & vbCr & vbLf &
"Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
"from dbo.Products" & vbCr & vbLf & vbCr & vbLf &
"WaitFor Delay '00:00:06';" & vbCr & vbLf & vbCr & vbLf &
"Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
"from dbo.Products"
conn.Open()
Using tran As SqlTransaction = conn.BeginTransaction(level, "PhantomReadFirst")
Using command As New SqlCommand(cmdText, conn)
command.Transaction = tran
Using reader As SqlDataReader = command.ExecuteReader()
Dim isFirstReader As [Boolean] = True
Do
Console.WriteLine("It's the result of {0} read:", If(isFirstReader, "first", "second"))
TransactionIsolationLevels.DisplayData(reader)
isFirstReader = Not isFirstReader
Loop While reader.NextResult() AndAlso Not isFirstReader
End Using
End Using
tran.Commit()
End Using
End Using
Console.WriteLine("Exit from the PhantomReadFirstThread.....")
End Sub
Public Shared Sub PhantomReadSecondThread(connStrig As [String], level As IsolationLevel)
Console.WriteLine("Begin the PhantomReadSecondThread.....")
Using conn As New SqlConnection(connStrig)
Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
"WaitFor Delay '00:00:03'; " & vbCr & vbLf & vbCr & vbLf &
"INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) " & vbCr & vbLf &
"VALUES (N'White Bike', 843, 1349.00)"
conn.Open()
Using tran As SqlTransaction = conn.BeginTransaction(level, "PhantomReadSecond")
Using command As New SqlCommand(cmdText, conn)
command.Transaction = tran
command.ExecuteNonQuery()
End Using
tran.Commit()
End Using
End Using
Console.WriteLine("Exit from the PhantomReadSecondThread.....")
End Sub
End Class
' Demonstrates if the specific transaction allows the following behaviors:
' 1. Dirty reads;
' 2. Non-repeatable reads;
' 3. Phantoms.
NotInheritable Class TransactionIsolationLevels
Private Sub New()
End Sub
Public Shared Sub DemonstrateIsolationLevel(connString As [String], level As IsolationLevel)
' Before connect the database, recreate the table.
OperateDatabase.CreateTable(connString)
DemonstrateIsolationLevel(connString, level, AddressOf DirtyReadThreads.DirtyReadFirstThread, AddressOf DirtyReadThreads.DirtyReadSecondThread)
DisplayData(connString)
Console.WriteLine()
OperateDatabase.CreateTable(connString)
DemonstrateIsolationLevel(connString, level, AddressOf NonrepeatableReadThreads.NonrepeatableReadFirstThread, AddressOf NonrepeatableReadThreads.NonrepeatableReadSecondThread)
Console.WriteLine()
OperateDatabase.CreateTable(connString)
DemonstrateIsolationLevel(connString, level, AddressOf PhantomReadThreads.PhantomReadFirstThread, AddressOf PhantomReadThreads.PhantomReadSecondThread)
Console.WriteLine()
End Sub
' Demonstrates if the specific transaction allows the specific behaviors.
Public Shared Sub DemonstrateIsolationLevel(connString As [String], level As IsolationLevel, firstThread As AsyncAccessDatabase, secondThread As AsyncAccessDatabase)
' Dim tasks As Task() = {Task.Factory.StartNew(Function() firstThread(connString, level)), Task.Factory.StartNew(Function() secondThread(connString, level))}
Dim tasks() As Task = {
Task.Factory.StartNew(Sub() firstThread(connString, level)),
Task.Factory.StartNew(Sub() secondThread(connString, level))
}
Task.WaitAll(tasks)
End Sub
Private NotInheritable Class ExchangeValuesThreads
Private Sub New()
End Sub
Public Shared Sub ExchangeValuesFirstThread(connStrig As [String], level As IsolationLevel)
Console.WriteLine("Begin the ExchangeValuesFirstThread.....")
Using conn As New SqlConnection(connStrig)
Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
"Declare @price money;" & vbCr & vbLf &
"select @price=Price from dbo.Products where ProductId=2;" & vbCr & vbLf & vbCr & vbLf &
"Update dbo.Products set Price=@price where ProductId=1;" & vbCr & vbLf & vbCr & vbLf &
"WaitFor Delay '00:00:06'; "
conn.Open()
Using tran As SqlTransaction = conn.BeginTransaction(level, "ExchangeValuesFirst")
Using command As New SqlCommand(cmdText, conn)
command.Transaction = tran
command.ExecuteNonQuery()
End Using
tran.Commit()
End Using
End Using
Console.WriteLine("Exit from the ExchangeValuesFirstThread.....")
End Sub
Public Shared Sub ExchangeValuesSecondThread(connStrig As [String], level As IsolationLevel)
Console.WriteLine("Begin the ExchangeValuesSecondThread.....")
Using conn As New SqlConnection(connStrig)
Dim cmdText As [String] = "Use DbDataIsolationLevel;" & vbCr & vbLf & vbCr & vbLf &
"WaitFor Delay '00:00:03'; " & vbCr & vbLf & vbCr & vbLf &
"Declare @price money;" & vbCr & vbLf &
"select @price=Price from dbo.Products where ProductId=1;" & vbCr & vbLf & vbCr & vbLf &
"Update dbo.Products set Price=@price where ProductId=2;"
conn.Open()
Using tran As SqlTransaction = conn.BeginTransaction(level, "ExchangeValuesSecond")
Using command As New SqlCommand(cmdText, conn)
command.Transaction = tran
command.ExecuteNonQuery()
End Using
tran.Commit()
End Using
End Using
Console.WriteLine("Exit from the ExchangeValuesSecondThread.....")
End Sub
End Class
' Demonstrates the difference between the Serializable and Snapshot transaction
Public Shared Sub DemonstrateBetweenSnapshotAndSerializable(connString As [String])
OperateDatabase.CreateTable(connString)
Console.WriteLine("Exchange Vaules in the Snapshot transaction:")
DemonstrateIsolationLevel(connString, IsolationLevel.Snapshot, AddressOf ExchangeValuesThreads.ExchangeValuesFirstThread, AddressOf ExchangeValuesThreads.ExchangeValuesSecondThread)
DisplayData(connString)
Console.WriteLine()
Console.WriteLine("Cannot Exchange Vaules in the Serializable transaction:")
OperateDatabase.CreateTable(connString)
DemonstrateIsolationLevel(connString, IsolationLevel.Serializable, AddressOf ExchangeValuesThreads.ExchangeValuesFirstThread, AddressOf ExchangeValuesThreads.ExchangeValuesSecondThread)
DisplayData(connString)
End Sub
Public Shared Sub DisplayData(connString As [String])
Using conn As New SqlConnection(connString)
Dim cmdText As [String] = "Use DbDataIsolationLevel; " & vbCr & vbLf & vbCr & vbLf &
"Select ProductId,ProductName,Quantity,Price" & vbCr & vbLf &
"from dbo.Products"
conn.Open()
Using command As New SqlCommand(cmdText, conn)
Using reader As SqlDataReader = command.ExecuteReader()
DisplayData(reader)
End Using
End Using
End Using
End Sub
Public Shared Sub DisplayData(reader As SqlDataReader)
Dim isFirst As [Boolean] = True
While reader.Read()
If isFirst Then
isFirst = False
For i As Integer = 0 To reader.FieldCount - 1
Console.Write("{0,-12} ", reader.GetName(i))
Next
Console.WriteLine()
End If
For i As Integer = 0 To reader.FieldCount - 1
Console.Write("{0,-12} ", reader(i))
Next
Console.WriteLine()
End While
End Sub
End Class
' This class includes database operations. If there's no database 'DbDataIsolationLevel', create the database.
NotInheritable Class OperateDatabase
Private Sub New()
End Sub
Public Shared Function CreateDatabase(connString As [String]) As [Boolean]
Using conn As New SqlConnection(connString)
Dim cmdText As [String] = "Use Master;" & vbCr & vbLf & vbCr & vbLf &
"if Db_Id('DbDataIsolationLevel') is null" & vbCr & vbLf &
"create Database [DbDataIsolationLevel];"
Using command As New SqlCommand(cmdText, conn)
conn.Open()
command.ExecuteNonQuery()
End Using
Console.WriteLine("Create the Database 'DbDataIsolationLevel'")
End Using
Return True
End Function
' If there's no table [dbo].[Products] in DbDataIsolationLevel, create the table; or recreate it.
Public Shared Function CreateTable(connString As [String]) As [Boolean]
Using conn As New SqlConnection(connString)
Dim cmdText As [String] = "Use DbDataIsolationLevel" & vbCr & vbLf & vbCr & vbLf &
"if Object_ID('[dbo].[Products]') is not null" & vbCr & vbLf &
"drop table [dbo].[Products]" & vbCr & vbLf & vbCr & vbLf &
"Create Table [dbo].[Products]" & vbCr & vbLf &
"(" & vbCr & vbLf &
"[ProductId] int IDENTITY(1,1) primary key," & vbCr & vbLf &
"[ProductName] NVarchar(100) not null," & vbCr & vbLf &
"[Quantity] int null," & vbCr & vbLf &
"[Price] money null" & vbCr & vbLf & " )"
Using command As New SqlCommand(cmdText, conn)
conn.Open()
command.ExecuteNonQuery()
End Using
End Using
Return InsertRows(connString)
End Function
' Insert some rows into [dbo].[Products] table.
Public Shared Function InsertRows(connString As [String]) As [Boolean]
Using conn As New SqlConnection(connString)
Dim cmdText As [String] = "Use DbDataIsolationLevel" & vbCr & vbLf & vbCr & vbLf &
"INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Blue Bike', 365,1075.00)" & vbCr & vbLf &
"INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Red Bike', 159, 1299.00)" & vbCr & vbLf &
"INSERT [dbo].[Products] ([ProductName], [Quantity], [Price]) VALUES (N'Black Bike', 638, 1159.00)"
Using command As New SqlCommand(cmdText, conn)
conn.Open()
command.ExecuteNonQuery()
End Using
End Using
Return True
End Function
' Turn on or off 'ALLOW_SNAPSHOT_ISOLATION'
Public Shared Function SetSnapshot(connString As [String], isOpen As [Boolean]) As [Boolean]
Using conn As New SqlConnection(connString)
Dim cmdText As [String] = Nothing
If isOpen Then
cmdText = "ALTER DATABASE DbDataIsolationLevel SET ALLOW_SNAPSHOT_ISOLATION ON"
Else
cmdText = "ALTER DATABASE DbDataIsolationLevel SET ALLOW_SNAPSHOT_ISOLATION OFF"
End If
Using command As New SqlCommand(cmdText, conn)
conn.Open()
command.ExecuteNonQuery()
End Using
End Using
Return True
End Function
End Class
Class Program
Public Shared Sub Main(args As String())
Dim connString As [String] = "Data Source=(local);Initial Catalog=master;Integrated Security=True;Asynchronous Processing=true;"
OperateDatabase.CreateDatabase(connString)
Console.WriteLine()
Console.WriteLine("Demonstrate the ReadUncommitted transaction: ")
TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.ReadUncommitted)
Console.WriteLine("-----------------------------------------------")
Console.WriteLine("Demonstrate the ReadCommitted transaction: ")
TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.ReadCommitted)
Console.WriteLine("-----------------------------------------------")
Console.WriteLine("Demonstrate the RepeatableRead transaction: ")
TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.RepeatableRead)
Console.WriteLine("-----------------------------------------------")
Console.WriteLine("Demonstrate the Serializable transaction: ")
TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.Serializable)
Console.WriteLine("-----------------------------------------------")
Console.WriteLine("Demonstrate the Snapshot transaction: ")
OperateDatabase.SetSnapshot(connString, True)
TransactionIsolationLevels.DemonstrateIsolationLevel(connString, System.Data.IsolationLevel.Snapshot)
Console.WriteLine("-----------------------------------------------")
Console.WriteLine("Demonstrate the difference between the Snapshot and Serializable transactions:")
TransactionIsolationLevels.DemonstrateBetweenSnapshotAndSerializable(connString)
OperateDatabase.SetSnapshot(connString, False)
Console.WriteLine()
End Sub
End Class
End Namespace
Remarques
Les IsolationLevel valeurs sont utilisées par un fournisseur de données .NET lors de l’exécution d’une transaction.
Le IsolationLevel reste en vigueur jusqu’à ce qu’il soit explicitement modifié, mais il peut être modifié à tout moment. La nouvelle valeur est utilisée au moment de l’exécution, et non au moment de l’analyse. En cas de modification au cours d’une transaction, le comportement attendu du serveur est d’appliquer le nouveau niveau de verrouillage à toutes les instructions restantes.
Lorsque vous utilisez OdbcTransaction, si vous ne définissez OdbcTransaction.IsolationLevel pas ou si vous le définissez Unspecifiedsur , la transaction s’exécute en fonction du niveau d’isolation déterminé par le pilote utilisé.
S’applique à
Collaborer avec nous sur GitHub
La source de ce contenu se trouve sur GitHub, où vous pouvez également créer et examiner des problèmes et des demandes de tirage (pull requests). Pour plus d’informations, consultez notre guide du contributeur.
