. NET code example on dirty read and unrepeatable and phantom read

Three possible problems of concurrency

Dirty read

Definition: During the execution of transaction A, transaction B reads the modification of transaction A, but transaction A does not end (COMMIT). Transaction A may succeed or fail later.

Metaphor: A modified the source code and did not submit it to the source code system. A sent the code to B directly through QQ. A then canceled the modification.

Sample Code

Copy codeThe Code is as follows: [TestMethod]
Public void dirty read _ test ()
{
// Prerequisites
Using (var context = new TestEntities ())
{
Assert. AreEqual (1, context. Tables. Count ());
}

Var autoResetEvent = new AutoResetEvent (false );

Var transactionOptions1 = new TransactionOptions {IsolationLevel = IsolationLevel. ReadCommitted };
Var transactionOptions2 = new TransactionOptions {IsolationLevel = IsolationLevel. ReadUncommitted };

Using (var ts1 = new TransactionScope (transactionscospontion. Required, transactionOptions1 ))
{
// Add data
Using (var context = new TestEntities ())
{
Context. Tables. Add (new Table () {Id = Guid. NewGuid (), Name = "Duan Guangwei "});
Context. SaveChanges ();
}

ThreadPool. QueueUserWorkItem (data =>
{
Using (var ts2 = new TransactionScope (transactionscospontion. Required, transactionOptions2 ))
{
// Dirty read Test
Using (var context = new TestEntities ())
{
Assert. AreEqual (2, context. Tables. Count ());
}
}

AutoResetEvent. Set ();
});

AutoResetEvent. WaitOne ();
}

// Prerequisites
Using (var context = new TestEntities ())
{
Assert. AreEqual (1, context. Tables. Count ());
}
}

Non-repeated read

Definition: Transaction A reads data twice. During these two reads, transaction B modifies the data, the data read by transaction A is different (repeated read is not allowed ).

Metaphor: A obtained the source code twice during the source code review. During the two review processes, B modified the source code, B may modify the code reviewed by A, which may not comply with the specification.

Sample Code

Copy codeThe Code is as follows: [TestMethod]
Public void cannot be read repeatedly _ test ()
{
Var autoResetEvent = new AutoResetEvent (false );

Var transactionOptions1 = new TransactionOptions {IsolationLevel = IsolationLevel. ReadCommitted };
Var transactionOptions2 = new TransactionOptions {IsolationLevel = IsolationLevel. ReadCommitted };

Using (var ts1 = new TransactionScope (transactionscospontion. Required, transactionOptions1 ))
{
// Prerequisites
Using (var context = new TestEntities ())
{
Assert. AreEqual ("Li yunniu", context. Tables. First (). Name );
}

ThreadPool. QueueUserWorkItem (data =>
{
Using (var ts2 = new TransactionScope (transactionscospontion. Required, transactionOptions2 ))
{
// Modify data
Using (var context = new TestEntities ())
{
Context. Tables. First (). Name = "Duan Guangwei ";
Context. SaveChanges ();
}

Ts2.Complete ();
}

AutoResetEvent. Set ();
});

AutoResetEvent. WaitOne ();

// Non-repeated read Test
Using (var context = new TestEntities ())
{
Assert. AreEqual ("Duan Guangwei", context. Tables. First (). Name );
}
}
}

Phantom read

Definition: Transaction A reads the data twice. During the two reads, transaction B adds the data, and the set read by transaction A is different (phantom read ).

Metaphor: A adds A file to the statistical file during the two statistical processes in order to accurately calculate the data in the statistical file, A finds that the number of these two statistics is different (phantom read). A will feel A headache in his head.

Sample Code

Copy codeThe Code is as follows: [TestMethod]
Public void phantom read_test ()
{
Var autoResetEvent = new AutoResetEvent (false );

Var transactionOptions1 = new TransactionOptions {IsolationLevel = IsolationLevel. RepeatableRead };
Var transactionOptions2 = new TransactionOptions {IsolationLevel = IsolationLevel. ReadCommitted };

Using (var ts1 = new TransactionScope (transactionscospontion. Required, transactionOptions1 ))
{
// Prerequisites
Using (var context = new TestEntities ())
{
Assert. AreEqual (1, context. Tables. Count ());
}

ThreadPool. QueueUserWorkItem (data =>
{
Using (var ts2 = new TransactionScope (transactionscospontion. Required, transactionOptions2 ))
{
// Add data
Using (var context = new TestEntities ())
{
Context. Tables. Add (new Table () {Id = Guid. NewGuid (), Name = "Duan Guangwei "});
Context. SaveChanges ();
}

Ts2.Complete ();
}

AutoResetEvent. Set ();
});

AutoResetEvent. WaitOne ();

// Phantom read Test
Using (var context = new TestEntities ())
{
Assert. AreEqual (2, context. Tables. Count ());
}
}
}

How to handle concurrency issues at the four isolation levels

	Dirty read	Non-repeated read	Phantom read
Read not submitted	Allow	Allow	Allow
Read committed	Not Allowed	Allow	Allow
Repeatable read	Not Allowed	Not Allowed	Allow
Serializing	Not Allowed	Not Allowed	Not Allowed