. code example for dirty read non-repeatable read and Phantom reading in net _ practical tips

Three kinds of problems that may arise from concurrency

Dirty Read

definition : A transaction has read the modification of a transaction during the execution, but a transaction is not finished (committed), and a transaction may succeed or fail later.

analogy : A modified the source code and did not submit to the source code system, a directly through the QQ code to the B,a later canceled the modification.

code example

Copy Code code as follows:

[TestMethod]
public void Dirty Read _ Test ()
{
Predecessor condition
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 (transactionscopeoption.required, transactionOptions1))
{
Add data
using (var context = new Testentities ())
{
Context. Tables.add (New Table () {Id = Guid.NewGuid (), Name = "De Guangwei"});
Context. SaveChanges ();
}

ThreadPool.QueueUserWorkItem (Data =>
{
using (var ts2 = new TransactionScope (transactionscopeoption.required, TransactionOptions2))
{
Dirty Read Test
using (var context = new Testentities ())
{
Assert.AreEqual (2, context. Tables.count ());
}
}

AutoResetEvent.Set ();
});

Autoresetevent.waitone ();
}

Predecessor condition
using (var context = new Testentities ())
{
Assert.AreEqual (1, context. Tables.count ());
}
}

Do not read repeatedly

definition : A transaction read two times, during which the B transaction modified the data, and the data from the two readings of a transaction was different (not repeatable).

analogy : A In doing the source code review, in the process of the review to obtain two times source code, in the two acquisition period B modified the source code, B is likely to modify a review of the code, and this part of the code may not conform to the specification.

code example

Copy Code code as follows:

[TestMethod]
public void non-repeatable read _ 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 (transactionscopeoption.required, transactionOptions1))
{
Predecessor condition
using (var context = new Testentities ())
{
Assert.AreEqual ("Li Yu Girl", context. Tables.first (). Name);
}

ThreadPool.QueueUserWorkItem (Data =>
{
using (var ts2 = new TransactionScope (transactionscopeoption.required, TransactionOptions2))
{
modifying data
using (var context = new Testentities ())
{
Context. Tables.first (). Name = "De Guangwei";
Context. SaveChanges ();
}

Ts2.complete ();
}

AutoResetEvent.Set ();
});

Autoresetevent.waitone ();

Non-repeatable READ test
using (var context = new Testentities ())
{
Assert.AreEqual ("De Guangwei", context. Tables.first (). Name);
}
}
}

Phantom Reading

definition : A transaction reads two of data, and during this two reads the B transaction adds data, and the collection of the two reads from a transaction is different (phantom reading).

analogy : A in the statistical file data, in order to statistical accurate a statistics two times, in these two statistics process B added a file, a found that the number of these two statistics is not the same (phantom reading), a will feel their head a little headache.

code example

Copy Code code 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 (transactionscopeoption.required, transactionOptions1))
{
Predecessor condition
using (var context = new Testentities ())
{
Assert.AreEqual (1, context. Tables.count ());
}

ThreadPool.QueueUserWorkItem (Data =>
{
using (var ts2 = new TransactionScope (transactionscopeoption.required, TransactionOptions2))
{
Add data
using (var context = new Testentities ())
{
Context. Tables.add (New Table () {Id = Guid.NewGuid (), Name = "De Guangwei"});
Context. SaveChanges ();
}

Ts2.complete ();
}

AutoResetEvent.Set ();
});

Autoresetevent.waitone ();

Phantom reading Test
using (var context = new Testentities ())
{
Assert.AreEqual (2, context. Tables.count ());
}
}
}

How to handle concurrency problems with four isolation levels

	Dirty Read	Do not read repeatedly	Phantom reading
Read not submitted	Allow	Allow	Allow
Read submitted	Not allowed	Allow	Allow
REPEATABLE READ	Not allowed	Not allowed	Allow
Serialization	Not allowed	Not allowed	Not allowed