I have the following very simple unit test that reproduces a case where DbContext.SaveChanges is not atomic. By not atomic I mean that the committed data can be read before all the commit was completed.
Add task: In a loop, adds a new TestEntity and a ReferencingEntity. Validate task: checks if there is a TestEntity that is not referenced by any ReferencingEntity - that is not supposed to happen because of the way I add the entities.
The unit test fails... any advice?
EDIT: According to the accepted answer - In order to run the unit test with the proposed solution add in the InitTest method:
using (var context = new TestContext())
{
var objectContext = (context as IObjectContextAdapter).ObjectContext;
objectContext.ExecuteStoreCommand(string.Format("ALTER DATABASE [{0}] SET READ_COMMITTED_SNAPSHOT ON", context.GetType().FullName));
}
Unit test:
using System.Data.Entity;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.VisualStudio.TestTools.UnitTesting;
namespace Atlit.Server.Tests.Integration.SessionProcessing
{
class TestContext : DbContext
{
public DbSet<TestEntity> TestEntities { get; set; }
public DbSet<ReferencingEntity> ReferencingEntities { get; set; }
}
class TestEntity
{
public int TestEntityId { get; set; }
}
class ReferencingEntity
{
public int ReferencingEntityId { get; set; }
public TestEntity TestEntity { get; set; }
}
[TestClass]
public class SaveChangesAtomicTest
{
private volatile int m_Count = 3000;
private volatile bool m_Failed = false;
[TestInitialize]
public void InitTest()
{
using (var context = new TestContext())
{
var dbInitializer = new DropCreateDatabaseAlways<TestContext>();
dbInitializer.InitializeDatabase(context);
}
}
private void AddEntities()
{
while (m_Count-- > 0 && !m_Failed)
{
var transactionOptions = new TransactionOptions { IsolationLevel = IsolationLevel.ReadCommitted };
using (var transactionScope = new TransactionScope(TransactionScopeOption.RequiresNew, transactionOptions))
{
using (var context = new TestContext())
{
var entity = context.TestEntities.Add(new TestEntity());
context.ReferencingEntities.Add(new ReferencingEntity { TestEntity = entity });
context.SaveChanges();
}
transactionScope.Complete();
}
}
}
private void ValidateEntities()
{
while (m_Count > 0 && !m_Failed)
{
if (FreeEntitiesExist())
{
m_Failed = true;
}
}
}
[TestMethod]
public void TestIsSaveChangesAtomic()
{
var addTask = Task.Factory.StartNew(AddEntities);
var readTask = Task.Factory.StartNew(ValidateEntities);
addTask.Wait();
readTask.Wait();
Assert.IsFalse(FreeEntitiesExist(), "sanity failed");
Assert.IsFalse(m_Failed, "test failed");
}
private static bool FreeEntitiesExist()
{
using (var context = new TestContext())
{
return (from entity in context.TestEntities
where !context.ReferencingEntities.Any(re => re.TestEntity.TestEntityId == entity.TestEntityId)
select entity)
.ToArray().Any();
}
}
}
}
