There is a performance and lock issue when using EF for a update-from-query case on MSSQL 2008. So I put ReadUncommitted transaction isolationlevel, hoping to resolve it, like this,
Before
using (MyEntities db = new MyEntities())
{
// large dataset
var data = from _Contact in db.Contact where _Contact.MemberId == 13 select _Contact;
for (var item in data)
item.Flag = 0;
// Probably db lock
db.SaveChanges();
}
After
using (var scope =
new TransactionScope(TransactionScopeOption.RequiresNew,
new TransactionOptions() { IsolationLevel = IsolationLevel.ReadUncommitted }))
{
using (MyEntities db = new MyEntities())
{
// large dataset but with NOLOCK
var data = from _Contact in db.Contact where _Contact.MemberId == 13 select _Contact;
for (var item in data)
item.Flag = 0;
// Try avoid db lock
db.SaveChanges();
}
}
We use SQL profiler to trace. However, got these scripts in order,
(Expect read-uncommitted for the 1st script.)
Audit Login
set transaction isolation level read committed
SP:StmtStarting
SELECT
[Extent1].[ContactId] AS [ContactId],
[Extent1].[MemberId] AS [MemberId],
FROM [dbo].[Contact] AS [Extent1]
WHERE [Extent1].[MemberId] = #p__linq__0
Audit Login
set transaction isolation level read uncommitted
Though I could resend this request and make it right order (will show read-uncommitted for the following requests, same SPID), I wonder why it sent read-uncommitted command after read-committed command and how to fix by using EF and TransactionScope ? Thanks.
I think this is a red herring caused by relying on the Audit Login Event. This is not showing the moment when client tells server 'set transaction isolation level read uncommitted'. It is showing you what the isolation level is later on, when that connection is picked out of the pool and reused.
I verify this by adding Pooling=false to my connection string. Then, audit login always shows transaction isolation level read committed.
I have so far found no way, in SQL Profiler, of seeing the moment when EF sets the transaction level, nor any explicit begin tran.
I can kind of confirm that it is being set somewhere, by reading and logging the level:
const string selectIsolationLevel = #"SELECT CASE transaction_isolation_level WHEN 0 THEN 'Unspecified' WHEN 1 THEN 'ReadUncommitted' WHEN 2 THEN 'ReadCommitted' WHEN 3 THEN 'Repeatable' WHEN 4 THEN 'Serializable' WHEN 5 THEN 'Snapshot' END AS TRANSACTION_ISOLATION_LEVEL FROM sys.dm_exec_sessions where session_id = ##SPID";
static void ReadUncommitted()
{
using (var scope =
new TransactionScope(TransactionScopeOption.RequiresNew,
new TransactionOptions{ IsolationLevel = IsolationLevel.ReadUncommitted }))
using (myEntities db = new myEntities())
{
Console.WriteLine("Read is about to be performed with isolation level {0}",
db.Database.SqlQuery(typeof(string), selectIsolationLevel).Cast<string>().First()
);
var data = from _Contact in db.Contact where _Contact.MemberId == 13 select _Contact; // large results but with nolock
foreach (var item in data)
item.Flag = 0;
//Using Nuget package https://www.nuget.org/packages/Serilog.Sinks.Literate
//logger = new Serilog.LoggerConfiguration().WriteTo.LiterateConsole().CreateLogger();
//logger.Information("{#scope}", scope);
//logger.Information("{#scopeCurrentTransaction}", Transaction.Current);
//logger.Information("{#dbCurrentTransaction}", db.Database.CurrentTransaction);
//db.Database.ExecuteSqlCommand("-- about to save");
db.SaveChanges(); // Try avoid db lock
//db.Database.ExecuteSqlCommand("-- finished save");
//scope.Complete();
}
}
(I say ‘kind of’ because the statements each run in their own session)
Perhaps this is a long way of saying, yes EF transactions work correctly even if you can't prove it via Profiler.
According to the following note in the ADO.NET documentation Snapshot Isolation in SQL Server, the Isolation Level is not bound to the Transaction Scope as long as the underlying connection is pooled:
If a connection is pooled, resetting its isolation level does not
reset the isolation level at the server. As a result, subsequent
connections that use the same pooled inner connection start with their
isolation levels set to that of the pooled connection. An alternative
to turning off connection pooling is to set the isolation level
explicitly for each connection.
Thus I conclude that until SQL Server 2012, setting the isolation to any other level than ReadCommitted requires to either turn of connection pooling when creating the questionable SqlConnection or to set the Isolation Level in each connection explicitly to avoid unexpected behavior, including deadlocks. Alternatively the Connection Pool could be cleared by calling the ClearPool Method, but since this method is neither bound to the Transaction Scope nor the underlying connection, I don't think that it's approriate when several connections run simultaneously against the same pooled inner connection.
Referencing the post SQL Server 2014 reseting isolation level in the SQL forum and my own tests, such workarounds are obsolete when using SQL Server 2014 and a client driver with TDS 7.3 or higher.
I think a better solution is to perform update by generating a direct query (not selection and update entity by entity). In order to work with objects and not queries, you can use EntityFramework.Extended:
db.Contact.Update(C => c.MemberId == 13, c => new Contact { Flag = 0 });
This should generate something like UPDATE Contact SET Flag = 0 WHERE MemberId = 13 which is much more faster than your current solution.
If I remember correctly, this should generate its own transaction. If this must be executed in a transaction with other queries, `TransactionScope can still be used (you will have two transactions).
Also, isolation level can remain untouched (ReadCommitted).
[EDIT]
Chris's analysis shows exactly what happens. To make it even more relevant the following code shows the difference inside and outside of TransactionScope:
using (var db = new myEntities())
{
// this shows ReadCommitted
Console.WriteLine($"Isolation level outside TransactionScope = {db.Database.SqlQuery(typeof(string), selectIsolationLevel).Cast<string>().First()}");
}
using (var scope =
new TransactionScope(TransactionScopeOption.RequiresNew,
new TransactionOptions() { IsolationLevel = IsolationLevel.ReadUncommitted }))
{
// this show ReadUncommitted
Console.WriteLine($"Isolation level inside TransactionScope = {db.Database.SqlQuery(typeof(string), selectIsolationLevel).Cast<string>().First()}");
using (myEntities db = new myEntities ())
{
var data = from _Contact in db.Contact where _Contact.MemberId == 13 select _Contact; // large results but with nolock
for (var item I data)
item.Flag = 0;
db.SaveChanges(); // Try avoid db lock
}
// this should be added to actually Commit the transaction. Otherwise it will be rolled back
scope.Complete();
}
Coming back to the actual problem (getting deadlocks), if we take a look of what Profiler is outputting during the whole thing, we see something like this (removed GOs):
BEGIN TRANSACTION
SELECT <all columns> FROM Contact
exec sp_reset_connection
exec sp_executesql N'UPDATE Contact
SET [Flag] = #0
WHERE ([Contact] = #1)
',N'#0 nvarchar(1000),#1 int',#0=N'1',#1=1
-- lots and lots of other UPDATEs like above
-- or ROLLBACK if scope.Complete(); is missed
COMMIT
This has two disadvantages:
Many round-trips - many queries are issued against the database, which puts more pressure on database engine and also takes much longer for the client
Long transaction - long transactions should be avoid as a tentative of minimizing deadlocks
So, the suggested solution should work better in your particular case (simple update).
In more complex cases, changing the isolation level might be needed.
I think that, if one deals with large processing of data (select millions, do something, update back etc.) a stored procedure might be the solution, since everything executes server-side.
Related
Changing isolation level within database transaction is ok, including case when you join one transaction to already running one.
You just change the way you handle locks from now on. Using Sql Server, this runs without problems:
begin transaction
set transaction isolation level serializable;
select * from FooTable;
set transaction isolation level read committed;
select * from FooTable;
begin transaction
set transaction isolation level serializable;
select * from FooTable;
--transaction_isolation_level can be observed as 4 (serializable)
But, when using .NET TransactionScope to create transaction in said Sql Server like this(C#, xUnit):
[Theory]
[AutoFixtureMagicToGetParameterInstances]
void ZmenaIzolacniUrovneVedeKVyjimce(IFooDao sut, Foo foo)
{
var tranOpts = new TransactionOptions()
{
IsolationLevel = IsolationLevel.Serializable,
Timeout = TimeSpan.FromSeconds(60)
};
var tranOpts2 = new TransactionOptions()
{
IsolationLevel = IsolationLevel.ReadCommitted,
Timeout = TimeSpan.FromSeconds(60)
};
using (var transactionScope = new TransactionScope(TransactionScopeOption.Required, tranOpts))
{
sut.SelectFoos();
using (var transactionScope2 = new TransactionScope(TransactionScopeOption.Required, tranOpts2))
{
sut.SelectFoos();
}
}
}
leads to exception:
System.ArgumentException : The transaction specified for TransactionScope has a different IsolationLevel than the value requested for the scope.
Parameter name: transactionOptions.IsolationLevel
Why designer of TransactionScope deemed it necessary to immediately throw an exception?
I'd expect behavior would be the same at least as long only database resources are involved.
Is there something about TransactionScope I'm missing or is it just because reasonable behavior cannot be guaranteed across all possible enlisted resources?
As stated in comments here Inner TransactionScope with different IsolationLevel, how can it be achieved?
TransactionScope is not limited to use with SQL Server, it can allow
distributed transactions across processes/systems. So it is stricter
than what SQL Server allows, likely to simplify the complexity of
ensuring consistency across the systems than support distributed
transactions. – AaronLS
So answer basically seems to boil down to "TransactionScope may have way more on it's plate than just database transaction and so it forbids complexities like changing isolation levels".
I suspect I don't understand fully what's going on or something weird is happening. (The first case is more likely I guess.)
The big picture:
I'm trying to have a web-service perform certain operations asynchronously as they can be time consuming and I don't want the client to wait for the operations to finish (just query for the results every now and again to see the operation is done).
The async code is wrapped in a transaction - in case something goes wrong, I want to be able to rollback any changes.
Unfortunately the last step of the async code is to call a DIFFERENT service which queries the same database.
Despite wrapping the whole thing in a Snapshot transaction, the last step fails as the service cannot read from the database.
For that matter, while the async operation is underway I cannot perform a simple SELECT statements from the database either.
Here's a sample of the code which I'm currently using to test out the transactions (using Entity Framework 5 model first):
using (var transaction = new System.Transactions.TransactionScope(System.Transactions.TransactionScopeOption.RequiresNew, new System.Transactions.TransactionOptions() { IsolationLevel = System.Transactions.IsolationLevel.Snapshot }))
{
var db = new DataModelContainer();
Log test = new Log();
test.Message = "TEST";
test.Date = DateTime.UtcNow;
test.Details = "asd";
test.Type = "test";
test.StackTrace = "asd";
db.LogSet.Add(test);
db.SaveChanges();
using (var suppressed = new System.Transactions.TransactionScope(System.Transactions.TransactionScopeOption.Suppress))
{
var newDb = new DataModelContainer();
var log = newDb.LogSet.ToArray(); //deadlock here... WHY?
}
test = db.LogSet.Where(l => l.Message == "TEST").Single();
db.LogSet.Remove(test);
db.SaveChanges();
transaction.Complete();
}
The code creates a simple Log entry in the database (yeah, I'm playing around at the moment so the values are rubbish). I've set the SQL database to allow snapshot isolation, and to my knowledge reads should still be permitted (these are being tested in this code by using a new, suppressed transaction and a new DataModelContainer). However, I cannot query the LogSet in the suppressed transaction or in SQL Management Studio - the whole table is locked!
So... why? Why is it locked if the transaction scope is defined as such? I've also tried other isolation levels (like ReadUncommited) and I still cannot query the table.
Could someone please provide an explanation for this behavior?
In SSMS, set your current isolation level to SNAPSHOT and see if that corrects your problem - it's probably set to READ COMMITTED and would therefore still block due to pending updates.
Update:
You can allow READ COMMITTED to access versioned rows DB-wide by altering the following option (and avoiding having to constantly set the current isolation level to SNAPSHOT):
ALTER DATABASE MyDatabase
SET READ_COMMITTED_SNAPSHOT ON
I'm having trouble suppressing part of a transaction using Sql Server CE 4 with Entity Framework and System.Transactions.TransactionScope.
The simplified code below is from a unit test demonstrating the problem.
The idea is to enable the innerScope block (with no transaction) to succeed or fail without affecting the outerScope block (the "ambient" transaction). This is the stated purpose of TransactionScopeOption.Suppress.
However, the code fails because it seems that the entire SomeTable table is locked by the first insert in outerScope. At the point indicated in the code, this error is thrown:
"SQL Server Compact timed out waiting for a lock. The default lock time is 2000ms for devices and 5000ms for desktops. The default lock timeout can be increased in the connection string using the ssce: default lock timeout property. [ Session id = 2,Thread id = 2248,Process id = 13516,Table name = SomeTable,Conflict type = x lock (x blocks),Resource = PAG (idx): 1046 ]"
[TestMethod()]
[DeploymentItem("MyLocalDb.sdf")]
public void MyLocalDb_TransactionSuppressed()
{
int count = 0;
// This is the ambient transaction
using (TransactionScope outerScope = new TransactionScope(TransactionScopeOption.Required))
{
using (MyObjectContext outerContext = new MyObjectContext())
{
// Do something in the outer scope
outerContext.Connection.Open();
outerContext.AddToSomeTable(CreateSomeTableRow());
outerContext.SaveChanges();
try
{
// Ambient transaction is suppressed for the inner scope of SQLCE operations
using (TransactionScope innerScope = new TransactionScope(TransactionScopeOption.Suppress))
{
using (MyObjectContext innerContext = new MyObjectContext())
{
innerContext.Connection.Open();
// This insert will work
innerContext.AddToSomeTable(CreateSomeTableRow());
innerContext.SaveChanges(); // ====> EXCEPTION THROWN HERE
// There will be other, possibly failing operations here
}
innerScope.Complete();
}
}
catch { }
}
outerScope.Complete();
}
count = GetCountFromSomeTable();
// The insert in the outer scope should succeed, and the one from the inner scope
Assert.AreEqual(2, count);
}
So, it seems that "a transaction in a transaction scope executes with isolation level set to Serializable", according to http://msdn.microsoft.com/en-us/library/ms172001
However, using the following code snippet to change the isolation level of the TransactionScope does not help:
public void MyLocalDb_TransactionSuppressed()
{
TransactionOptions opts = new TransactionOptions();
opts.IsolationLevel = IsolationLevel.ReadCommitted;
int count = 0;
// This is the ambient transaction
using (TransactionScope outerScope = new TransactionScope(TransactionScopeOption.Required, opts))
...
The same exception is thrown at the same location.
It seems the only way to avoid this is to call outerScope.Complete() before entering the innerScope block. But this would defeat the purpose.
What am I missing here?
Thanks.
AFAIK SQL Server Compact does not support nested transactions.
And why do you do that this way? If I look at your code there is no difference between running the second transaction scope inside the first one and running them in sequence.
IMHO this is not a problem of SQL Compact, TransactionScope or isolation level. This is a problem of your wrong application logic.
Each SaveChanges runs in transaction - either outer transaction defined by TransactionScope or inner DbTransaction. Even if it would not create transaction every database command has its own implicit transaction. If you use Suppress on the inner code block you are creating two concurrent transactions which are trying to insert into same table and moreover the first cannot complete without completing the second and the second cannot complete without completing the first => deadlock.
The reason is that insert command always locks part of the table not allowing new inserts until it is committed or rolled back. I'm not sure if this can be avoided by changing transaction isolation level - if it does, you will most probably need Read.Uncommitted.
I have some problem with this piece of code. When I run it, I want it to NOT lock the table(s) used by the transaction. To achieve this goal I set the isolation level to ReadUncommited.
The problem is that it still locks the table, it acts like if the isolationLevel were Serializable. I'm using SQL server 2008
Here is the code:
using (TransactionScope scope = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions() { IsolationLevel = System.Transactions.IsolationLevel.ReadUncommitted }))
{
while (true)
{
using (SqlConnection connection = new SqlConnection(ConnectionString))
{
connection.Open();
Console.WriteLine(Transaction.Current.IsolationLevel);
SqlUtils.ExecuteNonQuery(connection, "INSERT INTO test4 (test) VALUES ('ASDASDASD')");
}
Thread.Sleep(1000);
}
scope.Complete();
}
The transaction isolation level Read Uncommitted only applies when you read data (as the name says). It will read data that's not been committed yet.
There's no way I know of to stop SQL Server from putting locks on tables when you INSERT or UPDATE data.
ReadUncommitted, like the name suggests, impacts readers - i.e. should read operations take read-locks and key-range-locks; should they respect existing locks, etc.
I wonder whether IsolationLevel.Chaos would offer anything here, but please don't use that. Please.
If your competing reader needs to see uncommitted data, then change the isolation-level of the reader. Also, it goes without saying, but long-running transactions (and DTC/LTM transactions in particular) are not recommended.
if writing below codes: Error returns.i do like advise : http://stackoverflow.com/questions/794364/how-do-i-use-transactionscope-in-c
But only error change:The partner transaction manager has disabled its support for remote/network transactions Exception from HRESULT: 0x8004D025
i am using windows server 2003.
using (var stockMovementCtx = new StockMovementCtxDataContext())
{
using (var scope = new TransactionScope())
{
// do something....
}
scope.Complete();
}
but if i changed my codes ; every thing is ok:
using (var stockMovementCtx = new StockMovementCtxDataContext())
{
// do something....
}
How can i solve below error. This is really important.Please help me:((((
TransactionScope will elevate to DTC if necessary. It sounds like DTC is not correctly configured, usually due to firewall restrictions. Try a dtcping between the servers (important: in both directions).
DataContext by default wraps all operations within a Transaction, so you don't need to explicitly do Transaction while working with DataContext. Read this.
http://weblogs.asp.net/scottgu/archive/2007/07/11/linq-to-sql-part-4-updating-our-database.aspx
using (var stockMovementCtx = new StockMovementCtxDataContext())
{
// do something....
//everything until this point is within the transaction.
stockMovementCtx.SubmitChange();
}
Why we need TransactionScope ?
TransactionScope enables you to perform transactions beyond dataabse. Say you have series of operations as below and all are atomic and they need be performed within a transaction.
1.) Add record in Table 1
2.) Add record in Table 2
3.) Write a NEW file on the Disk
4.) Rename file X on the disk
5.) Update file record in Table 3
If you use SqlTransaction then only the operaration 1,3 and 5 can participate in the transaction but 3 and 4 cannot because they do not relate database at all. In such cases TrasanctionScope can help you. TransactionScope leverages the MSDTC (Distributed Trasanction co-coordinator) that can help you do transactions beyond database context. You can wrap all five operations within a TransactionScope transaction execute them atomically. It is also worth noting that TransactionScope supports nested transactions, that means one block of transaction can contain multiple set of transactions.