I'm trying to find a definitive answer for this. If you select and then update, using Linq on SQL Server, in a serializable transaction, is there a deadlock risk? eg:
using (var trans = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions { IsolationLevel = IsolationLevel.Serializable, Timeout = new TimeSpan(0, 0, 10) }))
{
using (var con = new MyContext())
{
var record = con.MyTable.Single(t => t.id == 1); // share lock?
record.field = 99;
con.SubmitChanges(); // upgraded to exclusive lock? possible deadlock
trans.Complete();
}
}
And since this is such a common pattern, is the only way to avoid it something like:
var record = con.ExecuteQuery<MyTable>("select * from MyTable with (updlock) where id = {0}", 1).Single();
Yes, this is a classic deadlock. Two transactions both read, then both attempt to update. Both must wait for the other to release the shared lock.
With an optimistic concurrency model this would result in a write conflict.
The UPDLOCK, HOLDLOCK, ROWLOCK hint combo is what I usually use in these cases. Note, that for convenience reasons you can execute this locking query using ExecuteQuery, discard the results and then use LINQ normally. You don't need to switch everything into SQL.
L2S and EF do not allow you to set table hints. This is unfortunate. You have to resort to SQL on some level. Either manually or using a function or a wrapper view.
Related
I am creating a DB synchronization engine using SQL CLR Triggers in Microsoft SQL Server 2012. These triggers do not call a stored procedure or function (and thereby have access to the INSERTED and DELETED pseudo-tables but do not have access to the ##procid).
Differences here, for reference.
This "sync engine" uses mapping tables to determine what the table and field maps are for this sync job. In order to determine the target table and fields (from my mapping table) I need to get the source table name from the trigger itself. I have come across many answers on Stack Overflow and other sites that say that this isn't possible. But, I've found one website that provides a clue:
Potential Solution:
using (SqlConnection lConnection = new SqlConnection(#"context connection=true")) {
SqlCommand cmd = new SqlCommand("SELECT object_name(resource_associated_entity_id) FROM sys.dm_tran_locks WHERE request_session_id = ##spid and resource_type = 'OBJECT'", lConnection);
cmd.CommandType = CommandType.Text;
var obj = cmd.ExecuteScalar();
}
This does in fact return the correct table name.
Question:
My question is, how reliable is this potential solution? Is the ##spid actually limited to this single trigger execution? Or is it possible that other simultaneous triggers will overlap within this process id? Will it stand up to multiple executions of the same and/or different triggers within the database?
From these sites, it seems the process Id is in fact limited to the open connection, which doesn't overlap: here, here, and here.
Will this be a safe method to get my source table?
Why?
As I've noticed similar questions, but all without a valid answer for my specific situation (except that one). Most of the comments on those sites ask "Why?", and in order to preempt that, here is why:
This synchronization engine operates on a single DB and can push changes to target tables, transforming the data with user-defined transformations, automatic source-to-target type casting and parsing and can even use the CSharpCodeProvider to execute methods also stored in those mapping tables for transforming data. It is already built, quite robust and has good performance metrics for what we are doing. I'm now trying to build it out to allow for 1:n table changes (including extension tables requiring the same Id as the 'master' table) and am trying to "genericise" the code. Previously each trigger had a "target table" definition hard coded in it and I was using my mapping tables to determine the source. Now I'd like to get the source table and use my mapping tables to determine all the target tables. This is used in a medium-load environment and pushes changes to a "Change Order Book" which a separate server process picks up to finish the CRUD operation.
Edit
As mentioned in the comments, the query listed above is quite "iffy". It will often (after a SQL Server restart, for example) return system objects like syscolpars or sysidxstats. But, it seems that in the dm_tran_locks table there's always an associated resource_type of 'RID' (Row ID) with the same object_name. My current query which works reliably so far is the following (will update if this changes or doesn't work under high load testing):
select t1.ObjectName FROM (
SELECT object_name(resource_associated_entity_id) as ObjectName
FROM sys.dm_tran_locks WHERE resource_type = 'OBJECT' and request_session_id = ##spid
) t1 inner join (
SELECT OBJECT_NAME(partitions.OBJECT_ID) as ObjectName
FROM sys.dm_tran_locks
INNER JOIN sys.partitions ON partitions.hobt_id = dm_tran_locks.resource_associated_entity_id
WHERE resource_type = 'RID'
) t2 on t1.ObjectName = t2.ObjectName
If this is always the case, I'll have to find that out during testing.
How reliable is this potential solution?
While I do not have time to set up a test case to show it not working, I find this approach (even taking into account the query in the Edit section) "iffy" (i.e. not guaranteed to always be reliable).
The main concerns are:
cascading (whether recursive or not) Trigger executions
User (i.e. Explicit / Implicit) transactions
Sub-processes (i.e. EXEC and sp_executesql)
These scenarios allow for multiple objects to be locked, all at the same time.
Is the ##SPID actually limited to this single trigger execution? Or is it possible that other simultaneous triggers will overlap within this process id?
and (from a comment on the question):
I think I can join my query up with the sys.partitions and get a dm_trans_lock that has a type of 'RID' with an object name that will match up to the one in my original query.
And here is why it shouldn't be entirely reliable: the Session ID (i.e. ##SPID) is constant for all of the requests on that Connection). So all sub-processes (i.e. EXEC calls, sp_executesql, Triggers, etc) will all be on the same ##SPID / session_id. So, between sub-processes and User Transactions, you can very easily get locks on multiple resources, all on the same Session ID.
The reason I say "resources" instead of "OBJECT" or even "RID" is that locks can occur on: rows, pages, keys, tables, schemas, stored procedures, the database itself, etc. More than one thing can be considered an "OBJECT", and it is possible that you will have page locks instead of row locks.
Will it stand up to multiple executions of the same and/or different triggers within the database?
As long as these executions occur in different Sessions, then they are a non-issue.
ALL THAT BEING SAID, I can see where simple testing would show that your current method is reliable. However, it should also be easy enough to add more detailed tests that include an explicit transaction that first does some DML on another table, or have a trigger on one table do some DML on one of these tables, etc.
Unfortunately, there is no built-in mechanism that provides the same functionality that ##PROCID does for T-SQL Triggers. I have come up with a scheme that should allow for getting the parent table for a SQLCLR Trigger (that takes into account these various issues), but haven't had a chance to test it out. It requires using a T-SQL trigger, set as the "first" trigger, to set info that can be discovered by the SQLCLR Trigger.
A simpler form can be constructed using CONTEXT_INFO, if you are not already using it for something else (and if you don't already have a "first" Trigger set). In this approach you would still create a T-SQL Trigger, and then set it as the "first" Trigger using sp_settriggerorder. In this Trigger you SET CONTEXT_INFO to the table name that is the parent of ##PROCID. You can then read CONTEXT_INFO() on a Context Connection in a SQLCLR Trigger. If there are multiple levels of Triggers then the value of CONTEXT INFO will get overwritten, so reading that value must be the first thing you do in each SQLCLR Trigger.
This is an old thread, but it is an FAQ and I think I have a better solution. Essentially it uses the schema of the inserted or deleted table to find the base table by doing a hash of the column names and comparing the hash with the hashes of tables with a CLR trigger on them.
Code snippet below - at some point I will probably put the whole solution on Git (it sends a message to Azure Service Bus when the trigger fires).
private const string colqry = "select top 1 * from inserted union all select top 1 * from deleted";
private const string hashqry = "WITH cols as ( "+
"select top 100000 c.object_id, column_id, c.[name] "+
"from sys.columns c "+
"JOIN sys.objects ot on (c.object_id= ot.parent_object_id and ot.type= 'TA') " +
"order by c.object_id, column_id ) "+
"SELECT s.[name] + '.' + o.[name] as 'TableName', CONVERT(NCHAR(32), HASHBYTES('MD5',STRING_AGG(CONVERT(NCHAR(32), HASHBYTES('MD5', cols.[name]), 2), '|')),2) as 'MD5Hash' " +
"FROM cols "+
"JOIN sys.objects o on (cols.object_id= o.object_id) "+
"JOIN sys.schemas s on (o.schema_id= s.schema_id) "+
"WHERE o.is_ms_shipped = 0 "+
"GROUP BY s.[name], o.[name]";
public static void trgSendSBMsg()
{
string table = "";
SqlCommand cmd;
SqlDataReader rdr;
SqlTriggerContext trigContxt = SqlContext.TriggerContext;
SqlPipe p = SqlContext.Pipe;
using (SqlConnection con = new SqlConnection("context connection=true"))
{
try
{
con.Open();
string tblhash = "";
using (cmd = new SqlCommand(colqry, con))
{
using (rdr = cmd.ExecuteReader(CommandBehavior.SingleResult))
{
if (rdr.Read())
{
MD5 hash = MD5.Create();
StringBuilder hashstr = new StringBuilder(250);
for (int i=0; i < rdr.FieldCount; i++)
{
if (i > 0) hashstr.Append("|");
hashstr.Append(GetMD5Hash(hash, rdr.GetName(i)));
}
tblhash = GetMD5Hash(hash, hashstr.ToString().ToUpper()).ToUpper();
}
rdr.Close();
}
}
using (cmd = new SqlCommand(hashqry, con))
{
using (rdr = cmd.ExecuteReader(CommandBehavior.SingleResult))
{
while (rdr.Read())
{
string hash = rdr.GetString(1).ToUpper();
if (hash == tblhash)
{
table = rdr.GetString(0);
break;
}
}
rdr.Close();
}
}
if (table.Length == 0)
{
p.Send("Error: Unable to find table that CLR trigger is on. Message not sent!");
return;
}
….
HTH
Using SQL Server 2008 R2 I am getting deadlocks when the same update statement (with different parameters) is running concurrently. Here is the deadlock graph (sorry cannot post images on here yet):
http://i.stack.imgur.com/E6JBK.png
And here is the actual execution plan:
http://i.stack.imgur.com/emm9i.png
The update is like this:
exec sp_executesql N'UPDATE mapping.IssuerAlternateName
SET
UseCount = UseCount + 1,
MostRecentlyAppeared = GETDATE(),
MostRecentlyAppearedUnderlyingAssetName = #p1
WHERE ID = #p0
',N'#p0 int,#p1 nvarchar(4000)',#p0=1234,#p1=N'blah blah blah'
If I have understood things correctly we are trying to read and write from the same index (PK_IssuerAlternateName_1).
Is there any way to resolve this? I was wondering if adding an additional index to the primary key and using WITH INDEX might fix it by stopping the read of PK_IssuerAlternateName_1 (sorry the full name is truncated in the execution plan screenshot).
Or is the best option just to live with this and retry the transaction, which is how the error is currently handled in .NET client. It is certainly successful on retry, but it would be good to avoid the deadlock if possible.
Thanks
In situations similar to this, I have used the UPDLOCK hint to let the database know I intend to update this row. It is not implied by the UPDATE statement. Without the lock hint, it will first obtain a "shared" lock, and then try to escalate. However, this causes deadlocks in certain scenarios.
You will need to do this within your own TransactionScope to ensure everything works correctly.
var sql = #"UPDATE mapping.IssuerAlternateName with (UPDLOCK)
SET
UseCount = UseCount + 1,
MostRecentlyAppeared = GETDATE(),
MostRecentlyAppearedUnderlyingAssetName = #p1
WHERE ID = #p0";
var options = new TransactionOptions()
{
IsolationLevel = IsolationLevel.ReadCommitted // don't use Serializable!
};
using (var scope = new TransactionScope(TransactionScopeOption.RequiresNew, options))
{
using (var context = new YourDbContext())
{
// execute your command here
}
}
I have the following SQL Server 2012 query:
var sql = #"Select question.QuestionUId
FROM Objective,
ObjectiveDetail,
ObjectiveTopic,
Problem,
Question
where objective.examId = 1
and objective.objectiveId = objectiveDetail.objectiveId
and objectiveDetail.ObjectiveDetailId = ObjectiveTopic.ObjectiveDetailId
and objectiveTopic.SubTopicId = Problem.SubTopicId
and problem.ProblemId = question.ProblemId";
var a = db.Database.SqlQuery<string>(sql).ToList();
Can someone help explain to me if it would be a good idea to put this into a
stored procedure and if so then how could I do that and then call it from my C# code. It was
suggested to me that if it is in a stored procedure then it would run more
efficiently as it would not be recompiled often. Is that the case?
Yes, there is. For starters, a stored procedure is precompiled and stored within your database. Being precompiled, the database engine can execute it more efficiently, since no on-the-fly compilation necessary. Also, database optimizations can be added to support a precompiled procedure. A stored procedure also allows business logic to be encapsulated within the database.
If you decide to go the stored procedure route, then consider the following:
First of all, you will need to create a stored procedure that encapsulates your existing SQL query.
CREATE PROCEDURE ListQuestionIds
#ExamId int
AS
BEGIN
SELECT Question.QuestionUId
FROM Objective
INNER JOIN ObjectiveDetail
ON ( Objective.objectiveId = ObjectiveDetail.objectiveId )
INNER JOIN ObjectiveTopic
ON ( ObjectiveDetail.ObjectiveDetailId = ObjectiveTopic.ObjectiveDetailId )
INNER JOIN Problem
ON ( ObjectiveTopic.SubTopicId = Problem.SubTopicId )
INNER JOIN Question
ON ( Problem.ProblemId = Question.ProblemId )
WHERE Objective.examId = #ExamId;
END;
Please make sure that the tables called by your procedure (Objective, Problem, etc,) have all of the relevant primary keys and indexes in place to enhance the performance of your query.
Next, you will need to call that stored procedure from within your C# code. One way--but by no means the only way--is to create a connection to your database using the SqlConnection object and then executing your procedure via the SqlCommand object.
I would recommend that you take a look at How to execute a stored procedure within C# program for some on-topic examples. But a simple example of such might look like:
string connectionString = "your_connection_string";
using (var con = new SqlConnection(connectionString))
{
using (var cmd = new SqlCommand("ListQuestionIds", con)) {
cmd.CommandType = CommandType.StoredProcedure
cmd.Parameters.Add(new SqlParameter("#ExamId", examId))
con.Open();
using (SqlDataReader rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
// Loop through the returned SqlDataReader object (aka. rdr) and
// then evaluate & process the returned question id value(s) here
}
}
}
}
Please note that this sample code does not (intentionally) include any error handling. I leave that up to you to integrate into your application.
Finally, just as an FYI... many of the more modern ORMs (e.g., Entity Framework, NHibernate, etc.) allow you to execute stored procedure-like queries from your C# code without requiring an explicit stored procedure. If you are already using an ORM in your application, then you may want to forgo the stored procedure altogether. Whatever you decide to do, a little research on your end will help you make an informed decision.
I hope this helps you get started. Good luck.
I'm trying to use linq to sql for integration testing of stored procedures. I'm trying to call an updating stored procedure and after that retrieving the updated row from db to verify the change. All this should happen in one transaction so that I can rollback the transaction after the verification.
The code fails in assert, because the the row I retrieved does not seem to be updated. I know that my SP works when called from ordinary code. Is it even possible see the updated row in same transaction?
I'm using Sql Server 2008 and used sqlmetal.exe to create linq-to-sql mapping.
I've tried many different things, and right now my code looks following:
DbTransaction transaction = null;
try
{
var context =
new DbConnection(
ConfigurationManager.ConnectionStrings["MyConnectionString"].ConnectionString);
context.Connection.Open();
transaction = context.Connection.BeginTransaction();
context.Transaction = transaction;
const string newUserName= "TestUserName";
context.SpUpdateUserName(136049 , newUserName);
context.SubmitChanges();
// select to verify
var user=
(from d in context.Users where d.NUserId == 136049 select d).First();
Assert.IsTrue(user.UserName == newUserName);
}
finally
{
if (transaction != null) transaction.Rollback();
}
I believe you are coming acress a stale datacontext issue.
Your update is done through a stored procedure so your context does not "see" the changes and has no way to update the Users.
If you use a new datacontext to do the assert, it usually works well. However, since you are using a transaction you probably have to add the second datacontext to the same transaction.
We're having a strange problem in Oracle. I'll sketch some (simplified) context first:
Consider this mapping to an Entity:
public EntityMap()
{
Table("EntityTable");
Id(x => x.Id)
.Column("entityID")
.GeneratedBy.Native("ENTITYID").UnsavedValue(0);
Map(x => x.SomeBoolean).Column("SomeBoolean");
}
and this code:
var entity = new Entity();
using (var transaction = new TransactionScope(TransactionScopeOption.Required))
{
Session.Save(entity);
transaction.Complete();
}
//A lot of code
if(someCondition)
{
using (var transaction = new TransactionScope(TransactionScopeOption.Required))
{
enitity.SomeBoolean = true;
Session.Update(entity);
transaction.Complete();
}
}
This code is called a few times. The first time it generates the following queries:
select ENTITYID.nextval from dual
INSERT INTO Entity
(SomeBoolean, EntityID)
VALUES (0, 1216)
UPDATE Entity
SET SomeBoolean = 1
WHERE EntityID = 1216
The second time it is called these queries are generated (someCondition is false)
select ENTITYID.nextval from dual
INSERT INTO Entity
(SomeBoolean, EntityID)
VALUES (0, 1217)
And now the trouble begins. From now on, each insert will use the correct autoincremented value, but the update will always use 1217
select ENTITYID.nextval from dual
INSERT INTO Entity
(SomeBoolean, EntityID)
VALUES (0, 1218)
UPDATE Entity
SET SomeBoolean = 1
WHERE EntityID = 1217
And of course, this is not what we want to happen. If I inspect the value of the Id while debugging, it contains the correct autoincremented value. Somehow, deep in the bowels of NHibernate, the incorrect is is assigned to the WHERE clause...
The strange part is that this only happens on Oracle. If I switch NHibernate to MsSql, everything works like a charm.
So I found out what happened. NHibernate changed it's default connection release mode between versions 1.x and 2.x. Instead of closing the connection when the session is Disposed, the connections is now closed after each transaction. However, we were manually coordinating our transactions which apparently caused troubles in Oracle.
This question has some extra information and this entry in the NHibernate documentation also clarifies how the connections are handeled:
As of NHibernate, if your application manages transactions through .NET APIs such as System.Transactions library, ConnectionReleaseMode.AfterTransaction may cause NHibernate to open and close several connections during one transaction, leading to unnecessary overhead and transaction promotion from local to distributed. Specifying ConnectionReleaseMode.OnClose will revert to the legacy behavior and prevent this problem from occuring.
This blog post is what got me looking in the right direction.