CODEWITHSUNDEEP
sqlsql-servertransactionslockingisolation-level
Novitzky
Novitzky

Reputation: 4876

SQL Server: how to acquire exclusive lock to prevent race condition?

I have the following T-SQL code:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
BEGIN TRANSACTION T1_Test

    /*This is a dummy table used for "locking" 
    and it doesn't contain any meaningful data.*/        
    UPDATE lockTable 
        SET ID = 1    
        WHERE ID = 1

    DECLARE @Count TINYINT 

    SELECT @Count = COUNT(*)
    FROM debugSP 

    WAITFOR DELAY '00:00:5';

    INSERT INTO debugSP 
        (DateCreated, ClientId, Result)
    SELECT 
        GETDATE(), @@SPID, @Count

COMMIT TRANSACTION T1_Test

I am using "locking" hack marked with comment to acquire the exclusive lock.

NOTE: using TABLOCKX or UPDLOCK hints will not work because I have broken ATOMIC-ity by splitting statements and adding WAITFOR command in the middle for testing purposes. I don't want something like that:

INSERT INTO debugSP (DateCreated, ClientId, Result)
SELECT GETDATE(), @@SPID, COUNT(*) 
FROM debugSP

This is the correct result after running two simultaneous sessions (with lock table)

Id DateCreated           ClientId Result
-- ----------------------- -------- ------
 1 2011-03-17 15:52:12.287       66      0
 2 2011-03-17 15:52:24.534       68      1

and that is the incorrect result of running the code with lock commented out

Id DateCreated           ClientId Result
-- ----------------------- -------- ------
 1 2011-03-17 15:52:43.128       66      0
 2 2011-03-17 15:52:46.341       68      0

Is there a better way of acquiring transaction-wide exclusive lock without such hacks?

Upvotes: 3

Views: 9737

Answers (2)

KeithS
KeithS

Reputation: 71573

You can use the locking hint WITH(XLOCK, ROWLOCK) within the scope of a transaction with Repeatable Read isolation. At Serializable isolation the exclusive lock is obtained by default on a read operation, so if you needed a particular transaction to play nice in parallel, you could specify an increased serialization level for that one transaction when creating it (which you are doing; this isn't a hack, just the way things are done depending on the situation).

Upvotes: 1

Martin Smith
Martin Smith

Reputation: 453278

I'm not exactly sure what you are trying to do from the posted code. I presume you are just trying to serialize access to that piece of code? If so sp_getapplock should do what you need instead of creating a new dummy table that you just use to take locks on.

Details here

Upvotes: 7

Related Questions