Detecting dirty reads from a stored procedure

Question

I\'ve got 100 threads that are each calling the stored procedure as defined below.

How do I prevent dirty reads?

SET QUOTED_IDENTIFIER OFF
SET ANSI_NULLS         


        

Answer 1

You need to replace this statement

UPDATE CfgCerealNumber Set CerealNumber = CerealNumber + 1  
WHERE CerealNumberID = @TableID

by this:

declare @CerealNumber int

SELECT @CerealNumber = CerealNumber  + 1
FROM CfgCerealNumber WITH (READCOMMITTED, READPAST, ROWLOCK) 
WHERE CerealNumberID = @TableID

if @CerealNumber is not null
    UPDATE CfgCerealNumber Set CerealNumber = @CerealNumber
    WHERE CerealNumberID = @TableID
else
    raiserror ('Row was locked by another update (no dirty read and no deadlock happen) or no Table Record Exists in CfgCerealNumber for ID:%d   ', 
              16,1, @TableID)

these table hints READCOMMITTED, READPAST, ROWLOCK will make sure that you have no dirty read and no deadlock

it will also let you decide if you still want to do an update

READCOMMITTED
Specifies that read operations comply with the rules for the READ COMMITTED isolation level by using either locking or row versioning. If the database option READ_COMMITTED_SNAPSHOT is OFF, the Database Engine acquires shared locks as data is read and releases those locks when the read operation is completed. If the database option READ_COMMITTED_SNAPSHOT is ON, the Database Engine does not acquire locks and uses row versioning.

READPAST
Specifies that the Database Engine not read rows that are locked by other transactions. When READPAST is specified, row-level locks are skipped. That is, the Database Engine skips past the rows instead of blocking the current transaction until the locks are released. For example, assume table T1 contains a single integer column with the values of 1, 2, 3, 4, 5. If transaction A changes the value of 3 to 8 but has not yet committed, a SELECT * FROM T1 (READPAST) yields values 1, 2, 4, 5. READPAST is primarily used to reduce locking contention when implementing a work queue that uses a SQL Server table. A queue reader that uses READPAST skips past queue entries locked by other transactions to the next available queue entry, without having to wait until the other transactions release their locks.

ROWLOCK
Specifies that row locks are taken when page or table locks are ordinarily taken. When specified in transactions operating at the SNAPSHOT isolation level, row locks are not taken unless ROWLOCK is combined with other table hints that require locks, such as UPDLOCK and HOLDLOCK.

Source MSDN Table Hints (Transact-SQL)

You might also need to use UPDLOCK and/or HOLDLOCK