How Can I Prevent Race Conditions in SQL Server's Order Processing Queue?

Addressing Race Conditions in SQL Server's Order Processing Queue

Challenge: Concurrent order processors accessing an order queue through a stored procedure encounter race conditions. This results in duplicate order retrievals and processing errors. The current stored procedure attempts to mitigate this by locking 20 orders at a time, but this proves insufficient.

Problematic Query:

The original query uses a two-step process (UPDATE followed by SELECT) with row locking, creating a window for race conditions:

<code class="language-sql">BEGIN TRAN
    UPDATE  OrderTable WITH ( ROWLOCK )
    SET     ProcessorID = @PROCID
    WHERE   OrderID IN ( SELECT TOP ( 20 )
                                        OrderID
                                FROM    OrderTable WITH ( ROWLOCK )
                                WHERE   ProcessorID = 0)
COMMIT TRAN


SELECT  OrderID, ProcessorID, etc...
FROM    OrderTable
WHERE   ProcessorID = @PROCID</code>

Root Cause: The UPDATE statement attempts to lock rows after the SELECT statement has identified them. In a multi-processor environment, this timing difference allows multiple processors to acquire the same orders.

Effective Solution: Employing the READPAST and UPDLOCK hints within a single UPDATE statement resolves the race condition. READPAST allows the SELECT portion of the UPDATE to bypass locked rows, while UPDLOCK ensures that only unlocked rows are updated.

Revised Query:

This streamlined approach eliminates the race condition:

<code class="language-sql">UPDATE TOP (20)
    OrderTable
SET
    ProcessorID = @PROCID
FROM
    OrderTable WITH (ROWLOCK, READPAST, UPDLOCK)
WHERE
    ProcessorID = 0</code>

This revised query efficiently and reliably assigns orders to processors, preventing duplicate processing.

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