How to Resolve a SQL Server Race Condition in an Order Processing Queue?

Addressing SQL Server Race Conditions in Order Processing

This article tackles a common problem: multiple order processors encountering race conditions when accessing an order queue through a stored procedure. The stored procedure employs a unique ID to lock the next 20 orders for processing. However, concurrent access can lead to multiple processors claiming the same order, resulting in processing errors.

The root cause is the visibility of uncommitted transactions. Even with ROWLOCK, which usually prevents simultaneous row access, uncommitted changes aren't visible to other processors checking for locks. This creates the race condition.

The solution involves using the READPAST and UPDLOCK hints in your SELECT and UPDATE statements. READPAST tells the database to bypass locked rows, avoiding interference from uncommitted updates. UPDLOCK ensures an exclusive lock on updated rows until the transaction commits, preventing other processors from modifying them.

Here's the improved code incorporating these hints:

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

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

For enhanced efficiency, consider using the OUTPUT clause. This merges the SELECT and UPDATE operations, updating the table and returning the modified rows in a single step. This eliminates the need for a separate SELECT statement.

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