SQL Server Process Queue Race Condition

Question

I have an order queue that is accessed by multiple order processors through a stored procedure. Each processor passes in a unique ID which is used to lock the next 20 orders

Answer 1

You can use Service Broker. Also you can use sp_getapplock to serialize access to your rows - that will eliminate race conditions:

"Assisting Concurrency by creating your own Locks (Mutexs in SQL) " http://sqlblogcasts.com/blogs/tonyrogerson/archive/2006/06/30/855.aspx

Answer 2

Edit:

I googled to check my answer: "Processing Data Queues in SQL Server with READPAST and UPDLOCK". It's been years since I read about and played with this solution.

Original:

If you use the READPAST hint, then locked rows are skipped. You've used ROWLOCK so you should avoid lock escalation. You also need UPDLOCK, as I found out.

So process 1 locks 20 rows, process 2 will take the next 20, process 3 takes rows 41 to 60, etc

The update can also be written like this:

UPDATE TOP (20)
    foo
SET
    ProcessorID = @PROCID
FROM
    OrderTable foo WITH (ROWLOCK, READPAST, UPDLOCK)
WHERE
    ProcessorID = 0

Refresh, Oct 2011

This can be done more elegantly with the OUTPUT clause if you need a SELECT and an UPDATE in one go.