Michael J. Swart

January 26, 2021

Avoid This Pitfall When Using sp_getapplock

Filed under: Miscelleaneous SQL,SQL Scripts,SQLServerPedia Syndication,Technical Articles — Michael J. Swart @ 11:10 am

Takeaway: For most use cases, using sp_releaseapplock is unnecessary. Especially when using @LockOwner = 'Transaction (which is the default).

The procedure sp_getapplock is a system stored procedure that can be helpful when developing SQL for concurrency. It takes a lock on an imaginary resource and it can be used to avoid race conditions.

But I don’t use sp_getapplock a lot. I almost always depend on SQL Server’s normal locking of resources (like tables, indexes, rows etc…). But I might consider it for complicated situations (like managing sort order in a hierarchy using a table with many different indexes).

In that case, I might use it something like this:

BEGIN TRAN
 
exec sp_getapplock 
    @Resource = @LockResourceName, 
    @LockMode = 'Exclusive', 
    @LockOwner = 'Transaction';
 
/* read stuff (e.g. "is time slot available?") */
/* change stuff (e.g. "make reservation") */
 
exec sp_releaseapplock
    @Resource = @LockResourceName, 
    @LockOwner = 'Transaction';
 
COMMIT

But there’s a problem with this pattern, especially when using RCSI. After sp_releaseapplock is called, but before the COMMIT completes, another process running the same code can read the previous state. In the example above, both processes will think a time slot is available and will try to make the same reservation.

What I really want is to release the applock after the commit. But because I specified the lock owner is 'Transaction'. That gets done automatically when the transaction ends! So really what I want is this:

BEGIN TRAN
 
exec sp_getapplock 
    @Resource = @LockResourceName, 
    @LockMode = 'Exclusive', 
    @LockOwner = 'Transaction';
 
/* read stuff (e.g. "is time slot available?") */
/* change stuff (e.g. "make reservation") */
 
COMMIT -- all locks are freed after this commit

1 Comment »

  1. […] Michael J Swart has a tip for those who have RCSI turned on and are using app locks: […]

    Pingback by App Locks and Read Committed Snapshot Isolation – Curated SQL — January 27, 2021 @ 8:00 am

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