Question

I’m running into a deadlock problem when trying to lock some records so that no process (Windows service) picks the items to service them, then update the status and then return a recordset.

Can you please let me know why am I getting the deadlock issue when this proc is invoked?

CREATE PROCEDURE [dbo].[sp_LoadEventsTemp]
(
   @RequestKey varchar(20),
   @RequestType varchar(20),
   @Status varchar(20),
   @ScheduledDate smalldatetime = null
)
AS
BEGIN
   declare @LoadEvents table
   (
      id int
   )

   BEGIN TRANSACTION

   if (@scheduledDate is null)
   Begin
      insert into @LoadEvents (id)
      (
         Select eventqueueid FROM eventqueue
         WITH (HOLDLOCK, ROWLOCK)
         WHERE requestkey = @RequestKey
         and requesttype = @RequestType
         and [status] = @Status
      )
   END
   else
   BEGIN
      insert into @LoadEvents (id)
      (
         Select eventqueueid FROM eventqueue
         WITH (HOLDLOCK, ROWLOCK)
         WHERE requestkey = @RequestKey
         and requesttype = @RequestType
         and [status] = @Status
         and (convert(smalldatetime,scheduleddate) <= @ScheduledDate)
      )
   END

   update eventqueue set [status] = 'InProgress'
   where eventqueueid in (select id from @LoadEvents)

   IF @@Error 0
   BEGIN
      ROLLBACK TRANSACTION
   END
   ELSE
   BEGIN
      COMMIT TRANSACTION
      select * from eventqueue
      where eventqueueid in (select id from @LoadEvents)
   END
END

Thanks in advance.

Answer 1

Deadlocks happen most often (in my experience) when differnt resources are locked within differnt transactions in different orders.

Imagine 2 processes using resource A and B, but locking them in different orders.
– Process 1 locks resource A, then resource B
– Process 2 locks resource B, then resource A

The following then becomes possible:
– Process 1 locks resource A
– Process 2 locks resource B
– Process 1 tries to lock resource B, then stops and waits as Process 2 has it
– Process 2 tries to lock resource A, then stops and waits as Process 1 has it
– Both proceses are waiting for each other, Deadlock

In your case we would need to see exactly where the SP falls over due to a deadlock (the update I’d guess?) and any other processes that reference that table. It could be an trigger or something, which then gets deadlocked on a different table, not the table you’re updating.

What I would do is use SQL Server 2005’s OUTPUT syntaxt to avoid having to use the transaction…

UPDATE
  eventqueue
SET
  status = 'InProgress'
WHERE
      requestkey  = @RequestKey
  AND requesttype = @RequestType
  AND status      = @Status
  AND (convert(smalldatetime,scheduleddate) <= @ScheduledDate OR @ScheduledDate  IS NULL)
OUTPUT
  inserted.*