Paul Williams Paul Williams - 3 months ago 18
SQL Question

How to simulate a deadlock in SQL Server in a single process?

Our client side code detects deadlocks, waits for an interval, then retries the request up to 5 times. The retry logic detects the deadlocks based on the error number 1205.

My goal is to test both the deadlock retry logic and deadlock handling inside of various stored procedures. I can create a deadlock using two different connections. However, I would like to simulate a deadlock inside of a single stored procedure itself.

A deadlock raises the following error message:


Msg 1205, Level 13, State 51, Line 1

Transaction (Process ID 66) was
deadlocked on lock resources with another process and has been chosen
as the deadlock victim. Rerun the transaction.


I see this error message is in
sys.messages
:

select * from sys.messages where message_id = 1205 and language_id = 1033

message_id language_id severity is_event_logged text
1205 1033 13 0 Transaction (Process ID %d) was deadlocked on %.*ls resources with another process and has been chosen as the deadlock victim. Rerun the transaction.


I can't raise this error using
RAISERROR
:

raiserror(1205, 13, 51)



Msg 2732, Level 16, State 1, Line 1

Error number 1205 is invalid.
The number must be from 13000 through 2147483647 and it cannot be 50000.


Our deadlock retry logic checks if the error number is 1205. The deadlock needs to have the same message ID, level, and state as a normal deadlock.

Is there a way to simulate a deadlock (with RAISERROR or any other means) and get the same message number out with just one process?

Our databases are using SQL 2005 compatibility, though our servers vary from 2005 through 2008 R2.

Answer

As many have pointed out, the answer is no, a single process cannot reliably deadlock itself. I came up with the following solution to simulate a deadlock on a development or test system..

Run the script below in a SQL Server Management Studio window. (Tested on 2008 R2 only.) You can leave it running as long as necessary.

In the place you want to simulate a deadlock, insert a call to sp_simulatedeadlock. Run your process, and the deadlock should occur.

When done testing, stop the SSMS query and run the cleanup code at the bottom.

/*
This script helps simulate deadlocks.  Run the entire script in a SQL query window.  It will continue running until stopped.
In the target script, insert a call to sp_simulatedeadlock where you want the deadlock to occur.
This stored procedure, also created below, causes the deadlock.
When you are done, stop the execution of this window and run the code in the cleanup section at the bottom.
*/
set nocount on

if object_id('DeadlockTest') is not null
    drop table DeadlockTest

create table DeadlockTest
(
    Deadlock_Key int primary key clustered,
    Deadlock_Count int
)
go

if exists (select * from sysobjects where id = object_id(N'sp_simulatedeadlock')
           AND objectproperty(id, N'IsProcedure') = 1)
drop procedure sp_simulatedeadlock
GO

create procedure sp_simulatedeadlock
(
    @MaxDeadlocks int = -1 -- specify the number of deadlocks you want; -1 = constant deadlocking
)
as begin

    set nocount on

    if object_id('DeadlockTest') is null
        return

    -- Volunteer to be a deadlock victim.
    set deadlock_priority low

    declare @DeadlockCount int

    select @DeadlockCount = Deadlock_Count -- this starts at 0
    from DeadlockTest
    where Deadlock_Key = 2

    -- Trace the start of each deadlock event.
    -- To listen to the trace event, setup a SQL Server Profiler trace with event class "UserConfigurable:0".
    -- Note that the user running this proc must have ALTER TRACE permission.
    -- Also note that there are only 128 characters allowed in the trace text.
    declare @trace nvarchar(128)

    if @MaxDeadlocks > 0 AND @DeadlockCount > @MaxDeadlocks
    begin

        set @trace = N'Deadlock Test @MaxDeadlocks: ' + cast(@MaxDeadlocks as nvarchar) + N' @DeadlockCount: ' + cast(@DeadlockCount as nvarchar) + N' Resetting deadlock count.  Will not cause deadlock.'
        exec sp_trace_generateevent
            @eventid = 82,  -- 82 = UserConfigurable:0 through 91 = UserConfigurable:9
            @userinfo = @trace

        -- Reset the number of deadlocks.
        -- Hopefully if there is an outer transaction, it will complete and persist this change.
        update DeadlockTest
        set Deadlock_Count = 0
        where Deadlock_Key = 2
        return
    end

    set @trace = N'Deadlock Test @MaxDeadlocks: ' + cast(@MaxDeadlocks as nvarchar) + N' @DeadlockCount: ' + cast(@DeadlockCount as nvarchar) + N' Simulating deadlock.'
    exec sp_trace_generateevent
        @eventid = 82,  -- 82 = UserConfigurable:0 through 91 = UserConfigurable:9
        @userinfo = @trace

    declare @StartedTransaction bit
    set @StartedTransaction = 0
    if @@trancount = 0
    begin
        set @StartedTransaction = 1
        begin transaction
    end

    -- lock 2nd record
    update DeadlockTest
    set Deadlock_Count = Deadlock_Count
    from DeadlockTest
    where Deadlock_Key = 2

    -- lock 1st record to cause deadlock
    update DeadlockTest
    set Deadlock_Count = Deadlock_Count
    from DeadlockTest
    where Deadlock_Key = 1

    if @StartedTransaction = 1
        rollback    
end
go

insert into DeadlockTest(Deadlock_Key, Deadlock_Count)
select 1, 0
union select 2, 0

-- Force other processes to be the deadlock victim.
set deadlock_priority high

begin transaction

while 1 = 1
begin

    begin try

        begin transaction

        -- lock 1st record
        update DeadlockTest
        set Deadlock_Count = Deadlock_Count
        from DeadlockTest
        where Deadlock_Key = 1

        waitfor delay '00:00:10'

        -- lock 2nd record (which will be locked when the target proc calls sp_simulatedeadlock)
        update DeadlockTest
        set Deadlock_Count = Deadlock_Count
        from DeadlockTest
        where Deadlock_Key = 2

        rollback

    end try
    begin catch
        print 'Error ' + convert(varchar(20), ERROR_NUMBER()) + ': ' + ERROR_MESSAGE()
        goto cleanup
    end catch

end

cleanup:

if @@trancount > 0
    rollback

drop procedure sp_simulatedeadlock
drop table DeadlockTest