sp_MSacquireSlotLock (Transact-SQL MetaData) Definition

Please note: that the following source code is provided and copyrighted by Microsoft and is for educational purpose only.
The meta data is from an SQL 2012 Server.

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Definition:

sys.sp_MSacquireSlotLock(nvarchar @process_name
, int @concurrent_max
, int @queue_timeout
, bit @return_immediately
, nvarchar @DbPrincipal)

MetaData:

   
create procedure sys.sp_MSacquireSlotLock   
@process_name                   sysname,  
@concurrent_max                 int,  
@queue_timeout                  int = 0,  
--  means wait in definitely  
@return_immediately             bit = 0,  
--  if set to 1, take a peek at the server and return immediately.  
@DbPrincipal                    sysname = NULL  
AS  
declare @entry_date     datetime  
declare @slot_name      nvarchar(150)  --  OUTPUT   
--  must give back slot acquired to caller so caller can later release.  
declare @basetime       datetime  
declare @delaytime      datetime  
declare @retcode        int  
declare @i              int  
declare @lock_acquired  bit  
  
--  Security Checking   
--  sysadmin or db_owner or replication agent have access  
exec @retcode = sys.sp_MSrepl_PAL_rolecheck   
if (@retcode <> 0) or (@@error <> 0)  
begin  
        RAISERROR (14126, 11, -1)  
       return 1  
end  
  
select @lock_acquired = 0  
if @queue_timeout<0  
    begin  
        raiserror(21344, 16, -1, '@queue_timeout')  
        return (1)        
    end  
      
if @concurrent_max<=0  
    begin  
        raiserror(21344, 16, -1, '@concurrent_max')  
        return (1)        
    end  
  
if @DbPrincipal is NULL   
begin  
    if exists (select * from sys.database_principals where name=N'MSmerge_PAL_role' and type = 'R')  
        select @DbPrincipal = N'MSmerge_PAL_role'  
    else  
        select @DbPrincipal = N'db_owner'  
end  
  
select @entry_date=getdate()  
select @delaytime = '00:00:02'   
-- polling interval is defaulted to 2 seconds  
  
SET @slot_name=NULL --  If terminate anywhere unexpectedly, dont want to give   
                    --  caller a lock they didnt really get.  
  
--  We are at front of queue, so check if any available 'slot' is open.     
--  We do not wait at all for these locks, and hence   
--  expect either it was granted or timed out (-1).  
  
SET @i=1  
  
WHILE (@i <= @concurrent_max)  
    BEGIN  
        -- the process has waited long enough. quit now and try later.   
        --  If @queue_timeout is 0, keep waiting until succeeds.  
        if @queue_timeout>0 and DATEADD(second, -@queue_timeout,  getdate())   
            > @entry_date  
            begin  
                select @lock_acquired = 1 --  not a peek but has waited as specified  
                select @slot_name = NULL --  waited but failed to get one  
                BREAK  
            end  
  
        SET @slot_name=@process_name+convert(varchar,@i)  
          
        -- the call is not blocking, return immediately having acquired   
        --  the lock or not  
        exec @retcode=sys.sp_getapplock @Resource=@slot_name,@LockMode=N'Exclusive',  
            @LockOwner='Session',@LockTimeout=0,@DbPrincipal=@DbPrincipal  
        IF (@retcode <> 0 AND @retcode <> -1)  
            BEGIN  
                RAISERROR(21414,16,-1)  
                RETURN(@retcode)  
            END  
      
        IF (@retcode = 0)       --  got lock for that slot - cleanup and leave.  
            BEGIN  
                --  Release the "Im first lock"  
                exec @retcode=sys.sp_releaseapplock @process_name,@LockOwner=N'Session',@DbPrincipal=@DbPrincipal  
                IF (@retcode <> 0)  
                BEGIN  
                    SET @slot_name=NULL  
                    RAISERROR(21415, 16, -1)  
                    RETURN(@retcode)  
                END  
                select @lock_acquired = 1  
                --  We got our slot and released the Im first lock.  We're done.  
                BREAK  
            END  
        ELSE  
        IF (@retcode = -1)  --  Couldn't immediately get the lock.    
                            --  So try the next one.  
            BEGIN  
            SET @i=@i+1  
                IF @i <= @concurrent_max  
                    CONTINUE   --  restart the loop  
                ELSE  
                    BEGIN       --  Sleep and start over.  
                    if @return_immediately=1  
                        begin  
                            select @lock_acquired = 0   
                            -- slot name does not matter in this case  
                            BREAK  
                        end  
                    else  
                        begin  
                            WAITFOR DELAY @delaytime  
                            SET @i=1  
                            CONTINUE   --  restart the loop  
                        end  
                    END  
            END  
          
    END  
      
    -- output the slot name for the purpose of releasing the lock by the caller  
    -- if the value if NULL, the caller does not acquire the lock  
    select @lock_acquired, @slot_name  
RETURN(0)