SQL Locking not implemented correctly - sql

Question

I have implemented a SQL tablockx locking in a Procedure.It is working fine when this is running on one server.But duplicate policyNumber occurs when request comes from two different servers.
declare #plantype varchar(max),
#transid varchar(max),
#IsStorySolution varchar(10),
#outPolicyNumber varchar(max) output,
#status int output -- 0 mean error and 1 means success
)
as
begin
BEGIN TRANSACTION
Declare #policyNumber varchar(100);
Declare #polseqid int;
-- GET POLICY NUMBER ON THE BASIS OF STORY SOLUTION..
IF (UPPER(#IsStorySolution)='Y')
BEGIN
select top 1 #policyNumber=Policy_no from PLAN_POL_NO with (tablockx, holdlock) where policy_no like '9%'
and pol_id_seq is null and status='Y';
END
ELSE
BEGIN
select top 1 #policyNumber=pp.Policy_no from PLAN_POL_NO pp with (tablockx, holdlock) ,PLAN_TYP_MST pt where pp.policy_no like PT.SERIES+'%'
and pt.PLAN_TYPE in (''+ISNULL(#plantype,'')+'') and pol_id_seq is null and pp.status='Y'
END
-- GET POL_SEQ_ID NUMBER
select #polseqid=dbo.Sequence();
--WAITFOR DELAY '00:00:03';
set #policyNumber= ISNULL(#policyNumber,'');
-- UPDATE POLICY ID INFORMATION...
Update PLAN_POL_NO set status='N',TRANSID =#transid , POL_ID_SEQ=ISNULL(#polseqid,0) where Policy_no =#policyNumber
set #outPolicyNumber=#policyNumber;
if(##ERROR<>0) begin GOTO Fail end
COMMIT Transaction
set #status=1;
return;
Fail:
If ##TRANCOUNT>0
begin
Rollback transaction
set #status=0;
return;
This is function which i have called::
CREATE function [dbo].[Sequence]()
returns int
as
begin
declare #POL_ID int
/***************************************
-- Schema name is added with table name in below query
-- as there are two table with same name (PLAN_POL_NO)
-- on different schema (dbo & eapp).
*****************************************/
select #POL_ID=isnull(MAX(POL_ID_SEQ),2354) from dbo.PLAN_POL_NO
return #POL_ID+1
end

Answer 1

The problem you are facing is because concurrent requests are both getting the same POL_ID_SEQ from your table dbo.PLAN_POL_NO.
There are multiple solutions to your problem, but I can think of two that might help you and require none/small code changes:
Using a higher transaction isolation level instead of table hints.
In your stored procedure you can use the following:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
This will make sure that any data read/modified during the SP block is transactionally consistent and avoids phantom reads, duplicate records etc. It will potentially create higher deadlocks and if these tables are heavily queried/updated throughout your application you might have a whole new set of issues.
Make sure your update to the dbo.PLAN_POL_NO only succeeds if the sequence has not changed. If it has changed, error out (if it changed it means a concurrent transaction obtained the ID and completed first)
Something like this:
Update dbo.PLAN_POL_NO
SET status ='N',
TRANSID = #transid,
POL_ID_SEQ = ISNULL(#polseqid,0)
WHERE Policy_no = #policyNumber
AND POL_ID_SEW = #polseqid - 1
IF ##ROWCOUNT <> 1
BEGIN
-- Update failed, error out and let the SP rollback the transaction
END

Answer 2

A WITH (HOLDLOCK) option in the function might suffice.
The HOLDLOCK from the first queries may be being applied at a row or page level that then DOESN'T include the row of interest that is queried in the function.
However, the function is not reliable since it is not self-contained. I'd be looking to redesign this such that the Sequence can generate AND "take" the sequence number before returning it. The current design is fragile at best.