Database polling, prevent duplicate fetches - c++

Question

I have a system whereby a central MSSQL database keeps in a table a queue of jobs that need to be done.
For the reasons that processing requirements would not be that high, and that there would not be a particularly high frequency of requests (probably once every few seconds at most) we made the decision to have the applications that utilise the queue simply query the database whenever one is needed; there is no message queue service at this time.
A single fetch is performed by having the client application run a stored procedure, which performs the query(ies) involved and returns a job ID. The client application then fetches the job information by querying by ID and sets the job as handled.
Performance is fine; the only snag we have felt is that, because the client application has to query for the details and perform a check before the job is marked as handled, on very rare occasions (once every few thousand jobs), two clients pick up the same job.
As a way of solving this problem, I was suggesting having the initial stored procedure that runs "tag" the record it pulls with the time and date. The stored procedure, when querying for records, will only pull records where this "tag" is a certain amount of time, say 5 seconds, in the past. That way, if the stored procedure runs twice within 5 seconds, the second instance will not pick up the same job.
Can anyone foresee any problems with fixing the problem this way or offer an alternative solution?

Answer 1

Use a UNIQUEIDENTIFIER field as your marker. When the stored procedure runs, lock the row you're reading and update the field with a NEWID(). You can mark your polling statement using something like WITH(READPAST) if you're worried about deadlocking issues.
The reason to use a GUID here is to have a unique identifier that will serve to mark a batch. Your NEWID() call is guaranteed to give you a unique value, which will be used to prevent you from accidentally picking up the same data twice. GETDATE() wouldn't work here because you could end up having two calls that resolve to the same time; BIT wouldn't work because it wouldn't uniquely mark off batches for picking up or reporting.
For example,
declare #ReadID uniqueidentifier
declare #BatchSize int = 20; -- make a parameter to your procedure
set #ReadID = NEWID();
UPDATE tbl WITH (ROWLOCK)
SET HasBeenRead = #ReadID -- your UNIQUEIDENTIFIER field
FROM (
SELECT TOP (#BatchSize) Id
FROM tbl WITH(UPDLOCK ROWLOCK READPAST )
WHERE HasBeenRead IS null ORDER BY [Id])
AS t1
WHERE ( tbl.Id = t1.Id)
SELECT Id, OtherCol, OtherCol2
FROM tbl WITH(UPDLOCK ROWLOCK READPAST )
WHERE HasBeenRead = #ReadID
And then you can use a polling statement like
SELECT COUNT(*) FROM tbl WITH(READPAST) WHERE HasBeenRead IS NULL
Adapted from here: https://msdn.microsoft.com/en-us/library/cc507804%28v=bts.10%29.aspx