T-SQL select and update with lock - transaction or table hint - sql-server

Question

How to achieve following transaction lock?
In big simplification - I have a table of "Tasks" with statuses (Created, Started, Completed). I want to create stored procedure GetNext to get top 1 task that wasn't yet started (has Created status).
In this procedure I want to mark the task as Started. Obviously I want to avoid the situation when two processes call this procedure and get the same task.
The procedure will not be called frequently so performance is not an issue, keeping data uncorrupted is an issue.
So I want to do something like this:
UPDATE tblTasks
SET Status = 'Started'
WHERE TaskId = (SELECT TOP 1 TaskId
FROM tblTasks
WHERE Status = 'Created')
I also want to receive the task that I just updated so rather than what is above I need something like:
DECLARE #TaskId AS INT = (SELECT TOP 1 TaskId FROM tblTasks WHERE Status = 'Created')
UPDATE tblTasks
SET Status = 'Started'
WHERE TaskId = #TaskId
[... - Do something with #TaskId - not relevant]
OR
DECLARE #TaskIds AS TABLE(Id INT)
UPDATE tblTasks
SET Status = 'Started'
OUTPUT INSERTED.Id INTO #TaskIdS
WHERE TaskId = #TaskId
[... - Do something with #TaskIds - not relevant]
So assuming that I need select + update to achieve what I need - how can I assure that no other process will execute even first operation (select) until existing process is done?
As far as I understand even Serializable isolation level of transaction is not enough here because other process can read data, then wait until I finish (because its update is being held by lock) and update the data that I just updated.
I feel that table hints XLOCK or HOLDLOCK might help but I'm no expert and MS doc scared me with :
Caution
Because the SQL Server query optimizer typically selects the best execution plan for a query, we recommend that hints be used only as a last resort by experienced developers and database administrators.
(from https://learn.microsoft.com/en-us/sql/t-sql/queries/hints-transact-sql-table)
So how do I make sure that two processes will not update one item and also how do I make sure that if one process is running the other will wait and do its job after the first finishes instead of failing?

Answer 1

Typically SQL Server locks automatically for each step until you either have a GOor reaches the end of the script.
From what I understand, what you want/need is a way to SELECT/UPDATE "in one go". You should be able to achieve that with a combination of TRANSACTION, TRY ... CATCH and CTE.
DECLARE #TaskIds AS TABLE (TaskId INT);
BEGIN TRANSACTION;
BEGIN TRY
WITH myTasks (TaskId) AS (
SELECT TOP 1 t.TaskId
FROM tblTasks AS t
WHERE t.Status = 'Created'
)
UPDATE t
SET t.Status = 'Started'
OUTPUT INSERTED.TaskId INTO #TaskIds
FROM tblTasks AS t
INNER JOIN myTasks AS mt
ON mt.TaskId = t.TaskId;
END TRY
BEGIN CATCH
IF ##TRANCOUNT > 0
ROLLBACK TRANSACTION;
THROW;
END CATCH;
IF ##TRANCOUNT > 0
BEGIN
COMMIT TRANSACTION;
SELECT TaskId FROM #TaskIds;
[.. do other stuff ..]
END
GO