Using SELECT ... FOR UPDATE to poll for a value change - sql

Question

I have a table that contains tasks and their status, akin to:
| task_id | task_status |
+---------+-------------+
| 71 | 1 |
| 85 | 3 |
| 110 | 2 |
Let's call the table TASKS.
Status is an enumerated value, for example:
= SCHEDULED
= RUNNING
= DONE
I need to poll this status to inform the user about a task he started. Currently, I'm just polling it on the server using a while loop, like this pseudocode:
status = old_status
while(timeout_not_expired and status==old_status) {
status = get_status("SELECT task_status FROM TASKS WHERE task_id=%1", task_id)
wait(check_interval)
}
return status
That's nasty, not only it spams the Oracle SQL server, it also spams our log of SQL queries.
So I did a bit of googling and found about SELECT ... FOR UPDATE. I tried to run this statement:
SELECT
task_status
FROM TASKS
WHERE task_id = 361
FOR UPDATE OF task_status
But it returns immediately. So the question:
Is this even what FOR UPDATE is for?
If yes, how do I get it to wait on the row with a timeout?

Answer 1

No, that isn't what that clause is for. From the documentation:
The FOR UPDATE clause lets you lock the selected rows so that other users cannot lock or update the rows until you end your transaction.
Your query selects the current status for that task and locks the row, essentially on the assumption that you plan to update it, and don't want anyone else to be able to change it between your select and subsequent update.
So after you perform that query, no-one else can update the status of that task until you commit or rollback - kind of the opposite of what you're trying to achieve.
You could look at alert or queueing mechanisms, but you might want to investigate continuous query notification, though it could be overkill for this.