How can I ensure that a job will never have more than one instance running?
For example, if I have a job that is scheduled to run once a minute, but it actually runs for three minutes, I don't want a second and third instance of the job to start and run concurrently.
Best Answer
Using
dbms_job
, you're never going to have two instances of a job running at the same time. If I do something like this with aninterval
that is 1 minute in the future and a procedure that takes 3 minutes to runthe
interval
expression is only evaluated after the prior run finishes. There won't be 3 instances of the job running simultaneously. The job will just run once every 4 minutes.This is why you frequently hear people about jobs "drifting" over time-- a job is scheduled to run at 4 AM with an interval of "sysdate + 1" so every day it runs at a slightly later time. If the job takes 1 minute to run, after 2 months, it's now running just after 5 AM. And that's why you'd generally want your
interval
to be something like "trunc(sysdate+1) + interval '4' hour" if you want to run every day at 4 AM.If you want to provide additional protection in case, say, someone tries to run the procedure in a different session at the same time that the job is running, you can put a
dbms_lock.request
call in the procedure that tries to acquire a user-defined lock before doing the actual work.