Обсуждение: Are advisory locks guaranteed to be First Come First Serve? And can the behavior be relied upon?
I was playing with Advisory locks. I opened three connections to the database And obtained a lock with the same key. I noticed that the locks were obtained in the order of requests. I was wondering whether the locks are guaranteed to be First Come First Serve and can this behaviour be relied upon.
I read the docs here. But it didn't mention anything.
Thanks a lot for your time and efforts
Thanks a lot
On Thu 27 Aug, 2020, 12:23 PM Laurenz Albe, <laurenz.albe@cybertec.at> wrote:
On Thu, 2020-08-27 at 10:10 +0530, Hemil Ruparel wrote:
> I was playing with Advisory locks. I opened three connections to the database And obtained
> a lock with the same key. I noticed that the locks were obtained in the order of requests.
> I was wondering whether the locks are guaranteed to be First Come First Serve and can this
> behaviour be relied upon.
Yes - lock requests that block are queued, with later lock requests queued behind previous
ones. Once a lock gets free, the first in line gets it.
This is the same for all heavyweight locks in PostgreSQL.
Yours,
Laurenz Albe
--
Cybertec | https://www.cybertec-postgresql.com
Laurenz Albe <laurenz.albe@cybertec.at> writes:
> On Thu, 2020-08-27 at 10:10 +0530, Hemil Ruparel wrote:
>> I was playing with Advisory locks. I opened three connections to the database And obtained
>> a lock with the same key. I noticed that the locks were obtained in the order of requests.
>> I was wondering whether the locks are guaranteed to be First Come First Serve and can this
>> behaviour be relied upon.
> Yes - lock requests that block are queued, with later lock requests queued behind previous
> ones. Once a lock gets free, the first in line gets it.
> This is the same for all heavyweight locks in PostgreSQL.
There is an exception: the queue order will be rearranged if we find
that doing so is necessary to avoid a deadlock failure. See
src/backend/storage/lmgr/README
regards, tom lane