| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> |
| Cc: | Hemil Ruparel <hemilruparel2002(at)gmail(dot)com>, pgsql-general(at)lists(dot)postgresql(dot)org |
| Subject: | Re: Are advisory locks guaranteed to be First Come First Serve? And can the behavior be relied upon? |
| Date: | 2020-08-27 13:44:33 |
| Message-ID: | 3028225.1598535873@sss.pgh.pa.us |
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| Lists: | pgsql-general |
Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)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
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