Can a long running procedure detect when smart shutdown is pending?

Στις 5/7/24 21:12, ο/η Dennis White έγραψε:

My project's DB has a mutli-step stored procedure using Transaction
Control that may take 30 minutes or more to complete.
I am curious if there is a way to make it more smart shutdown friendly
so it can stop between steps?

We are using both PG 14 and PG 16 on Rhel 8.
Pardon me if it's obvious but is there a function to call or a table
that could be checked after a commit within the procedure to determine
a shutdown is pending?

Maybe somehow checking the log for a message like :

received smart shutdown request

Or use tail_n_mail against the log and then implement some logic using
NOTIFY , ideally you want your procedure to be interrupted rather than
do polling.

Thanks,
Dennis

--
Achilleas Mantzios
IT DEV - HEAD
IT DEPT
Dynacom Tankers Mgmt (as agents only)

Thanks for responding.
This will be a stored procedure written in plpgsql that's kicked off by
pg_cron.
I wasn't sure a normal smart shutdown would stop it.

Thanks

On Fri, Jul 5, 2024 at 4:57 PM Achilleas Mantzios <
a.mantzios@cloud.gatewaynet.com> wrote:

so 6. 7. 2024 v 1:36 odesílatel Dennis White <dwhite@seawardmoon.com>
napsal:

Thanks for responding.
This will be a stored procedure written in plpgsql that's kicked off by
pg_cron.
I wasn't sure a normal smart shutdown would stop it.

shutdown try to cancel any query. The plpgsql routine should be canceled
without problems.

On Fri, 2024-07-05 at 14:12 -0400, Dennis White wrote:

My project's DB has a mutli-step stored procedure using Transaction Control that may take 30 minutes or more to complete.
I am curious if there is a way to make it more smart shutdown friendly so it can stop between steps?

I don't think there is a direct way to do that in SQL; that would require a new
system function that exposes canAcceptConnections() in SQL.

What you could do is use the dblink extension to connect to the local database.
If you get an error "the database system is shutting down", there is a smart
shutdown in progress.

Yours,
Laurenz Albe

Στις 6/7/24 08:39, ο/η Pavel Stehule έγραψε:

so 6. 7. 2024 v 1:36 odesílatel Dennis White <dwhite@seawardmoon.com>
napsal:

Thanks for responding.
This will be a stored procedure written in plpgsql that's kicked
off by pg_cron.
I wasn't sure a normal smart shutdown would stop it.

shutdown try to cancel any query. The plpgsql routine should be
canceled without problems.

fast mode will have this effect. smart mode not, from my testing. The
running queries do not get interrupted in smart shutdown.

Thanks

On Fri, Jul 5, 2024 at 4:57 PM Achilleas Mantzios
<a.mantzios@cloud.gatewaynet.com> wrote:

Στις 5/7/24 21:12, ο/η Dennis White έγραψε:

My project's DB has a mutli-step stored procedure using
Transaction Control that may take 30 minutes or more to complete.
I am curious if there is a way to make it more smart shutdown
friendly so it can stop between steps?

We are using both PG 14 and PG 16 on Rhel 8.
Pardon me if it's obvious but is there a function to call or
a table that could be checked after a commit within the
procedure to determine a shutdown is pending?

Maybe somehow checking the log for a message like :

received smart shutdown request

Or use tail_n_mail against the log and then implement some
logic using NOTIFY , ideally you want your procedure to be
interrupted rather than do polling.

Thanks,
Dennis

--
Achilleas Mantzios
IT DEV - HEAD
IT DEPT
Dynacom Tankers Mgmt (as agents only)

--
Achilleas Mantzios
IT DEV - HEAD
IT DEPT
Dynacom Tankers Mgmt (as agents only)

so 6. 7. 2024 v 8:06 odesílatel Achilleas Mantzios <
a.mantzios@cloud.gatewaynet.com> napsal:

Στις 6/7/24 08:39, ο/η Pavel Stehule έγραψε:

so 6. 7. 2024 v 1:36 odesílatel Dennis White <dwhite@seawardmoon.com>
napsal:

Thanks for responding.
This will be a stored procedure written in plpgsql that's kicked off by
pg_cron.
I wasn't sure a normal smart shutdown would stop it.

shutdown try to cancel any query. The plpgsql routine should be canceled
without problems.

fast mode will have this effect. smart mode not, from my testing. The
running queries do not get interrupted in smart shutdown.

ok

but you can set timeout there. It doesn't help?

You can create some aux postgresql connection with specific app name, and
then you can monitor if this process still live from pg_stat_activity

one process

connect
set application_name to 'xxxx';

second process
if not exists(select * from pg_stat_activity where appname = 'xxxx') then
exit
end if;

so 6. 7. 2024 v 8:19 odesílatel Pavel Stehule <pavel.stehule@gmail.com>
napsal:

so 6. 7. 2024 v 8:06 odesílatel Achilleas Mantzios <
a.mantzios@cloud.gatewaynet.com> napsal:

Στις 6/7/24 08:39, ο/η Pavel Stehule έγραψε:

so 6. 7. 2024 v 1:36 odesílatel Dennis White <dwhite@seawardmoon.com>
napsal:

Thanks for responding.
This will be a stored procedure written in plpgsql that's kicked off by
pg_cron.
I wasn't sure a normal smart shutdown would stop it.

shutdown try to cancel any query. The plpgsql routine should be canceled
without problems.

fast mode will have this effect. smart mode not, from my testing. The
running queries do not get interrupted in smart shutdown.

ok

but you can set timeout there. It doesn't help?

You can create some aux postgresql connection with specific app name, and
then you can monitor if this process still live from pg_stat_activity

one process

connect
set application_name to 'xxxx';

second process
if not exists(select * from pg_stat_activity where appname = 'xxxx') then
exit
end if;

sorry - it cannot to help too

probably only one solution can be to write some extension and read some
internal state

Στις 6/7/24 09:22, ο/η Pavel Stehule έγραψε:

so 6. 7. 2024 v 8:19 odesílatel Pavel Stehule
<pavel.stehule@gmail.com> napsal:

so 6. 7. 2024 v 8:06 odesílatel Achilleas Mantzios
<a.mantzios@cloud.gatewaynet.com> napsal:

Στις 6/7/24 08:39, ο/η Pavel Stehule έγραψε:

so 6. 7. 2024 v 1:36 odesílatel Dennis White
<dwhite@seawardmoon.com> napsal:

Thanks for responding.
This will be a stored procedure written in plpgsql that's
kicked off by pg_cron.
I wasn't sure a normal smart shutdown would stop it.

shutdown try to cancel any query. The plpgsql routine should
be canceled without problems.

fast mode will have this effect. smart mode not, from my
testing. The running queries do not get interrupted in smart
shutdown.

ok

but you can set timeout there. It doesn't help?

You can create some aux postgresql connection with specific app
name, and then you can monitor if this process still live from
pg_stat_activity

one process

connect
set application_name to 'xxxx';

second process
if not exists(select * from pg_stat_activity where appname =
'xxxx') then
  exit
end if;

sorry - it cannot to help too

probably only one solution can be to write some extension and read
some internal state

One idea was to have a process reading the log for " received smart
shutdown request " and then send a

NOTIFY shutdown_channel, 'shutdown in progress'

The question I have not yet answered is how to get the asynchronous
notification from within plpgsql. I mean this is possible with python or
Go or Java / JDBC or some other programming language , but cannot seem
to find anything in plpgsql .

Thanks

On Fri, Jul 5, 2024 at 4:57 PM Achilleas Mantzios
<a.mantzios@cloud.gatewaynet.com> wrote:

Στις 5/7/24 21:12, ο/η Dennis White έγραψε:

My project's DB has a mutli-step stored procedure
using Transaction Control that may take 30 minutes
or more to complete.
I am curious if there is a way to make it more smart
shutdown friendly so it can stop between steps?

We are using both PG 14 and PG 16 on Rhel 8.
Pardon me if it's obvious but is there a function to
call or a table that could be checked after a commit
within the procedure to determine a shutdown is pending?

Maybe somehow checking the log for a message like :

received smart shutdown request

Or use tail_n_mail against the log and then implement
some logic using NOTIFY , ideally you want your
procedure to be interrupted rather than do polling.

Thanks,
Dennis

--
Achilleas Mantzios
IT DEV - HEAD
IT DEPT
Dynacom Tankers Mgmt (as agents only)

--
Achilleas Mantzios
IT DEV - HEAD
IT DEPT
Dynacom Tankers Mgmt (as agents only)

--
Achilleas Mantzios
IT DEV - HEAD
IT DEPT
Dynacom Tankers Mgmt (as agents only)

Laurenz Albe <laurenz.albe@cybertec.at> writes:

On Fri, 2024-07-05 at 14:12 -0400, Dennis White wrote:

My project's DB has a mutli-step stored procedure using Transaction Control that may take 30 minutes or more to complete.
I am curious if there is a way to make it more smart shutdown friendly so it can stop between steps?

I don't think there is a direct way to do that in SQL; that would require a new
system function that exposes canAcceptConnections() in SQL.

It's worse than that: the state variables involved are local to the
postmaster, so you wouldn't get the right answer in a backend even
if the function were reachable.

What you could do is use the dblink extension to connect to the local database.
If you get an error "the database system is shutting down", there is a smart
shutdown in progress.

This'd probably work. Ugly, but ...

regards, tom lane

Thanks everyone. A more direct way to check via a sql function would be
better but I suppose the dblink extension method will work.

Thanks again,
Dennis

On Sat, Jul 6, 2024 at 9:38 AM Tom Lane <tgl@sss.pgh.pa.us> wrote: