Re: Is Recovery actually paused?

On Wed, Jan 13, 2021 at 3:35 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Wed, Jan 13, 2021 at 3:27 PM Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp> wrote:
> >
> > On Thu, 10 Dec 2020 11:25:23 +0530
> > Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > > > > However, I wonder users don't expect pg_is_wal_replay_paused to wait.
> > > > > Especially, if max_standby_streaming_delay is -1, this will be blocked forever,
> > > > > although this setting may not be usual. In addition, some users may set
> > > > > recovery_min_apply_delay for a large. If such users call pg_is_wal_replay_paused,
> > > > > it could wait for a long time.
> > > > >
> > > > > At least, I think we need some descriptions on document to explain
> > > > > pg_is_wal_replay_paused could wait while a time.
> > > >
> > > > Ok
> > >
> > > Fixed this, added some comments in .sgml as well as in function header
> >
> > Thank you for fixing this.
> >
> > Also, is it better to fix the description of pg_wal_replay_pause from
> > "Pauses recovery." to "Request to pause recovery." in according with
> > pg_is_wal_replay_paused?
>
> Okay
>
> >
> > > > > Also, how about adding a new boolean argument to pg_is_wal_replay_paused to
> > > > > control whether this waits for recovery to get paused or not? By setting its
> > > > > default value to true or false, users can use the old format for calling this
> > > > > and the backward compatibility can be maintained.
> > > >
> > > > So basically, if the wait_recovery_pause flag is false then we will
> > > > immediately return true if the pause is requested? I agree that it is
> > > > good to have an API to know whether the recovery pause is requested or
> > > > not but I am not sure is it good idea to make this API serve both the
> > > > purpose? Anyone else have any thoughts on this?
> > > >
> >
> > I think the current pg_is_wal_replay_paused() already has another purpose;
> > this waits recovery to actually get paused. If we want to limit this API's
> > purpose only to return the pause state, it seems better to fix this to return
> > the actual state at the cost of lacking the backward compatibility. If we want
> > to know whether pause is requested, we may add a new API like
> > pg_is_wal_replay_paluse_requeseted(). Also, if we want to wait recovery to actually
> > get paused, we may add an option to pg_wal_replay_pause() for this purpose.
> >
> > However, this might be a bikeshedding. If anyone don't care that
> > pg_is_wal_replay_paused() can make user wait for a long time, I don't care either.
>
> I don't think that it will be blocked ever, because
> pg_wal_replay_pause is sending the WakeupRecovery() which means the
> recovery process will not be stuck on waiting for the WAL.
>
> > > > > As another comment, while pg_is_wal_replay_paused is blocking, I can not cancel
> > > > > the query. I think CHECK_FOR_INTERRUPTS() is necessary in the waiting loop.
> >
> > How about this fix? I think users may want to cancel pg_is_wal_replay_paused() during
> > this is blocking.
>
> Yeah, we can do this. I will send the updated patch after putting
> some more thought into these comments. Thanks again for the feedback.
>

Please find the updated patch.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com