Re: Is Recovery actually paused?

At Tue, 9 Feb 2021 09:58:30 +0530, Bharath Rupireddy <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote in
> On Tue, Feb 9, 2021 at 9:48 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Tue, Feb 9, 2021 at 8:54 AM Yugo NAGATA <nagata(at)sraoss(dot)co(dot)jp> wrote:
> > >
> > > On Tue, 09 Feb 2021 10:58:04 +0900 (JST)
> > > Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > > > If we are going to introduce that complexity, I'd like to re-propose
> > > > to introduce interlocking between the recovery side and the
> > > > pause-requestor side instead of introducing the intermediate state,
> > > > which is the cause of the complexity.
> > > >
> > > > The attached PoC patch adds:
> > > >
> > > > - A solid checkpoint just before calling rm_redo. It doesn't add a
> > > > info_lck since the check is done in the existing lock section.
> > > >
> > > > - Interlocking between the above and SetRecoveryPause without adding a
> > > > shared variable.
> > > > (This is what I called "synchronous" before.)
> > >
> > > I think waiting in pg_wal_replay_pasue is a possible option, but this will
> > > also introduce other complexity to codes such as possibility of waiting for
> > > long or for ever. For example, waiting in SetRecoveryPause as in your POC
> > > patch appears to make recovery stuck in RecoveryRequiresIntParameter.
> > >
> >
> > I agree with this, I think we previously discussed these approaches
> > where we can wait in pg_wal_replay_pasue() or
> > pg_is_wal_replay_pasued(). In fact, we had an older version where we
> > put the wait in pg_is_wal_replay_pasued(). But it appeared that doing
> > so will add extra complexity as well as instead of waiting in these
> > APIs the wait logic can be implemented in the application code which
> > is actually using these APIs and IMHO that will give better control to
> > the users.
>
> And also, having waiting logic in pg_wal_replay_pasue() or
> pg_is_wal_replay_pasued() required changes to the existing API such as
> a timeout to not allow them infinitely waiting.

I don't understand that. pg_wal_replay_pause() is defined as "pausees
recovery". so it is the correct behavior to wait actual pause.
pg_is_wal_replay_paused() doesn't wait for anything at all.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center