Re: Time delayed LR (WAS Re: logical replication restrictions)

On Thu, Mar 2, 2023 at 7:38 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Wed, Mar 1, 2023 at 6:21 PM Hayato Kuroda (Fujitsu)
> <kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
> >
> > >
> > > Apart from a bad-use case example I mentioned, in general, piling up
> > > WAL files due to the replication slot has many bad effects on the
> > > system. I'm concerned that the side effect of this feature (at least
> > > of the current design) is too huge compared to the benefit, and afraid
> > > that users might end up using this feature without understanding the
> > > side effect well. It might be okay if we thoroughly document it but
> > > I'm not sure.
> >
> > One approach is that change max_slot_wal_keep_size forcibly when min_send_delay
> > is set. But it may lead to disable the slot because WALs needed by the time-delayed
> > replication may be also removed. Just the right value cannot be set by us because
> > it is quite depends on the min_send_delay and workload.
> >
> > How about throwing the WARNING when min_send_delay > 0 but
> > max_slot_wal_keep_size < 0? Differ from previous, version the subscription
> > parameter min_send_delay will be sent to publisher. Therefore, we can compare
> > min_send_delay and max_slot_wal_keep_size when publisher receives the parameter.
>
> Since max_slot_wal_keep_size can be changed by reloading the config
> file, each walsender warns it also at that time?
>

I think Kuroda-San wants to emit a WARNING at the time of CREATE
SUBSCRIPTION. But it won't be possible to emit a WARNING at the time
of ALTER SUBSCRIPTION. Also, as you say if the user later changes the
value of max_slot_wal_keep_size, then even if we issue LOG/WARNING in
walsender, it may go unnoticed. If we really want to give WARNING for
this then we can probably give it as soon as user has set non-default
value of min_send_delay to indicate that this can lead to retaining
WAL on the publisher and they should consider setting
max_slot_wal_keep_size.

Having said that, I think users can always tune max_slot_wal_keep_size
and min_send_delay (as none of these requires restart) if they see any
indication of unexpected WAL size growth. There could be multiple ways
to check it but I think one can refer wal_status in
pg_replication_slots, the extended value can be an indicator of this.

> Not sure it's
> helpful. I think it's a legitimate use case to set min_send_delay > 0
> and max_slot_wal_keep_size = -1, and users might not even notice the
> WARNING message.
>

I think it would be better to tell about this in the docs along with
the 'min_send_delay' description. The key point is whether this would
be an acceptable trade-off for users who want to use this feature. I
think it can harm only if users use this without understanding the
corresponding trade-off. As we kept the default to no delay, it is
expected from users using this have an understanding of the trade-off.

--
With Regards,
Amit Kapila.