From: "Takamichi Osumi (Fujitsu)" <osumi.takamichi@fujitsu.com>
To: 'Euler Taveira' <euler@eulerto.com>, Peter Smith <smithpb2250@gmail.com>
CC: vignesh C <vignesh21@gmail.com>, Kyotaro Horiguchi
	<horikyota.ntt@gmail.com>, "Hayato Kuroda (Fujitsu)"
	<kuroda.hayato@fujitsu.com>, "shveta.malik@gmail.com"
	<shveta.malik@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Amit Kapila
	<amit.kapila16@gmail.com>, Melih Mutlu <m.melihmutlu@gmail.com>, Andres
 Freund <andres@anarazel.de>, Marcos Pegoraro <marcos@f10.com.br>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: RE: Time delayed LR (WAS Re: logical replication restrictions)
Thread-Topic: Time delayed LR (WAS Re: logical replication restrictions)
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Date: Tue, 24 Jan 2023 14:57:22 +0000
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Precedence: bulk

On Tuesday, January 24, 2023 8:32 AM Euler Taveira <euler@eulerto.com> wrot=
e:
> Good to know that you keep improving this patch. I have a few suggestions=
 that
> were easier to provide a patch on top of your latest patch than to provid=
e an
> inline suggestions.
Thanks for your review ! We basically adopted your suggestions.


> There are a few documentation polishing. Let me comment some of them abov=
e.
>=20
> -       The length of time (ms) to delay the application of changes.
> +       Total time spent delaying the application of changes, in millisec=
onds
>=20
> I don't remember if I suggested this description for catalog but IMO the
> suggestion reads better for me.
Adopted the above change.


> -       For time-delayed logical replication (i.e. when the subscription =
is
> -       created with parameter min_apply_delay > 0), the apply worker sen=
ds a
> -       Standby Status Update message to the publisher with a period of
> -       <literal>wal_receiver_status_interval</literal>. Make sure to set
> -       <literal>wal_receiver_status_interval</literal> less than the
> -       <literal>wal_sender_timeout</literal> on the publisher, otherwise=
, the
> -       walsender will repeatedly terminate due to the timeout errors. If
> -       <literal>wal_receiver_status_interval</literal> is set to zero, t=
he apply
> -       worker doesn't send any feedback messages during the subscriber's
> -       <literal>min_apply_delay</literal> period. See
> -       <xref linkend=3D"sql-createsubscription"/> for details.
> +       For time-delayed logical replication, the apply worker sends a fe=
edback
> +       message to the publisher every
> +       <varname>wal_receiver_status_interval</varname> milliseconds. Mak=
e sure
> +       to set <varname>wal_receiver_status_interval</varname> less than =
the
> +       <varname>wal_sender_timeout</varname> on the publisher, otherwise=
, the
> +       <literal>walsender</literal> will repeatedly terminate due to tim=
eout
> +       error. If <varname>wal_receiver_status_interval</varname> is set =
to
> +       zero, the apply worker doesn't send any feedback messages during =
the
> +       <literal>min_apply_delay</literal> interval.
>=20
> I removed the parenthesis explanation about time-delayed logical replicat=
ion.
> If you are reading the documentation and does not know what it means you =
should
> (a) read the logical replication chapter or (b) check the glossary (maybe=
 a new
> entry should be added). I also removed the Standby status Update message =
but it
> is a low level detail; let's refer to it as feedback message as the other
> sentences do. I changed "literal" to "varname" that's the correct tag for
> parameters. I replace "period" with "interval" that was the previous
> terminology. IMO we should be uniform, use one or the other.
Adopted.

Also, I added the glossary for time-delayed replication (one for
applicable to both physical replication and logical replication).
Plus, I united the term "interval" to period, because it would clarify the =
type for this feature.
I think this is better.
> -   The subscriber replication can be instructed to lag behind the publis=
her
> -   side changes by specifying the <literal>min_apply_delay</literal>
> -   subscription parameter. See <xref linkend=3D"sql-createsubscription"/=
> for
> -   details.
> +   A logical replication subscription can delay the application of chang=
es by
> +   specifying the <literal>min_apply_delay</literal> subscription parame=
ter.
> +   See <xref linkend=3D"sql-createsubscription"/> for details.
>=20
> This feature refers to a specific subscription, hence, "logical replicati=
on
> subscription" instead of "subscriber replication".
Adopted.

> +           if (IsSet(opts->specified_opts, SUBOPT_MIN_APPLY_DELAY))
> +               errorConflictingDefElem(defel, pstate);
> +
>=20
> Peter S referred to this missing piece of code too.
Added.


> -int
> +static int
> defGetMinApplyDelay(DefElem *def)
> {
>=20
> It seems you forgot static keyword.
Fixed.


> -       elog(DEBUG2, "time-delayed replication for txid %u, min_apply_del=
ay =3D %lld ms, Remaining wait time: %ld ms",
> -            xid, (long long) MySubscription->minapplydelay, diffms);
> +       elog(DEBUG2, "time-delayed replication for txid %u, min_apply_del=
ay =3D " INT64_FORMAT " ms, remaining wait time: %ld ms",
> +            xid, MySubscription->minapplydelay, diffms);
> int64 should use format modifier INT64_FORMAT.
Fixed.


> -                     (long) wal_receiver_status_interval * 1000,
> +                     wal_receiver_status_interval * 1000L,
>=20
> Cast is not required. I added a suffix to the constant.
Fixed.


> -   elog(DEBUG2, "sending feedback (force %d) to recv %X/%X, write %X/%X,=
 flush %X/%X in-delayed: %d",
> +   elog(DEBUG2, "sending feedback (force %d) to recv %X/%X, write %X/%X,=
 flush %X/%X, apply delay: %s",
>          force,
>          LSN_FORMAT_ARGS(recvpos),
>          LSN_FORMAT_ARGS(writepos),
>          LSN_FORMAT_ARGS(flushpos),
> -        in_delayed_apply);
> +        in_delayed_apply? "yes" : "no");
>=20
> It is better to use a string to represent the yes/no option.
Fixed.


> -                             gettext_noop("Min apply delay (ms)"));
> +                             gettext_noop("Min apply delay"));
>=20
> I don't know if it was discussed but we don't add units to headers. When =
I
> think about this parameter representation (internal and external), I deci=
ded to
> use the previous code because it provides a unit for external representat=
ion. I
> understand that using the same representation as recovery_min_apply_delay=
 is
> good but the current code does not handle the external representation
> accordingly. (recovery_min_apply_delay uses the GUC machinery to adds the=
 unit
> but for min_apply_delay, it doesn't).
Adopted.


> # Setup for streaming case
> -$node_publisher->append_conf('postgres.conf',
> +$node_publisher->append_conf('postgresql.conf',
>     'logical_decoding_mode =3D immediate');
> $node_publisher->reload;
>=20
> Fix configuration file name.
Fixed.


> Maybe tests should do a better job. I think check_apply_delay_time is fra=
gile
> because it does not guarantee that time is not shifted. Time-delayed
> replication is a subscriber feature and to check its correctness it shoul=
d
> check the logs.
>=20
> # Note that we cannot call check_apply_delay_log() here because there is =
a
> # possibility that the delay is skipped. The event happens when the WAL
> # replication between publisher and subscriber is delayed due to a mechan=
ical
> # problem. The log output will be checked later - substantial delay-time =
case.
>=20
> If you might not use the logs for it, it should adjust the min_apply_dela=
y, no?
Yes. Adjusted.


> It does not exercise the min_apply_delay vs parallel streaming mode.
>=20
> +                   /*
> +                    * The combination of parallel streaming mode and
> +                    * min_apply_delay is not allowed.
> +                    */
> +                   if (opts.streaming =3D=3D LOGICALREP_STREAM_PARALLEL)
> +                       if ((IsSet(opts.specified_opts, SUBOPT_MIN_APPLY_=
DELAY) && opts.min_apply_delay > 0) ||
> +                           (!IsSet(opts.specified_opts, SUBOPT_MIN_APPLY=
_DELAY) && sub->minapplydelay > 0))
> +                           ereport(ERROR,
> +                                   errcode(ERRCODE_OBJECT_NOT_IN_PREREQU=
ISITE_STATE),
> +                                   errmsg("cannot enable %s mode for sub=
scription with %s",
> +                                          "streaming =3D parallel", "min=
_apply_delay"));
> +
>=20
> Is this code correct? I also didn't like this message. "cannot enable str=
eaming
> =3D parallel mode for subscription with min_apply_delay" is far from a go=
od error
> message. How about refer parallelism to "parallel streaming mode".
Yes. opts is the input for alter command and sub object is the existing def=
inition.
We need to check those combinations like when streaming is set to parallel
and min_apply_delay also gets set, then, min_apply_delay should not be bigg=
er than 0, for example.
Besides, adopted your suggestion to improve the comments.


Attach the patch in [1]. Kindly have a look at it.


[1] - https://www.postgresql.org/message-id/TYCPR01MB8373DC1881F382B4703F26=
E0EDC99%40TYCPR01MB8373.jpnprd01.prod.outlook.com


Best Regards,
	Takamichi Osumi