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

Hi,


On Thursday, January 19, 2023 7:55 PM vignesh C <vignesh21@gmail.com> wrote:
> On Thu, 19 Jan 2023 at 12:06, Takamichi Osumi (Fujitsu)
> <osumi.takamichi@fujitsu.com> wrote:
> >
> > Updated the comment and the function call.
> >
> > Kindly have a look at the updated patch v17.
> 
> Thanks for the updated patch, few comments:
> 1) min_apply_delay was accepting values like '600 m s h', I was not sure if we
> should allow this:
> alter subscription sub1 set (min_apply_delay = ' 600 m s h');
> 
> +                       /*
> +                        * If no unit was specified, then explicitly
> add 'ms' otherwise
> +                        * the interval_in function would assume 'seconds'.
> +                        */
> +                       if (strspn(tmp, "-0123456789 ") == strlen(tmp))
> +                               val = psprintf("%sms", tmp);
> +                       else
> +                               val = tmp;
> +
> +                       interval =
> DatumGetIntervalP(DirectFunctionCall3(interval_in,
> +
> 
> CStringGetDatum(val),
> +
> 
> ObjectIdGetDatum(InvalidOid),
> +
>                                                   Int32GetDatum(-1)));
> 
FYI, the input can be accepted by the interval type.
Now we changed the direction of the type from interval to integer
but plus some unit can be added like recovery_min_apply_delay.
Please check.


> 3) There is one check at parse_subscription_options and another check in
> AlterSubscription, this looks like a redundant check in case of alter
> subscription, can we try to merge and keep in one place:
> /*
> * The combination of parallel streaming mode and min_apply_delay is not
> * allowed.
> */
> if (IsSet(supported_opts, SUBOPT_MIN_APPLY_DELAY) &&
> opts->min_apply_delay > 0)
> {
> if (opts->streaming == LOGICALREP_STREAM_PARALLEL) ereport(ERROR,
> errcode(ERRCODE_SYNTAX_ERROR), errmsg("%s and %s are mutually
> exclusive options",
>    "min_apply_delay > 0", "streaming = parallel")); }
> 
> if (IsSet(opts.specified_opts, SUBOPT_MIN_APPLY_DELAY)) {
> /*
> * The combination of parallel streaming mode and
> * min_apply_delay is not allowed.
> */
> if (opts.min_apply_delay > 0)
> if ((IsSet(opts.specified_opts, SUBOPT_STREAMING) && opts.streaming ==
> LOGICALREP_STREAM_PARALLEL) ||
> (!IsSet(opts.specified_opts, SUBOPT_STREAMING) && sub->stream ==
> LOGICALREP_STREAM_PARALLEL))
> ereport(ERROR,
> errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
> errmsg("cannot enable %s for subscription in %s mode",
>    "min_apply_delay", "streaming = parallel"));
> 
> values[Anum_pg_subscription_subminapplydelay - 1] =
> Int64GetDatum(opts.min_apply_delay);
> replaces[Anum_pg_subscription_subminapplydelay - 1] = true; }
We can't. For create subscription, we need to check the patch
from parse_subscription_options, while for alter subscription,
we need to refer the current MySubscription value for those tests
in AlterSubscription.

 
> 4) typo "execeeds" should be "exceeds"
> 
> +          time on the subscriber. Any overhead of time spent in
> logical decoding
> +          and in transferring the transaction may reduce the actual wait time.
> +          It is also possible that the overhead already execeeds the
> requested
> +          <literal>min_apply_delay</literal> value, in which case no
> additional
> +          wait is necessary. If the system clocks on publisher and subscriber
> +          are not synchronized, this may lead to apply changes earlier
> + than
Fixed.

Kindly have a look at the v18 patch in [1].


[1] -
https://www.postgresql.org/message-id/TYCPR01MB8373BED9E390C4839AF56685EDC59%40TYCPR01MB8373.jpnprd01.prod.outlook.com


Best Regards,
    Takamichi Osumi