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

On Mon, Feb 27, 2023 at 11:11 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Thu, Feb 23, 2023 at 9:10 PM Hayato Kuroda (Fujitsu)
> <kuroda(dot)hayato(at)fujitsu(dot)com> wrote:
> >
> > Thank you for reviewing! PSA new version.
> >
> >
>
> Thank you for updating the patch. Here are some comments on v7 patch:
>
> + *
> + * LOGICALREP_PROTO_MIN_SEND_DELAY_VERSION_NUM is the minimum protocol version
> + * with support for delaying to send transactions. Introduced in PG16.
> */
> #define LOGICALREP_PROTO_MIN_VERSION_NUM 1
> #define LOGICALREP_PROTO_VERSION_NUM 1
> #define LOGICALREP_PROTO_STREAM_VERSION_NUM 2
> #define LOGICALREP_PROTO_TWOPHASE_VERSION_NUM 3
> #define LOGICALREP_PROTO_STREAM_PARALLEL_VERSION_NUM 4
> -#define LOGICALREP_PROTO_MAX_VERSION_NUM
> LOGICALREP_PROTO_STREAM_PARALLEL_VERSION_NUM
> +#define LOGICALREP_PROTO_MIN_SEND_DELAY_VERSION_NUM 4
> +#define LOGICALREP_PROTO_MAX_VERSION_NUM
> LOGICALREP_PROTO_MIN_SEND_DELAY_VERSION_NUM
>
> What is the usecase of the old macro,
> LOGICALREP_PROTO_STREAM_PARALLEL_VERSION_NUM, after adding
> LOGICALREP_PROTO_MIN_SEND_DELAY_VERSION_NUM ? I think if we go this
> way, we will end up adding macros every time when adding a new option,
> which seems not a good idea. I'm really not sure we need to change the
> protocol version or the macro. Commit
> 366283961ac0ed6d89014444c6090f3fd02fce0a introduced the 'origin'
> subscription parameter that is also sent to the publisher, but we
> didn't touch the protocol version at all.
>

Right, I also don't see a reason to do anything for this. We have
previously bumped the protocol version when we send extra/additional
information from walsender but here that is not the requirement, so
this change doesn't seem to be required.

> ---
> Why do we not to delay sending COMMIT PREPARED messages?
>

I think we need to either add delay for prepare or commit prepared as
otherwise, it will lead to delaying the xact more than required. The
patch seems to add a delay before sending a PREPARE as that is the
time when the subscriber will apply the changes.
--
With Regards,
Amit Kapila.