Re: Synchronizing slots from primary to standby

On Fri, Dec 29, 2023 at 10:25 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Fri, Dec 29, 2023 at 7:18 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Wed, Dec 27, 2023 at 7:13 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Dec 27, 2023 at 11:36 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > > I was not aware if there is any way to connect if we
> > > want to run SQL queries. I initially tried using 'PQconnectdbParams'
> > > but couldn't make it work. Perhaps it is to be used only by front-end
> > > and extensions as the header files indicate as well:
> > > * libpq-fe.h : This file contains definitions for structures and
> > > externs for functions used by frontend postgres applications.
> > > * libpq-be-fe-helpers.h: Helper functions for using libpq in
> > > extensions . Code built directly into the backend is not allowed to
> > > link to libpq directly.
> >
> > Oh I didn't know that. Thank you for pointing it out.
> >
> > But I'm still concerned it could confuse users that
> > pg_stat_replication keeps showing one entry that remains as "startup"
> > state.

Okay. I understand your concern. I have attached PoC patch
(v55_02-0004) which attempts to implement non-replication connection
in slotsync worker. By doing so, pg_stat_replication will not show its
entry while pg_stat_activity will still show it with 'state' as either
"active" or "idle". Currently, since we are not using any of the
replication cmds, thus non-replication connection suits well. But in
future, if there is a requirement to execute existing (or new) cmd in
slotsync worker, then that can not be done simply in non-replication
connection; it will need some changes in non-replication or will need
the replication connection itself.

>> It has the same application_name as the walreceiver uses. For
> > example, when users want to check the particular replication
> > connection, it's common to filter the entries by the application name.
> > But it will end up having duplicate entries having different states.
> >
>
> Valid point. The main reason for using cluster_name is that if
> multiple standby's connect to the same primary, all will have the same
> application_name as 'slotsyncworker'. The other alternative could be
> to use {cluster_name}_slotsyncworker, which will probably address your
> concern and we can have to provision to differentiate among
> slotsyncworkers from different standby's.

The topup patch has also changed app_name to
{cluster_name}_slotsyncworker so that we do not confuse between
walreceiver and slotsyncworker entry.

Please note that there is no change in rest of the patches, changes
are in additional 0004 patch alone.

> --
> With Regards,
> Amit Kapila.