Re: Synchronizing slots from primary to standby

On Mon, Nov 6, 2023 at 5:36 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Mon, Nov 6, 2023 at 1:57 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Mon, Nov 6, 2023 at 7:01 AM Zhijie Hou (Fujitsu)
> > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > >
>
> +static void
> +WalSndGetStandbySlots(List **standby_slots, bool force)
> +{
> + if (!MyReplicationSlot->data.failover)
> + return;
> +
> + if (standby_slot_names_list == NIL && strcmp(standby_slot_names, "") != 0)
> + SlotSyncInitConfig();
> +
> + if (force || StandbySlotNamesPreReload == NULL ||
> + strcmp(StandbySlotNamesPreReload, standby_slot_names) != 0)
> + {
> + list_free(*standby_slots);
> +
> + if (StandbySlotNamesPreReload)
> + pfree(StandbySlotNamesPreReload);
> +
> + StandbySlotNamesPreReload = pstrdup(standby_slot_names);
> + *standby_slots = list_copy(standby_slot_names_list);
> + }
> +}
>
> I find this code bit difficult to understand. I think we don't need to
> maintain a global variable like StandbySlotNamesPreReload. We can use
> a local variable for it on the lines of what we do in
> StartupRereadConfig(). Then, can we think of maintaining
> standby_slot_names_list in something related to decoding like
> LogicalDecodingContext as this will be used during decoding only?
>

Yes, agreed. This code part is now simplified in v31. PFA the patches.

The overall changes are:

1) Caching of the standby_slots list in the logical-decoding context
as suggested above. All the globals have been removed.
2) Dropping of local synced slots for obsolete dbs. Launcher now takes
care of that.
3) There was a repeated warning in the log file due to missing GUCs as
described in [1]. Fixed that.
4) Optimized code in slotsync.c and launcher.c to get rid of globals.
5) Adjusted patch003's wait-for-standby logic in slot-sync workers as
per changes in pt. 1. There is still one optimization left here (in
patch003) to avoid repeated parsing. I have mentioned the TODO
comment. Will be targeted in the next version.

The changes for 1 are in patch01. The changes for 2,3,4 are in patch02.

Thanks Hou-san for implementing the changes for 1 and assisting in 5.

[1]: https://www.postgresql.org/message-id/CAJpy0uDpV0suPbhCp%2B1aRLXEChD9uKp-ffBW_HfZro%3D53JKK5w%40mail.gmail.com

thanks
Shveta