Re: Synchronizing slots from primary to standby

On Thu, Jun 29, 2023 at 3:52 PM Hayato Kuroda (Fujitsu)
<kuroda.hayato@fujitsu.com> wrote:
>
> 2. I want to confirm the reason why new replication command is added.
>

Are you referring LIST_SLOTS command? If so, I don't think that is
required and instead, we can use a query to fetch the required
information.

> IIUC the
>    launcher connects to primary by using primary_conninfo connection string, but
>    it establishes the physical replication connection so that any SQL cannot be executed.
>    Is it right? Another approach not to use is to specify the target database via
>    GUC, whereas not smart. How do you think?
> 3. You chose the per-db worker approach, however, it is difficult to extend the
>    feature to support physical slots. This may be problematic. Was there any
>    reasons for that? I doubted ReplicationSlotCreate() or advance functions might
>    not be used from other databases and these may be reasons, but not sure.
>    If these operations can do without connecting to specific database, I think
>    the architecture can be changed.
>

I think this point needs some investigation but do we want just one
worker that syncs all the slots? That may lead to lag in keeping the
slots up-to-date. We probably need some tests.

> 4. Currently the launcher establishes the connection every time. Isn't it better
>    to reuse the same one instead?
>

I feel it is not the launcher but a separate sync slot worker that
establishes the connection. It is not clear to me what exactly you
have in mind. Can you please explain a bit more?

> Following comments are assumed the configuration, maybe the straightfoward:
>
> primary->standby
>    |->subscriber
>
> 5. After constructing the system, I dropped the subscription on the subscriber.
>    In this case the logical slot on primary was removed, but that was not replicated
>    to standby server. Did you support the workload or not?
>

This should work.

>
> 6. Current approach may delay the startpoint of sync.
>
> Assuming that physical replication system is created first, and then the
> subscriber connects to the publisher node. In this case the launcher connects to
> primary earlier than the apply worker, and reads the slot. At that time there are
> no slots on primary, so launcher disconnects from primary and waits a time period (up to 3min).
> Even if the apply worker creates the slot on publisher, but the launcher on standby
> cannot notice that. The synchronization may start 3 min later.
>

I feel this should be based on some GUC like
'wal_retrieve_retry_interval' which we are already using in the
launcher or probably a new one if that doesn't seem to match.

--
With Regards,
Amit Kapila.