Re: speed up a logical replica setup

On Mon, Mar 18, 2024 at 7:22 PM Peter Eisentraut <peter(at)eisentraut(dot)org> wrote:
>
> In check_subscriber(): All these permissions checks seem problematic
> to me. We shouldn't reimplement our own copy of the server's
> permission checks. The server can check the permissions. And if the
> permission checking in the server ever changes, then we have
> inconsistencies to take care of. Also, the error messages "permission
> denied" are inappropriate, because we are not doing the actual thing.
> Maybe we want to do a dry-run for the benefit of the user, but then we
> should do the actual thing, like try to create a replication slot, or
> whatever. But I would rather just remove all this, it seems too
> problematic.
>

If we remove all the checks then there is a possibility that we can
fail later while creating the actual subscription. For example, if
there are not sufficient max_replication_slots, then it is bound to
fail in the later steps which would be a costlier affair because by
that time the standby would have been promoted and the user won't have
any way to move forward but to re-create standby and then use this
tool again. I think here the patch tries to mimic pg_upgrade style
checks where we do some pre-checks.

This raises a question in my mind how are we expecting users to know
all these required values and configure it properly before using this
tool? IIUC, we are expecting that the user should figure out the
appropriate values for max_replication_slots,
max_logical_replication_workers, etc. by querying the number of
databases in the primary. Then either stop the standby to change these
parameters or use ALTER SYSTEM depending on the required parameters.
Similarly, there are some config requirements (like max_wal_senders,
max_replication_slots) for the primary which would be difficult for
users to know as these are tools are internal requirements.

The two possibilities that come to my mind are (a) pg_createsubscriber
should have some special mode/option using which the user can find out
all the required config settings, or (b) document how a user can find
the required settings. There are good chances of mistakes with option
(b).

--
With Regards,
Amit Kapila.