Re: Synchronizing slots from primary to standby

On Mon, Dec 4, 2023 at 10:07 PM Drouvot, Bertrand
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,
>
> On 12/4/23 6:10 AM, shveta malik wrote:
> > On Fri, Dec 1, 2023 at 5:40 PM Nisha Moond <nisha(dot)moond412(at)gmail(dot)com> wrote:
> >>
> >> Review for v41 patch.
> >
> > Thanks for the feedback.
> >
> >> ~~~
> >> 2.
> >> IIUC, the slotsyncworker's connection to the primary is to execute a
> >> query. Its aim is not walsender type connection, but at primary when
> >> queried, the 'backend_type' is set to 'walsender'.
> >> Snippet from primary db-
> >>
> >> datname | usename | application_name | wait_event_type | backend_type
> >> ---------+-------------+------------------+-----------------+--------------
> >> postgres | replication | slotsyncworker | Client | walsender
> >>
> >> Is it okay?
> >>
> >
> > Slot sync worker uses 'libpqrcv_connect' for connection which sends
> > 'replication'-'database' key-value pair as one of the connection
> > options. And on the primary side, 'ProcessStartupPacket' on the basis
> > of this key-value pair sets the process as walsender one (am_walsender
> > = true).
> > And thus this reflects as backend_type='walsender' in
> > pg_stat_activity. I do not see any harm in this backend_type for
> > slot-sync worker currently. This is on a similar line of connections
> > used for logical-replications. And since a slot-sync worker also deals
> > with wals-positions (lsns), it is okay to maintain backend_type as
> > walsender unless you (or others) see any potential issue in doing
> > that. So let me know.
>
> I don't see any issue as well (though I understand it might
> seems weird to see a walsender process being spawned doing non
> replication stuff)
>
> >
> >> ~~~
> >> 4. primary_slot_name GUC value test:
> >>
> >> When standby is started with a non-existing primary_slot_name, the
> >> wal-receiver gives an error but the slot-sync worker does not raise
> >> any error/warning. It is no-op though as it has a check 'if
> >> (XLogRecPtrIsInvalid(WalRcv->latestWalEnd)) do nothing'. Is this
> >> okay or shall the slot-sync worker too raise an error and exit?
> >>
> >> In another case, when standby is started with valid primary_slot_name,
> >> but it is changed to some invalid value in runtime, then walreceiver
> >> starts giving error but the slot-sync worker keeps on running. In this
> >> case, unlike the previous case, it even did not go to no-op mode (as
> >> it sees valid WalRcv->latestWalEnd from the earlier run) and keep
> >> pinging primary repeatedly for slots. Shall here it should error out
> >> or at least be no-op until we give a valid primary_slot_name?
> >>
> >
>
> Nice catch, thanks!
>
> > I reviewed it. There is no way to test the existence/validity of
> > 'primary_slot_name' on standby without making a connection to primary.
> > If primary_slot_name is invalid from the start, slot-sync worker will
> > be no-op (as you tested) as WalRecv->latestWalENd will be invalid, and
> > if 'primary_slot_name' is changed to invalid on runtime, slot-sync
> > worker will still keep on pinging primary. But that should be okay (in
> > fact needed) as it needs to sync at-least the previous slot's
> > positions (in case it is delayed in doing so for some reason earlier).
> > And once the slots are up-to-date on standby, even if worker pings
> > primary, it will not see any change in slots lsns and thus go for
> > longer nap. I think, it is not worth the effort to introduce the
> > complexity of checking validity of 'primary_slot_name' on primary from
> > standby for this rare scenario.
> >
>
> Maybe another option could be to have the walreceiver a way to let the slot sync
> worker knows that it (the walreceiver) was not able to start due to non existing
> replication slot on the primary? (that way we'd avoid the slot sync worker having
> to talk to the primary).

Few points:
1) I think if we do it, we should do it in generic way i.e. slotsync
worker should go to no-op if walreceiver is not able to start due to
any reason and not only due to invalid primary_slot_name.
2) Secondly, slotsync worker needs to make sure it has synced the
slots so far i.e. worker should not go to no-op immediately on seeing
missing WalRcv process if there are pending slots to be synced.

So the generic way I see to have this optimization is:
1) Slotsync worker can use 'WalRcv->pid' to figure out if WalReceiver
is running or not.
2) Slotsync worker should check null 'WalRcv->pid' only when
no-activity is observed for threshold time i.e. it can do it during
existing logic of increasing naptime.
3) On finding null 'WalRcv->pid', worker can mark a flag to go to
no-op unless WalRcv->pid becomes valid again. Marking this flag during
increasing naptime will guarantee that the worker has taken all the
changes so far i.e. standby is not lagging in terms of slots.

Thoughts?

thanks
Shveta