Re: Synchronizing slots from primary to standby

On Wed, Jan 10, 2024 at 5:53 PM Zhijie Hou (Fujitsu)
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>

> > IIUC on the standby we just want to overwrite what we get from primary no? If
> > so why we are using those APIs that are meant for the actual decoding slots
> > where it needs to take certain logical decisions instead of mere overwriting?
>
> I think we don't have a strong reason to use these APIs, but it was convenient to
> use these APIs as they can take care of updating the slots info and will call
> functions like, ReplicationSlotsComputeRequiredXmin,
> ReplicationSlotsComputeRequiredLSN internally. Or do you prefer directly overwriting
> the fields and call these manually ?

I might be missing something but do you want to call
ReplicationSlotsComputeRequiredXmin() kind of functions in standby? I
mean those will ultimately update the catalog xmin and replication
xmin in Procarray and that prevents Vacuum from cleaning up some of
the required xids. But on standby, those shared memory parameters are
not used IIUC.

In my opinion on standby, we just need to update the values in the
local slots and whatever we get from remote slots without taking all
the logical decisions in the hope that they will all fall into a
particular path, for example, if you see LogicalIncreaseXminForSlot(),
it is doing following steps of operations as shown below[1]. These
all make sense when you are doing candidate-based updation where we
first mark the candidates and then update the candidate to real value
once you get the confirmation for the LSN. Now following all this
logic looks completely weird unless this can fall in a different path
I feel it will do some duplicate steps as well. For example in
local_slot_update(), first you call LogicalConfirmReceivedLocation()
which will set the 'data.confirmed_flush' and then you will call
LogicalIncreaseXminForSlot() which will set the 'updated_xmin = true;'
and will again call LogicalConfirmReceivedLocation(). I don't think
this is the correct way of reusing the function unless you need to go
through those paths and I am missing something.

[1]
LogicalIncreaseXminForSlot()
{
if (TransactionIdPrecedesOrEquals(xmin, slot->data.catalog_xmin))
{
}
else if (current_lsn <= slot->data.confirmed_flush)
{
}
else if (slot->candidate_xmin_lsn == InvalidXLogRecPtr)
{
}

if (updated_xmin)
LogicalConfirmReceivedLocation(slot->data.confirmed_flush);
}

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com