Re: Synchronizing slots from primary to standby

On Thu, Jan 18, 2024 at 4:49 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:

> 2.
> +synchronize_one_slot(WalReceiverConn *wrconn, RemoteSlot *remote_slot)
> {
> ...
> + LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
> + xmin_horizon = GetOldestSafeDecodingTransactionId(true);
> + SpinLockAcquire(&slot->mutex);
> + slot->data.catalog_xmin = xmin_horizon;
> + SpinLockRelease(&slot->mutex);
> ...
> }
>
> Here, why slot->effective_catalog_xmin is not updated? The same is
> required by a later call to ReplicationSlotsComputeRequiredXmin(). I
> see that the prior version v60-0002 has the corresponding change but
> it is missing in the latest version. Any reason?

I think it was a mistake in v61. Added it back in v64..

>
> 3.
> + * Return true either if the slot is marked as RS_PERSISTENT (sync-ready) or
> + * is synced periodically (if it was already sync-ready). Return false
> + * otherwise.
> + */
> +static bool
> +update_and_persist_slot(RemoteSlot *remote_slot)
>
> The second part of the above comment (or is synced periodically (if it
> was already sync-ready)) is not clear to me. Does it intend to
> describe the case when we try to update the already created temp slot
> in the last call. If so, that is not very clear because periodically
> sounds like it can be due to repeated sync for sync-ready slot.

The comment was as per old functionality where this function was doing
persist and save both. In v61 code changed, but comment was not
updated. I have changed it now in v64.

> 4.
> +update_and_persist_slot(RemoteSlot *remote_slot)
> {
> ...
> + (void) local_slot_update(remote_slot);
> ...
> }
>
> Can we write a comment to state the reason why we don't care about the
> return value here?

Since it is the first time 'local_slot_update' is happening on any
slot, the return value must be true i.e. local_slot_update() should
not skip the update. I have thus added an Assert on return value now
(in v64).

thanks
Shveta