Re: Introduce XID age and inactive timeout based replication slot invalidation

On Mon, Sep 16, 2024 at 10:41 PM Bharath Rupireddy
<bharath.rupireddyforpostgres@gmail.com> wrote:
>
> Thanks for looking into this.
>
> On Mon, Sep 16, 2024 at 4:54 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > Why raise the ERROR just for timeout invalidation here and why not if
> > the slot is invalidated for other reasons? This raises the question of
> > what happens before this patch if the invalid slot is used from places
> > where we call ReplicationSlotAcquire(). I did a brief code analysis
> > and found that for StartLogicalReplication(), even if the error won't
> > occur in ReplicationSlotAcquire(), it would have been caught in
> > CreateDecodingContext(). I think that is where we should also add this
> > new error. Similarly, pg_logical_slot_get_changes_guts() and other
> > logical replication functions should be calling
> > CreateDecodingContext() which can raise the new ERROR. I am not sure
> > about how the invalid slots are handled during physical replication,
> > please check the behavior of that before this patch.
>
> When physical slots are invalidated due to wal_removed reason, the failure happens at a much later point for the
streamingstandbys while reading the requested WAL files like the following: 
>
> 2024-09-16 16:29:52.416 UTC [876059] FATAL:  could not receive data from WAL stream: ERROR:  requested WAL segment
000000010000000000000005has already been removed 
> 2024-09-16 16:29:52.416 UTC [872418] LOG:  waiting for WAL to become available at 0/5002000
>
> At this point, despite the slot being invalidated, its wal_status can still come back to 'unreserved' even from
'lost',and the standby can catch up if removed WAL files are copied either by manually or by a tool/script to the
primary'spg_wal directory. IOW, the physical slots invalidated due to wal_removed are *somehow* recoverable unlike the
logicalslots. 
>
> IIUC, the invalidation of a slot implies that it is not guaranteed to hold any resources like WAL and XMINs. Does it
alsoimply that the slot must be unusable? 
>

If we can't hold the dead rows against xmin of the invalid slot, then
how can we make it usable even after copying the required WAL?

--
With Regards,
Amit Kapila.