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

On Wed, Sep 4, 2024 at 9:17 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Tue, Sep 3, 2024 at 3:01 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
> >
> >

1)
It is related to one of my previous comments (pt 3 in [1]) where I
stated that inactive_since should not keep on changing once a slot is
invalidated.
Below is one side effect if inactive_since keeps on changing:

postgres=# SELECT * FROM pg_replication_slot_advance('mysubnew1_1',
pg_current_wal_lsn());
ERROR: can no longer get changes from replication slot "mysubnew1_1"
DETAIL: The slot became invalid because it was inactive since
2024-09-04 10:03:56.68053+05:30, which is more than 10 seconds ago.
HINT: You might need to increase "replication_slot_inactive_timeout.".

postgres=# select now();
now
---------------------------------
2024-09-04 10:04:00.26564+05:30

'DETAIL' gives wrong information, we are not past 10-seconds. This is
because inactive_since got updated even in ERROR scenario.

2)
One more issue in this message is, once I set
replication_slot_inactive_timeout to a bigger value, it becomes more
misleading. This is because invalidation was done in the past using
previous value while message starts showing new value:

ALTER SYSTEM SET replication_slot_inactive_timeout TO '36h';

--see 129600 secs in DETAIL and the current time.
postgres=# SELECT * FROM pg_replication_slot_advance('mysubnew1_1',
pg_current_wal_lsn());
ERROR: can no longer get changes from replication slot "mysubnew1_1"
DETAIL: The slot became invalid because it was inactive since
2024-09-04 10:06:38.980939+05:30, which is more than 129600 seconds
ago.
postgres=# select now();
now
----------------------------------
2024-09-04 10:07:35.201894+05:30

I feel we should change this message itself.

~~~~~

When invalidation is due to wal_removed, we get a way simpler message:

newdb1=# SELECT * FROM pg_replication_slot_advance('mysubnew1_2',
pg_current_wal_lsn());
ERROR: replication slot "mysubnew1_2" cannot be advanced
DETAIL: This slot has never previously reserved WAL, or it has been
invalidated.

This message does not mention 'max_slot_wal_keep_size'. We should have
a similar message for our case. Thoughts?

[1]: https://www.postgresql.org/message-id/CAJpy0uC8Dg-0JS3NRUwVUemgz5Ar2v3_EQQFXyAigWSEQ8U47Q%40mail.gmail.com

thanks
Shveta