Thread: BUG #17327: Postgres server does not correctly emit error for max_slot_wal_keep_size being breached

The following bug has been logged on the website:

Bug reference:      17327
Logged by:          Alex E
Email address:      alex@altmetric.com
PostgreSQL version: 13.5
Operating system:   Ubuntu 18.04
Description:

We have recently run into a situation where our pg_basebackup-based backups
started failing unexpectedly. These use WAL streaming to keep up with
changes (which uses a temporary replication slot server side). 

The only errors logged on the client side were as listed below:

pg_basebackup: error: could not receive data from WAL stream: SSL connection
has been closed unexpectedly
pg_basebackup: error: could not read COPY data: server closed the connection
unexpectedly
    This probably means the server terminated abnormally
    before or while processing the request.
pg_basebackup: removing contents of data directory "/backups/some/path/"

whilst on the server side we only got:

2021-12-03 16:21:54 UTC [29724-2647] LOG:  terminating process 42601 to
release replication slot "pg_basebackup_42601"
2021-12-03 16:21:54 UTC [42601-1] replicator@[unknown] FATAL:  terminating
connection due to administrator command
2021-12-03 16:21:54 UTC [42601-2] replicator@[unknown] STATEMENT:
START_REPLICATION SLOT "pg_basebackup_42601" 4721F/45000000 TIMELINE 3

The above was very unhelpful as it made us believe we might be dealing with
either a network interruption or another type of mysterious hardware
error.

We then proceeded to try several things to try and determine the root cause
of the problem and eventually realized (by trial and error and monitoring
various statistics) that we were breaching our max_slot_wal_keep_size limit
for the temporary replication slot whilst taking the pg_basebackup. The only
way we realized this was by using a permanent physical replication slot to
take the backup instead of a temporary one, and when doing this a relevant
error related to max_slot_wal_keep_size being breached was issued.

The core issue here then in our opinion is that Postgres server should log
an error when the max_slot_wal_keep_size limit is reached for temporary
replication slots as well as for permanent ones as otherwise
users/administrators are presented only with non-descript connection
termination errors which do not point to the actual cause of the problem.

On Wed, Dec 8, 2021 at 9:58 PM PG Bug reporting form
<noreply@postgresql.org> wrote:
>
> The following bug has been logged on the website:
>
> Bug reference:      17327
> Logged by:          Alex E
> Email address:      alex@altmetric.com
> PostgreSQL version: 13.5
> Operating system:   Ubuntu 18.04
> Description:
>
> We have recently run into a situation where our pg_basebackup-based backups
> started failing unexpectedly. These use WAL streaming to keep up with
> changes (which uses a temporary replication slot server side).
>
> The only errors logged on the client side were as listed below:
>
> pg_basebackup: error: could not receive data from WAL stream: SSL connection
> has been closed unexpectedly
> pg_basebackup: error: could not read COPY data: server closed the connection
> unexpectedly
>         This probably means the server terminated abnormally
>         before or while processing the request.
> pg_basebackup: removing contents of data directory "/backups/some/path/"
>
> whilst on the server side we only got:
>
> 2021-12-03 16:21:54 UTC [29724-2647] LOG:  terminating process 42601 to
> release replication slot "pg_basebackup_42601"
> 2021-12-03 16:21:54 UTC [42601-1] replicator@[unknown] FATAL:  terminating
> connection due to administrator command
> 2021-12-03 16:21:54 UTC [42601-2] replicator@[unknown] STATEMENT:
> START_REPLICATION SLOT "pg_basebackup_42601" 4721F/45000000 TIMELINE 3
>
> The above was very unhelpful as it made us believe we might be dealing with
> either a network interruption or another type of mysterious hardware
> error.
>
> We then proceeded to try several things to try and determine the root cause
> of the problem and eventually realized (by trial and error and monitoring
> various statistics) that we were breaching our max_slot_wal_keep_size limit
> for the temporary replication slot whilst taking the pg_basebackup. The only
> way we realized this was by using a permanent physical replication slot to
> take the backup instead of a temporary one, and when doing this a relevant
> error related to max_slot_wal_keep_size being breached was issued.
>
> The core issue here then in our opinion is that Postgres server should log
> an error when the max_slot_wal_keep_size limit is reached for temporary
> replication slots as well as for permanent ones as otherwise
> users/administrators are presented only with non-descript connection
> termination errors which do not point to the actual cause of the problem.

IIUC the LOG message you got, "terminating process 42601 to release
replication slot "pg_basebackup_42601"", is the message you want. That
is, it's emitted when the replication slot reached
max_slot_wal_keep_size and was invalidated. I don't think such
behavior varies depending on persistent slots and temporary slots. If
the message "terminating process XXXX to release replication slot
"YYY"" is not clear to indicate that the replication slot is
invalidated, probably we can improve the message or add details.

Regards,

-- 
Masahiko Sawada
EDB:  https://www.enterprisedb.com/

At Fri, 10 Dec 2021 15:46:11 +0000, Alex Enachioaie <alex@altmetric.com> wrote in 
> So, essentially the server side log emmitted on a temporary
> replication breaching max_slot_wal_keep_size limit is only:
> 
> 2021-12-03 16:21:54 UTC [29724-2647] LOG: terminating process 42601 to
> release replication slot "pg_basebackup_42601"
> 
> whereas for a persistent replication slot we get an additional line
> that clearly states _why_ the replication process was terminated:
> 
> 2021-12-03 00:57:16 UTC [29724-2645] LOG: terminating process 3899 to
> release replication slot "backup"
> 2021-12-03 00:57:16 UTC [29724-2646] LOG: invalidating slot "backup"
> because its restart_lsn 47198/1E000000 exceeds max_slot_wal_keep_size
> 
> I'm not sure if this means that in the case of a temporary slot it
> does not get invalidated at all (I've not looked at the code), or it's
> simply that we don't emit a log message when it does because the slot
> would be discarded anyway, but such a message would be very useful for
> diagnostic purposes imo.

The "invalidating slot" message is emitted when the slot needs to be
invalidated, that is, when the slot persists after the user process is
terminated.  Thus the message cannot be seen for temporary slots since
they are removed at process termination and no longer exist after
that.

At Wed, 08 Dec 2021 11:23:35 +0000, PG Bug reporting form <noreply@postgresql.org> wrote in 
> The core issue here then in our opinion is that Postgres server should log
> an error when the max_slot_wal_keep_size limit is reached for temporary
> replication slots as well as for permanent ones as otherwise
> users/administrators are presented only with non-descript connection
> termination errors which do not point to the actual cause of the problem.

If you mean the "invalidating slot" message by "an error", that
wouldn't happen since invalidation is actually doesn't happen.  Or, we
could change the message like this.  Does this make sense for you?

> LOG:  terminating process 42601 to release temporary replication slot "pg_basebackup_42601"
> DETAIL:  The slot will be dropped by the process termination.


> LOG:  terminating process 3899 to release persistent replication slot "backup" 
...
> LOG:  invalidating slot "backup" because its restart_lsn 47198/1E000000 exceeds max_slot_wal_keep_size

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Hello, Alex.

At Mon, 13 Dec 2021 09:45:51 +0000, Alex Enachioaie <alex@altmetric.com> wrote in 
> I think I'm happy to leave the method of resolution up to you, I think
> the main point for me would be that when a
> replication process gets terminated as a consequence of the underlying
> temporary replication slot reaching max_slot_wal_keep_size
> that we log a specific message to indicate to the user the cause of
> the termination rather than leave it ambiguous.

Ah.. I think I got your point.  I thought that the message
"terminating process to release replication slot X" is showing the
cause for process termination but actually it doesn't mention the
reason for releasing replication slot. For persistent slots the
following message "invalidating slot" does that but that message is
absent for temporary slots.  However, in that sense, the root reason
is not clearly stated in any case.

So I'm going to propose something like the following.

  LOG:  terminating process nnnnn to release replication slot "slot"
+ DETAIL:  The WAL files retained by the slot is going to exceed max_slot_wal_keep_size.

Thanks!

-- 
Kyotaro Horiguchi
NTT Open Source Software Center