Re: Postgres Logical Replication - how to see what subscriber is doing with received data?

Hi Muhammad,

On Mon, 2 Sep 2024, 07:08 Muhammad Ikram, <mmikram(at)gmail(dot)com> wrote:

> Hi Shaheed,
>
> Maybe these considerations could help you or give any hint to the problem ?
>
>
> Check if wal_receiver_timeout being set to 0 could potentially cause
> issues, like not detecting network issues quickly enough. Consider
> re-evaluating this setting if you see connection issues.
>
> If you notice that some data is missing on subscriber then could you
> increase max_slot_wal_keep_size on publisher so that WALs are not deleted
> until they are applied on subscriber.
>
> Do you have flexibility to increase max_worker_processes and
> max_logical_replication_workers, work_mem and maintenance_work_mem on
> subscriber (In case bottleneck exists on subscriber)
>
> If there's significant lag, consider whether it might be more efficient to
> drop the subscription and re-initialize it from scratch using a new base
> backup, depending on the data volume and how long it might take for the
> existing replication to catch up.
>

Thanks for the kind hints, I'll certainly look into those.

My main interest however was with the "visibility" question, i.e. to get an
understanding of the gap between the two ends of a replication slot,
ideally in human terms (e.g. tables x records).

I understand the difficulties of trying to produce a meaningful metric that
spans two (or more) systems but let's be honest, trying to diagnose which
knobs to tweak (whether in application, PG, the OS or the network) is
basically black magic when all we really have is a pair of opaque LSNs.

>
> Regards,
> Muhammad Ikram
>
>
> On Sun, Sep 1, 2024 at 9:22 PM Shaheed Haque <shaheedhaque(at)gmail(dot)com>
> wrote:
>
>> Since nobody more knowledgeable has replied...
>>
>> I'm very interested in this area and still surprised that there is no
>> official/convenient/standard way to approach this (see
>> https://www.postgresql.org/message-id/CAHAc2jdAHvp7tFZBP37awcth%3DT3h5WXCN9KjZOvuTNJaAAC_hg%40mail.gmail.com
>> ).
>>
>> Based partly on that thread, I ended up with a script that connects to
>> both ends of the replication, and basically loops while comparing the
>> counts in each table.
>>
>> On Fri, 30 Aug 2024, 12:38 Michael Jaskiewicz, <mjaskiewicz(at)ghx(dot)com>
>> wrote:
>>
>>> I've got two Postgres 13 databases on AWS RDS.
>>>
>>> - One is a master, the other a slave using logical replication.
>>> - Replication has fallen behind by about 350Gb.
>>> - The slave was maxed out in terms of CPU for the past four days
>>> because of some jobs that were ongoing so I'm not sure what logical
>>> replication was able to replicate during that time.
>>> - I killed those jobs and now CPU on the master and slave are both
>>> low.
>>> - I look at the subscriber via `select * from pg_stat_subscription;`
>>> and see that latest_end_lsn is advancing albeit very slowly.
>>> - The publisher says write/flush/replay lags are all 13 minutes
>>> behind but it's been like that for most of the day.
>>> - I see no errors in the logs on either the publisher or subscriber
>>> outside of some simple SQL errors that users have been making.
>>> - CloudWatch reports low CPU utilization, low I/O, and low network.
>>>
>>>
>>>
>>> Is there anything I can do here? Previously I set wal_receiver_timeout
>>> timeout to 0 because I had replication issues, and that helped things. I
>>> wish I had *some* visibility here to get any kind of confidence that
>>> it's going to pull through, but other than these lsn values and database
>>> logs, I'm not sure what to check.
>>>
>>>
>>>
>>> Sincerely,
>>>
>>> mj
>>>
>>
>
> --
> Muhammad Ikram
>
>