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

From: Michael Jaskiewicz <[email-masked]>
Date: Thu, 29 Aug 2024 01:58:06 +0000

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

---

From: Shaheed Haque <[email-masked]>
Date: Sun, 1 Sep 2024 17:22:01 +0100

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, <[email-masked]> wrote:

>    - 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
>

---

From: Muhammad Ikram <[email-masked]>
Date: Mon, 2 Sep 2024 11:08:20 +0500

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.

 Regards,
Muhammad Ikram

On Sun, Sep 1, 2024 at 9:22 PM Shaheed Haque <[email-masked]> wrote:

> 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.
>
>>    - The publisher says write/flush/replay lags are all 13 minutes
>>    behind but it's been like that for most of the day.
>> 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

---

From: Shaheed Haque <[email-masked]>
Date: Mon, 2 Sep 2024 08:42:05 +0100

Hi Muhammad,

> 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.

---

From: Muhammad Ikram <[email-masked]>
Date: Mon, 2 Sep 2024 13:45:16 +0500

Hi Shaheed,
I think you must have already analyzed the outcome of queries
on pg_replication_slots,  pg_current_wal_lsn(), pg_stat_subscription etc. I
could find a query SELECT
pg_size_pretty(pg_wal_lsn_diff('<publisher_restart_lsn>',
'<subscriber_replayed_lsn>'));

As a side note if you want to see what has been applied to subscribers vs
what exists on publisher then here is something from my previous
experience. We used to have a Data Validation tool for checking tables/rows
across publisher/subscriber. We also used pg_dump for another tool that was
meant for making copies of schemas.

Regards,
Muhammad Ikram

On Mon, Sep 2, 2024 at 12:42 PM Shaheed Haque <[email-masked]>
wrote:

>> 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
> 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,
> 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
>>>>    - I look at the subscriber via `select * from
>>>>    pg_stat_subscription;` and see that latest_end_lsn is advancing albeit very
>>>>    slowly.

-- 
Muhammad Ikram