Subject: Cascading Replication

From: Ilian Kostadinov <[email-masked]>
Date: Tue, 20 Aug 2024 14:55:16 +0300

Hello,

currently we have postgresql 16 with streaming replication. Is it possible
to create logical replication of the streaming replica?
So I have server A which is main database server with application connected
to it.
Also server B which is streaming replication of server A.  I succeded to
created server C
which is streaming replica of server B, but I want to convert it to logical
replica of server B.
Database is more then 50TB, so how to convert server C to logical replica
without need to sync all data from the beginning?

Best Regards,
Iliyan

---

From: Asad Ali <[email-masked]>
Date: Thu, 22 Aug 2024 16:22:16 +0500

Hello *Ilian Kostadinov,*

Here’s how you can achieve this without needing to sync all data from the
beginning:

Make sure logical replication is enabled on Server B. In your
postgresql.conf, set the following parameters on Server B:

*wal_level = logicalmax_replication_slots = <desired number>max_wal_senders
= <desired number>*
Restart PostgreSQL on Server B for these changes to take effect.

On Server B, create a publication for the tables you want to replicate:

*CREATE PUBLICATION my_publication FOR ALL TABLES;*
If you need specific tables, adjust the query accordingly.

Create a logical replication slot on Server B. This slot will be used by
Server C to stream changes:

*SELECT * FROM pg_create_logical_replication_slot('my_slot', 'pgoutput');*
On Server C, create a subscription to the publication on Server B:

On Server C, create a subscription to the publication on Server B:

*CREATE SUBSCRIPTION my_subscription CONNECTION 'host=<Server B IP>
dbname=<dbname> user=<replication_user> password=<password>'PUBLICATION
my_publicationWITH (copy_data = false);*
The WITH (copy_data = false) clause is crucial because it prevents Server C
from copying all the data from scratch, which would be inefficient given
your 50TB database size. Instead, it will start replicating changes from
the point when the subscription is created.

Since you're skipping the initial data copy, you need to ensure that the
data on Server C is in sync with Server B. If Server C was already a
streaming replica of Server B, this should already be the case.
However, if any discrepancies exist, you may need to manually sync specific
tables or sequences.

Best Regards,
Asad Ali

On Tue, Aug 20, 2024 at 4:55 PM Ilian Kostadinov <[email-masked]>
wrote:

> So I have server A which is main database server with application
> connected to it.
> which is streaming replica of server B, but I want to convert it to
> logical replica of server B.

---

From: rams nalabolu <[email-masked]>
Date: Thu, 22 Aug 2024 07:53:08 -0500

Along with Asad steps you need to add one extra parameter
hot_standby_feedback=on on Standby server(B)

And while creating subscriptions on server C using server B connection
information it will be on halt state and won’t finish fast. This will
happen while server B is waiting to sync the data from Server A. In this
case you need to refresh the replication state between server A and Server
B using “select pg_log_standby_snapshot()” and there after the server B
will allow the replication commands to be executed and subscription
creation will be finished.

Thanks
Veeru

On Thu, Aug 22, 2024 at 6:22 AM Asad Ali <[email-masked]> wrote:

> *wal_level = logicalmax_replication_slots = <desired
> number>max_wal_senders = <desired number>*
> The WITH (copy_data = false) clause is crucial because it prevents Server
> C from copying all the data from scratch, which would be inefficient given
> However, if any discrepancies exist, you may need to manually sync
> specific tables or sequences.
>
> Best Regards,
> Asad Ali
>
> On Tue, Aug 20, 2024 at 4:55 PM Ilian Kostadinov <
> [email-masked]> wrote:
>
>> Hello,
>>