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From: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Date: Mon, 29 Nov 2021 09:39:59 +0530
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Subject: Re: Synchronizing slots from primary to standby
To: SATYANARAYANA NARLAPURAM <satyanarlapuram@gmail.com>
Cc: Peter Eisentraut <peter.eisentraut@enterprisedb.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
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On Mon, Nov 29, 2021 at 1:48 AM SATYANARAYANA NARLAPURAM
<satyanarlapuram@gmail.com> wrote:
>
>> 3) Instead of the subscriber pulling the slot info, why can't the
>> publisher (via the walsender or a new bg worker maybe?) push the
>> latest slot info? I'm not sure we want to add more functionality to
>> the walsender, if yes, isn't it going to be much simpler?
>
> Standby pulling the information or at least making a first attempt to con=
nect to the  primary is a better design as primary doesn't need to spend it=
s cycles repeatedly connecting to an unreachable standby. In fact, primary =
wouldn't even need to know the followers, for example followers / log shipp=
ing standbys

My idea was to let the existing walsender from the primary/publisher
to send the slot info (both logical and physical replication slots) to
the standby/subscriber, probably by piggybacking the slot info with
the WAL currently it sends. Having said that, I don't know the
feasibility of it. Anyways, I'm not in favour of having a new bg
worker to just ship the slot info. The standby/subscriber, while
making connection to primary/publisher, can choose to get the
replication slot info.

As I said upthread, the problem I see with standby/subscriber pulling
the info is that: how frequently the standby/subscriber is going to
sync the slot info from primary/publisher? How can it ensure that the
latest information exists say when the subscriber is down/crashed
before it picks up the latest slot information?

IIUC, the initial idea proposed in this patch deals with only logical
replication slots not the physical replication slots, what I'm
thinking is to have a generic way to deal with both of them.

Note: In the above description, I used primary-standby and
publisher-subscriber to represent the physical and logical replication
slots respectively.

Regards,
Bharath Rupireddy.