Date: Tue, 29 Oct 2024 14:06:00 +0900 (JST)
Message-Id: <20241029.140600.1382327295267983604.ishii@postgresql.org>
To: peter@eisentraut.org
Cc: pgsql-hackers@postgresql.org
Subject: Re: protocol-level wait-for-LSN
From: Tatsuo Ishii <ishii@postgresql.org>
In-Reply-To: <8e6ba982-fd41-4779-8729-eb120df69ea0@eisentraut.org>
References: <8e6ba982-fd41-4779-8729-eb120df69ea0@eisentraut.org>
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> The patch adds a protocol extension called _pq_.wait_for_lsn as well
> as a libpq connection option wait_for_lsn to activate the same.  (Use
> e.g., psql -d 'wait_for_lsn=1'.)
> 
> With this protocol extension, two things are changed:
> 
> - The ReadyForQuery message sends back the current LSN.

If other protocol extension X tries to add something to the
ReadyForQuery message too, what would happen?
Currently ReadyForQuery message is like this:

Byte1('Z')
Int32
Byte1

With the wait_for_lsn extension, It becomes:

Byte1('Z')
Int32
Byte1
String

Suppose the X extension wants to extend like this:

Byte1('Z')
Int32
Byte1
Int32

It seems impossible to coexist both.

Does this mean once the wait_for_lsn extension is brought into the
frontend/backend protocol specification, no other extensions that touch
ReadyForQuery cannot be defined?

Best reagards,
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en/
Japanese:http://www.sraoss.co.jp