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Re: [Proposal] Adding Log File Capability to pg_createsubscriber

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Euler Taveira <euler@eulerto.com>
Cc: vignesh C <vignesh21@gmail.com>, "kuroda.hayato@fujitsu.com" <kuroda.hayato@fujitsu.com>, Gyan Sreejith <gyan.sreejith@gmail.com>, "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>, Peter Smith <smithpb2250@gmail.com>
Date: 2025-12-18T06:49:02Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. pg_createsubscriber: Don't use MAXPGPATH

  2. pg_createsubscriber: Remove separate logfile_open() function

  3. pg_createsubscriber: Use logging.c log file callback

  4. Add log file support to logging.c

  5. pg_createsubscriber: Add -l/--logdir option to redirect output to files.

  6. pg_createsubscriber: Introduce module-specific logging functions.

On Thu, Dec 18, 2025 at 6:59 AM Euler Taveira <euler@eulerto.com> wrote:
>
> On Wed, Dec 17, 2025, at 7:07 AM, vignesh C wrote:
> >
> > By providing this as an option, users can store the log files outside
> > the data directory, eliminating the need for any additional handling
> > during backups.
> >
>
> Do we really need an option to capture the stdout / stderr output to a file? I
> doubt it. There is already various ways to capture. psql and pg_upgrade are the
> only tools that have this option.
>

pg_ctl also has the -l option. I think any place where long
text/errors can be outputted, a log file is preferred because one
could later parse it to know the exact details. Also, splitting the
log as proposed here or in pg_upgrade helps to navigate the LOG like
is the problem in start/stop of the server or a pub-sub setup?
Similarly the log can be splitted for pub/sub specific information.
There appears to be some useful information like:

pg_createsubscriber: warning: two_phase option will not be enabled for
replication slots
pg_createsubscriber: detail: Subscriptions will be created with the
two_phase option disabled. Prepared transactions will be replicated at
COMMIT PREPARED.
pg_createsubscriber: hint: You can use the command-line option
--enable-two-phase to enable two_phase.

I think it will be useful to LOG this separately from the main LOG [1]
(which can contain server specific info as follows) so that users can
consider running pg_createsubscriber with additional options or
changing the subscriber configuration once setup is complete.

[1]:
[startup] LOG:  database system was interrupted; last known up at
2025-12-17 14:46:07 IST
[startup] LOG:  starting backup recovery with redo LSN 0/06000028,
checkpoint LSN 0/06000080, on timeline ID 1
[startup] LOG:  entering standby mode
[startup] LOG:  redo starts at 0/06000028
[startup] LOG:  completed backup recovery with redo LSN 0/06000028 and
end LSN 0/06000120
[startup] LOG:  consistent recovery state reached at 0/06000120

-- 
With Regards,
Amit Kapila.