Horton

Re: archive modules

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Nathan Bossart <nathandbossart@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, bharath.rupireddyforpostgres@gmail.com, tgl@sss.pgh.pa.us, peter.eisentraut@enterprisedb.com, benoit.lobreau@gmail.com, alvherre@alvh.no-ip.org, robertmhaas@gmail.com, bossartn@amazon.com, david@pgmasters.net, masao.fujii@oss.nttdata.com, pgsql-hackers@postgresql.org, magnus@hagander.net, sfrost@snowman.net
Date: 2022-11-07T06:20:31Z
Lists: pgsql-hackers 
On Sat, Nov 05, 2022 at 02:08:58PM -0700, Nathan Bossart wrote:
> Such a module could define a custom GUC that accepts a shell command.  I
> don't think we should overload the meaning of archive_command based on the
> whims of whatever archive module is loaded.  Besides the potential end-user
> confusion, your archive_command might be unexpectedly used incorrectly if
> you forget to set archive_library.

While mostly copying the logic from shell_archive.c to build the
command to execute (aka shell_archive_file), which is not great as
well.  But well, perhaps my whole line of argument is just moot..  

> Perhaps we could eventually move the archive_command functionality to a
> contrib module (i.e., "shell_archive") so that users must always set
> archive_library.  But until then, I suspect it's better to treat modules
> and commands as two separate interfaces to ease migration from older major
> versions (even though archive_command is now essentially a built-in archive
> module).

I agree that this is a fine long-term goal, removing all traces of the
archive_command from the backend core code.  This is actually an
argument in favor of having no traces of XLogArchiveCommand in
pgarch.c, no? ;p

I am not sure how long we should wait before being able to do that,
perhaps a couple of years of least?  I'd like to think the sooner the
better (like v17?) but we are usually conservative, and the removal of
the exclusive backup mode took 5~6 years if I recall correctly..
--
Michael

Commits

  1. Disallow setting archive_library and archive_command at the same time

  2. Restore archive_command documentation

  3. Doc: improve explanation of when custom GUCs appear in pg_settings.

  4. doc: Add note about re-archiving of same WAL files in docs.

  5. In basic_archive tests, insist on wal_level='replica'.

  6. Allow archiving via loadable modules.

  7. Move the code to archive files via the shell to a separate file.