Horton

Re: archive modules

Nathan Bossart <bossartn@amazon.com>

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From: "Bossart, Nathan" <bossartn@amazon.com>
To: Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael@paquier.xyz>
Cc: Fujii Masao <masao.fujii@oss.nttdata.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, Stephen Frost <sfrost@snowman.net>, "Magnus Hagander" <magnus@hagander.net>
Date: 2021-11-02T15:26:04Z
Lists: pgsql-hackers 
On 11/2/21, 8:11 AM, "Robert Haas" <robertmhaas@gmail.com> wrote:
> Why in the world would you want a plain hook rather than something
> closer to the way logical replication works?
>
> Plain hooks are annoying to use. If you load things at the wrong time,
> it silently doesn't work. It's also impossible to unload anything. If
> you want to change to a different module, you probably have to bounce
> the whole server instead of just changing the GUC and letting it load
> the new module when you run 'pg_ctl reload'.

Well, the current patch does require a reload since the modules are
preloaded, but maybe there is some way to avoid that.  However, I
agree with the general argument that a dedicated archive module
interface will be easier to use correctly.  With a hook, it could be
hard to know which library's archive function we are actually using.
With archive_library, we know the exact library's callback functions
we are using.

I think an argument for just adding a hook is that it's simpler and
easier to maintain.  But continuous archiving is a pretty critical
piece of functionality, so I think it's a great candidate for some
sturdy infrastructure.

Nathan

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.