Re: [PATCH] Add archive_mode=follow_primary to prevent unarchived WAL on standby promotion

On Fri, Oct 24, 2025 at 1:25 AM Andrey Borodin <x4mmm(at)yandex-team(dot)ru> wrote:
>
> Hi hackers,
>
> I'd like to propose a new archive_mode setting to address a gap in WAL
> archiving for high availability streaming replication configurations.
>
> ## Problem
>
> In HA setups using streaming replication, standbys can be
> promoted when primary has failed. Some WAL segments might be not yet
> archived. This creates gaps in the WAL archive, breaking point-in-time
> recovery:
>
> 1. Primary generates WAL, streams to standby
> 2. Standby receives WAL, marks segments as .done immediately
> 3. Standby deletes WAL during checkpoints
> 4. Primary hasn't archived yet (archiver lag, network issues, etc.)
> 5. Primary vanishes
> 6. Standby gets promoted
> 7. WAL history lost from archive
>
> This is particularly problematic in synchronous replication where
> promotion might happen while the primary is still catching up on archival.
>
> Promoted standby might have some WALs from walreceiver, some from archive. In
> this case we need to archive only those WALs which were received, but not
> confirmed to be archived by primary.
>
> ## Proposed Solution
>
> Add archive_mode=follow_primary, where standbys defer WAL deletion until
> the primary confirms archival:

Can't we achieve nearly the same behavior by setting archive_mode to
always and configuring archive_command on the standby to check
whether the WAL file already exists in the shared archive area
(e.g., test -f <archive directory>/%f (probably also the WAL file size
should be checked))? In this setup, archive_command would fail
until the WAL file appears in the archive, preventing the standby
from removing it while the command is failing.

Regards,

--
Fujii Masao