Re: what to do after a failover

On Wed, Jan 08, 2020 at 11:06:28PM -0500, Rita wrote:
> I run a master and standby setup with Postgresql 11. The systems are
> identical from a hardware and software setup. If the master goes down I
> can do a pg_ctl promote on the standby and point my applications to use the
> standby (new master).
>
> Once the original master is online, when is an appropriate time to fail
> back over? And are there any other things besides promote after the
> failover is done?

Make sure that you still have an HA configuration able to handle
multiple degrees of failures with always standbys available after a
promotion.

The options available to rebuild your HA configuration after a
failover depend on the version of PostgreSQL you are using. After a
failover the most simple solution would be to always recreate a new
standby from a base backup taken from the freshly-promoted primary,
though it can be costly depending on your instance. You could also
use pg_rewind (available in core since 9.5) to recycle the previous
primary and reuse it as a standby of the new promoted custer. Note
that there are community-based solutions for such things, like
pg_auto_failover or pacemaker-based stuff just to name two. These
rely on more complex architectures, where a third node is present to
monitor the others (any sane HA infra ought to do at least that to be
honest).
--
Michael