RE: Fast switchover

From: Ron Johnson <ronljohnsonjr@gmail.com>
Sent: Monday, September 8, 2025 6:10 PM
To: pgsql-general@lists.postgresql.org
Subject: Re: Fast switchover

On Mon, Sep 8, 2025 at 11:03 AM legrand legrand <legrand_legrand@hotmail.com> wrote:

Hello all the readers,

 

For some projects we need a fast manual switchover to address Near Zero downtime maintenance

(not speaking here about automated failover like those provided by HA tools, but just planned, controlled operations)

 

 

Database Physical replication switchover itself:

- initial replication (before switchover) should be synchronous or replication LAG should be controlled to prevent data loss.

- Switchover duration seems not "compressible" under a few seconds (because of primary shutdown, promotion, new standby catch up, ...)

- Application retry strategy (after disconnection) should be tuned using proper retry delay. Pooler or specific driver may help.

There will always be a few seconds delay while the applications reconnect.

Do the applications connect via a VIP?  That's simpler for the application.

This is what I do from the not-yet-new-primary:

1. psql -h  $CurrentPrimary -c "ALTER SYSTEM SET synchronous_standby_names TO '*';"
2. Wait a few seconds.
3. ssh $CurrentPrimary sudo ip del $VIP # cmd is more complicated, but you get the idea
4. ssh $CurrentPrimary pg_ctl stop -mfast # to kill connections, has to happen, no matter the solution.