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:
- psql -h $CurrentPrimary -c "ALTER SYSTEM SET synchronous_standby_names TO '*';"
- Wait a few seconds.
- ssh $CurrentPrimary sudo ip del $VIP # cmd is more complicated, but you get the idea
- ssh $CurrentPrimary pg_ctl stop -mfast # to kill connections, has to happen, no matter the solution.
- pg_ctl promote
- sudo ip add $VIP
- Replicate from new-primary to new-replica "at leisure".
No retry delay, since the application directly goes to the new server.
Steps 3-6 are in a script, and what pgpool does, except I do it. #4 is by far the slowest. ssh authentication delay in #3 and #4 are nonexistent if you have "pre-created" an ssh socket.
-- Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
