[pgpool-general: 7474] Promote specific backend to primary in streaming replication

[Nathan Ward]

Hi all,

I'm busy upgrading to postgres 13 and pgpool 4.1.4, and as part of that
reworking some of our scripts and improving some of our operational
procedures.

I'd like to have a way to promote a specific backend to primary, and have
the old primary use pg_rewind, then connect to the new primary.

I'm using scripts largely based on the samples, though without
pg_basebackup as a fallback to pg_rewind - our thinking is if pg_rewind
fails, we want to leave it in the current state and investigate, then
restore manually.

I can run pcp_detach_node to almost achieve this controlled primary
failover, but it doesn't let me specify which primary to fail over to.
pcp_promote_node explicitly says it's only about internal pgpool state, and
it runs follow_master_command, but not failover_command.

Is there a way to achieve this in pgpool at the moment? If not, could we
put together a new feature / command to achieve this? (Happy to contribute
code, if this is something that would be useful).
Thinking:
1) Detach old primary
2) Run failover_command on the new primary
3) Run follow_master_command on the standbys (including the old primary,
which gets reattached by the script)

Perhaps this is an extra flag on pcp_promote_node which causes
failover_command to run?

--
Nathan Ward
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