[pgpool-general: 7526] Re: Strange behavior on switchover with detach_false_primary enabled

Tatsuo Ishii ishii at sraoss.co.jp
Tue Apr 27 11:30:46 JST 2021

> Recently we've enabled the detach_false_primary option to prevent
> pgpool from picking the incorrect primary database in some situations.
> However, from the moment we've enabled this option, we are seeing
> erratic behavior during a switchover. We start with the following
> sitation:
> - 3 backends and 3 pgpool nodes
> - node 0 is primary database
> - node 0 is pgpool leader
> We first stop and restart pgpool on node 0, causing node 1 to become
> the leader. Next, we detach the primary database node with
> pcp_detach_node. This starts a failover, but during this failover,
> things happen that I cannot explain, causing the switchover to fail
> miserably. It looks like pgpool starts a second failover while it is
> still execution the first.
> I've attached the log from the new pgpool leader (at node 1). This is
> what I get from the logs:
> 04:03:29: pgpool stops at node 0 and node 1 is elected as the new leader
> 04:03:31: pgpool at node 0 rejoins
> 04:03:33: the detach_node command is received from (and for) node 0
> 04:03:33: failover starts, node 1 is indicated as the new primary
> 04:03:34: node 1 is promoted
> 04:03:34: follow_primary is executed for node 0 to follow node 1
> And here things go strange:
> 04:03:34: 2 additional failovers are started, both for node 0 and 1,
> but no new primary is selected
> 04:03:41: the follow_primary started at 03:34 is completed
> 04:03:41: a new follow_primary is started instructing node 1 to follow
> node 1. This breaks badly, because node 1 now tries to pg_rewind and
> pg_basebackup from itself.
> 04:03:57: we end with only node 0, running in standby.
> We would really appreciate it if we could get some help debugging this
> issue. If you need more information, please let me know.

Thanks for the report. It looks like a race condition between
detach_false_primary and follow_primary command. In your test there are
two primary nodes after pcp_detach_node command is executed and
detach_false_primary happily decided to detach one of them.

I think what we really need here is, let detach_false_primary
postpone until all follow_primary commands completed. Let me think
more on this.

In the meantime you can work around this by temporarily disabling
detach_false_primary until all follow primary commands completed. The
parameter can be changed by reloading pgpool.conf.

Best regards,
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php

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