pg_upgrade and logical replication

From: Julien Rouhaud <rjuju123(at)gmail(dot)com>
To: pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: pg_upgrade and logical replication
Date: 2023-02-17 07:54:33
Message-ID: 20230217075433.u5mjly4d5cr4hcfe@jrouhaud
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I was working on testing a major upgrade scenario using a mix of physical and
logical replication when I faced some unexpected problem leading to missing
rows. Note that my motivation is to rely on physical replication / physical
backup to avoid recreating a node from scratch using logical replication, as
the initial sync with logical replication is much more costly and impacting
compared to pg_basebackup / restoring a physical backup, but the same problem
exist if you just pg_upgrade a node that has subscriptions.

The problem is that pg_upgrade creates the subscriptions on the newly upgraded
node using "WITH (connect = false)", which seems expected as you obviously
don't want to try to connect to the publisher at that point. But then once the
newly upgraded node is restarted and ready to replace the previous one, unless
I'm missing something there's absolutely no possibility to use the created
subscriptions without losing some data from the publisher.

The reason is that the subscription doesn't have a local list of relation to
process until you refresh the subscription, but you can't refresh the
subscription without enabling it (and you can't enable it in a transaction),
which means that you have to let the logical worker start, consume and ignore
all changes that happened on the publisher side until the refresh happens.

An easy workaround that I tried is to allow something like

ALTER SUBSCRIPTION ... ENABLE WITH (refresh = true, copy_data = false)

so that the refresh internally happens before the apply worker is started and
you just keep consuming the delta, which works on naive scenario.

One concern I have with this approach is that the default values for both
"refresh" and "copy_data" for all other subcommands is "true, but we would
probably need a different default value in that exact scenario (as we know we
already have the data). I think that it would otherwise be safe in my very
specific scenario, assuming that you created the slot beforehand and moved the
slot's LSN at the promotion point, as even if you add non-empty tables to the
publication you will only need the delta whether those were initially empty or
not given your initial physical replica state. Any other scenario would make
this new option dangerous, if not entirely useless, but not more than any of
the current commands that lead to refreshing a subscription and have the same
options I guess.

All in all, currently the only way to somewhat safely resume logical
replication after a pg_upgrade is to drop all the subscriptions that were
transferred during pg_upgrade on all databases and recreate them (using the
existing slots on the publisher side obviously), allowing the initial
connection. But this approach only works in the exact scenario I mentioned
(physical to logical replication, or at least a case where *all* the tables
where logically replicated prior to the pg_ugprade), otherwise you have to
recreate the follower node from scratch using logical repication.

Is that indeed the current behavior, or did I miss something?

Is this "resume logical replication on pg_upgraded node" something we want to
support better? I was thinking that we could add a new pg_dump mode (maybe
only usable during pg_upgrade) that also restores the pg_subscription_rel
content in each subscription or something like that. If not, should pg_upgrade
keep preserving the subscriptions as it doesn't seem safe to use them, or at
least document the hazards (I didn't find anything about it in the


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