Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

On Fri, Jan 3, 2025, at 10:14 AM, Bertrand Drouvot wrote:
> If we don't want to force wal_level = logical to enable logical decoding (your
> second idea) then I think that it would be better to "hide" everything behind
> logical replication slot creation / deletion. That would mean that having at
> least one logical replication slot created would be synonym to "activate logical
> decoding" and zero logical replication slot created would be synonym to "deactivate
> logical decoding".

I like this idea. The logical replication slot existence already has the
required protections and guarantees (no running transactions from the past while
creating) for logical decoding.

ERROR: logical decoding requires "wal_level" >= "logical"
STATEMENT: select pg_create_logical_replication_slot('test', 'pgoutput');

FATAL: logical replication slot "test" exists, but "wal_level" < "logical"
HINT: Change "wal_level" to be "logical" or higher.

Having said that, you are basically folding 'logical' machinery into 'replica'.
The 'logical' option can still exists but it will be less attractive because it
increases the WAL volume even if you are not using logical replication. I don't
know if the current 'logical' behavior (WAL records for logical decoding even
if there is no active logical replication) has any use case (maybe someone
inspects these extra records for analysis) but one suggestion (separate patch)
is to make 'logical' synonymous with the new 'replica' behavior (logical
decoding capability). This opens the door to remove 'logical' in future
releases (accepted as synonym but not documented).

--
Euler Taveira
EDB https://www.enterprisedb.com/