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

On Tue, 31 Dec 2024 at 10:15, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> Hi all,
>
> Logical decoding (and logical replication) are available only when
> wal_level = logical. As the documentation says[1], Using the 'logical'
> level increases the WAL volume which could negatively affect the
> performance. For that reason, users might want to start with using
> 'replica', but when they want to use logical decoding they need a
> server restart to increase wal_level to 'logical'. My goal is to allow
> users who are using 'replica' level to use logical decoding without a
> server restart. There are other GUC parameters related to logical
> decoding and logical replication such as max_wal_senders,
> max_logical_replication_workers, and max_replication_slots, but even
> if users set these parameters >0, there would not be a noticeable
> performance impact. And their default values are already >0. So I'd
> like to focus on making only the wal_level dynamic GUC parameter.
> There are several earlier discussions[2][3] but no one has submitted
> patches unless I'm missing something.
>
> The first idea I came up with is to make the wal_level a PGC_SIGHUP
> parameter. However, it affects not only setting 'replica' to 'logical'
> but also setting 'minimal' to 'replica' or higher. I'm not sure the
> latter case is common and it might require a checkpoint. I don't want
> to make the patch complex for uncommon cases.
>
> The second idea is to somehow allow both WAL-logging logical info and
> logical decoding even when wal_level is 'replica'. I've attached a PoC
> patch for that. The patch introduces new SQL functions such as
> pg_activate_logical_decoding() and pg_deactivate_logical_decoding().
> These functions are available only when wal_level is 'repilca'(or
> higher). In pg_activate_logical_decoding(), we set the status of
> logical decoding stored on the shared memory from 'disabled' to
> 'xlog-logical-info', allowing all processes to write logical
> information to WAL records for logical decoding. But the logical
> decoding is still not allowed. Once we confirm all in-progress
> transactions completed, we switch the status to
> 'logical-decoding-ready', meaning that users can create logical
> replication slots and use logical decoding.

I felt that the only disadvantage with this approach is that we
currently wait for all in-progress transactions to complete before
enabling logical decoding. If a long-running transaction exists and
the session enabling logical decoding fails—due to factors like
statement_timeout, transaction_timeout,
idle_in_transaction_session_timeout, idle_session_timeout, or any
other failure. This would require restarting the wait. During this
time, there's a risk that a new long-running transaction could start,
further delaying the process. Probably this could be solved if the
waiting is done from any of the background processes through
PGC_SIGHUP. While this type of failure is likely rare, I’m unsure
whether we should consider this scenario.

Thoughts?

Regards,
Vignesh