synchronized_standby_slots used in logical replication

Hi,

I'm working with *logical replication* in a PostgreSQL 17 setup, and I'm
exploring the new synchronized_standby_slots parameter to make replication
slots failover safe in a highly available environment using physical
standby nodes managed by Patroni.

While testing this feature, I encountered a blocking behavior, when a *standby
is listed in synchronized_standby_slots* and that standby goes
offline, *logical
replication on the primary stops progressing*. From what I understand, the
primary node waits for the standby to acknowledge received wal records,
effectively stalling WAL decoding for the logical slot. I noticed that the
failover slot on the standby continue to be synced.

This raises several questions about the *tradeoffs and implications* of
using this feature:

- What are the *risks or limitations* if synchronized_standby_slots is *left
empty* (the default)? Is there a risk of data loss or inconsistency for
logical subscribers in such cases?
- Is it expected behavior that *any failure of a standby listed in
synchronized_standby_slots stalls logical decoding* on the primary? If
so, are there any ways to *avoid blocking* WAL decoding while still
having slot synchronization?
- Patroni is managing FO slots better than native Postgres
impletmentation?

Thanks for helping on those points

Regards,

Fabrice