On Fri, Jun 13, 2025 at 9:46 PM Amit Kapila <amit.kapil...@gmail.com> wrote:
> On Fri, Jun 13, 2025 at 10:52 PM Perumal Raj <peruci...@gmail.com> wrote: > > > > Thanks for explanation Shveta! > > > > ------------ > > As Summary in this original thread, > > > > Prerequisites for Setting Up a Logical Replication Slot sync in >= pg17 > > > > To successfully configure a logical replication slot, ensure the > following settings are applied: > > > > wal_level = 'logical' > > hot_standby = 'on' > > hot_standby_feedback = 'on' > > sync_replication_slots = 'on' > > > > Additionally, you need to configure primary_slot_name on the standby > and have dbname in primary_conninfo. For further details, you can > refer docs (1)(2). > *Thanks for the notes here. * *Yes, those parameters configured as part of Normal Physical replication setup. * > > Replication Slot Synchronization > > > > Logical replication slots can synchronize with all direct standby > servers of the primary but are not compatible with cascade standby servers. > > > > Temporary Status of New Standby Slots > > > > If a new standby server is created after the logical replication slot, > it will be marked as temporary=true until the reset_lsn of the primary > matches the confirmed_lsn of the new standby. > > > > It is restart_lsn on both nodes, but there are other things like > slot's catalog_xmin as well. As a user, you need to ensure that your > primary's logical slot is being consumed. And this is required > primarily at the initial sync time so that we sync the slot only if > the standby has required resources like WAL to allow decoding from the > synced slot after failover. > *Thanks for correcting,* > > Limitations on Using Logical Replication Slots > > > > While logical replication slots can synchronize on the direct standby > side, they cannot be utilized (as in the case of Debezium) until the > standby server is promoted to primary. Attempting to use a synchronized > logical slot on a standby server will result in the following error: > > > > org.postgresql.util.PSQLException: ERROR: cannot use replication slot > "kafka_logical_slot" for logical decoding > > Detail: This replication slot is being synchronized from the primary > server. > > > > I don't think we can call this a limitation. According to me, this is > a requirement for this feature to work. Consider if we allow the use > of this synced slot for decoding when sync is still in-progress, this > slot could be advanced ahead of the primary. Now, after the failover, > we won't be able to reuse this slot to allow the subscribers to > continue replication. > *Make sense, * > (1) - > https://www.postgresql.org/docs/devel/logicaldecoding-explanation.html#LOGICALDECODING-REPLICATION-SLOTS-SYNCHRONIZATION > (2) - > https://www.postgresql.org/docs/devel/logical-replication-failover.html > > -- > With Regards, > Amit Kapila. >
