On Fri, May 30, 2025 at 4:05 PM Amul Sul <sula...@gmail.com> wrote: > > Quick question -- due to my limited understanding of this area: why > can't we perform an action similar to pg_logical_slot_get_changes() > implicitly from pg_sync_replication_slots()? Would there be any > implications of doing so? >
Yes, there would be implications if we did it that way. It would mean that the consumer of the slot may not process those changes (for which sync_slot API has done the get_changes) and send it to the client. Consider a publisher-subscriber and physical standby setup. In this setup, the subscriber creates a logical slot corresponding to the subscription on the publisher. Now, the publisher process changes and sends it to the subscriber; then the slot is advanced (both its xmin and WAL locations) once the corresponding changes are sent to the client. If we allow pg_sync_replication_slots() to do pg_logical_slot_get_changes or equivalent in some way, then we may end up advancing the slot without sending the changes to the subscriber, which would be considered a data loss for the subscriber. I have explained in terms of built-in logical replication, but the external plugins using these APIs (pg_logical_*) should be doing something similar to process the changes and advance the slot. Does this answer your question and make sense to you? -- With Regards, Amit Kapila.
