synchronized_standby_slots
Also called: logical sender gating
synchronized_standby_slots is a PostgreSQL 17 setting on the primary. You list the physical replication slots of your failover-target standbys, and the primary's logical walsenders hold back changes until those standbys have confirmed the WAL, so a logical subscriber can never read past the node you'd promote.
What this means
A Postgres 17 setting on the primary. You list your failover-target standbys' slots, and the primary makes its logical senders hold changes back until those standbys have the WAL, so a logical subscriber can never read past the very node you'd promote in a failover.
Why it matters operationally
It closes a correctness trap: if a subscriber raced ahead of the failover-target standby and you then promoted that standby, the subscriber would be ahead of the new primary, an inconsistency. This gating guarantees the subscriber stays at or behind the future primary.