Also called: logical sender gating
In plain English
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.
Why it matters
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.