Also called: slot sync, replication slot sync
In plain English
Slot synchronization is the PostgreSQL 17 mechanism that copies failover-enabled replication slots from the primary onto a standby. A background slot sync worker (or the pg_sync_replication_slots() function) reads the primary’s slots and maintains local synced copies, advanced only to positions the standby has safely received.
Why it matters
It is what makes a failover slot actually survive: the bookmark is kept current on the standby in real time, so after promotion the new primary already has it. The synced copies are read-only placeholders until the standby is promoted.