Replication Slots are a new feature in PostgreSQL 9.4. They allow you to define your replication architecture explicitly. They also allow you to track details of nodes even when they are disconnected. Replication Slots work with both PSR and LSR.
Replication Slots make data persist about the downstream node. They are crash-safe, so if a connection is lost, the slot still continues to exist. By tracking data on downstream nodes, we avoid these problems:
hot_standby_feedbackis lost. When the standby reconnects, it may be sent cleanup records that result in query conflicts. Replication Slots remember the standby's
xminvalue even when disconnected, ensuring that cleanup ...