What is a replication slot and what is it used for?

The replication slot in PostgreSQL is a mechanism that allows maintaining the replication state for a specific subscriber (replica or logical client). It guarantees that the server will not delete WAL segments (transaction logs) until they are confirmed by the slot, meaning until the replica has read this data.

Main tasks of the replication slot:

• Ensure data safety for the replica so it can catch up with the master without data loss.
• Support logical or physical replication.
• Allow clients to receive changes in real-time (for example, for CDC — Change Data Capture).

Without a replication slot, the server may delete WAL segments that have not yet been transmitted to the replica, which can lead to errors and the need for full resynchronization.