What is Replication Slot in PostgreSQL: Explanation and Usage

A replication slot in PostgreSQL is a feature that ensures the database keeps the necessary data changes until a replica has received them. It prevents the database from removing data needed by replicas, helping maintain consistent replication without data loss.

How It Works

Think of a replication slot as a bookmark in a book that you share with a friend. The book is the database's write-ahead log (WAL), which records all changes. The bookmark marks the last page your friend (the replica) has read.

PostgreSQL uses replication slots to remember how far each replica has read in the WAL. This way, the database knows not to delete or recycle WAL data that replicas still need. It helps replicas catch up even if they temporarily disconnect.

This mechanism ensures data consistency and avoids losing changes during replication by keeping the required data until all replicas confirm they have received it.

Example

This example shows how to create a physical replication slot and check its status.

sql

SELECT * FROM pg_create_physical_replication_slot('my_slot');

SELECT slot_name, plugin, slot_type, active, restart_lsn FROM pg_replication_slots WHERE slot_name = 'my_slot';

Output

 slot_name | plugin | slot_type | active | restart_lsn 
-----------+--------+-----------+--------+-------------
 my_slot   |        | physical  | f      | 0/3000028
(1 row)

When to Use

Use replication slots when you have one or more replicas that need to stay in sync with the primary database. They are essential in streaming replication setups to avoid losing WAL data before replicas receive it.

For example, if a replica disconnects due to network issues, the replication slot keeps the WAL data until the replica reconnects and catches up. This prevents data loss and replication errors.

Replication slots are also useful in logical replication to track changes for specific subscribers.

Key Points

• Replication slots prevent WAL data from being removed before replicas receive it.
• They track how far each replica has processed changes.
• Essential for reliable streaming and logical replication.
• Must be monitored to avoid disk space issues if replicas lag too long.

Key Takeaways

Replication slots ensure replicas receive all necessary data changes without loss.

They act like bookmarks tracking replica progress in the WAL stream.

Use them in streaming or logical replication setups for data consistency.

Monitor slots to prevent disk space issues from accumulating unused WAL files.