Question: What are PostgreSQL replication slots and how do they work?

Answer

Replication slots are a feature of PostgreSQL, introduced in version 9.4, that provide a more robust way to handle data replication between a primary database server and one or more secondary servers (standbys). Replication slots ensure that the primary does not remove WAL (Write-Ahead Logging) segments until they have been received by all standbys using those slots, thus preventing data loss in case of delayed replication or network issues.

How Replication Slots Work

A replication slot is created on the primary database server for each standby server. The slot keeps track of the last confirmed receive location of WAL data on the standby. This mechanism allows the primary to know precisely how far each standby has replicated and thus which WAL files can be safely removed.

Creating a Replication Slot

To create a physical replication slot, you use the pg_create_physical_replication_slot function. Here's how:

SELECT * FROM pg_create_physical_replication_slot('my_slot');

This command creates a new replication slot named 'my_slot'. Remember, this action should be performed on the primary server.

Viewing Replication Slots

To view existing replication slots and their status, execute the following query:

SELECT * FROM pg_replication_slots;

This will list all replication slots along with details such as whether they are active, the last WAL file sent to the standby, and more.

Deleting a Replication Slot

When a standby server is no longer needed, its replication slot should be removed to prevent unnecessary retention of WAL files. To do this, use the pg_drop_replication_slot function:

SELECT pg_drop_replication_slot('my_slot');

Execute this command on the primary server, specifying the name of the slot you wish to remove.

Advantages of Replication Slots

1. Data Safety: Ensures no data loss by keeping necessary WAL files until all standbys have confirmed receipt.
2. Disk Space Management: Helps in managing disk space on the primary by allowing safe removal of WAL files that are confirmed to be applied on all standbys.
3. Monitoring: Facilitates monitoring of replication lag and other performance metrics.

Conclusion

Replication slots are a vital component in PostgreSQL replication setups, offering a reliable and efficient way to manage WAL file retention and ensuring data consistency across replicas. Proper management of replication slots is essential for maintaining the health and performance of your PostgreSQL replication environment.